[class] (original) (raw)

11 Classes [class]

11.1 Preamble [class.pre]

A class-name is inserted into the scope in which it is declared immediately after the class-name is seen.

Theclass-name is also inserted into the scope of the class itself; this is known as the injected-class-name.

For purposes of access checking, the injected-class-name is treated as if it were a public member name.

A class-specifier is commonly referred to as a class definition.

A class is considered defined after the closing brace of itsclass-specifier has been seen even though its member functions are in general not yet defined.

If a class-head-name contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which thenested-name-specifier refers, or in an element of the inline namespace set ([namespace.def]) of that namespace (i.e., not merely inherited or introduced by a using-declaration), and theclass-specifier shall appear in a namespace enclosing the previous declaration.

[ Note

:

The class-key determines whether the class is a union ([class.union]) and whether access is public or private by default ([class.access]).

A union holds the value of at most one data member at a time.

— end note

]

If a class is marked with the class-virt-specifier final and it appears as a class-or-decltype in a base-clause, the program is ill-formed.

Whenever aclass-key is followed by a class-head-name, theidentifier final, and a colon or left brace, final is interpreted as a class-virt-specifier.

[ Example

:

struct A; struct A final {};

struct X { struct C { constexpr operator int() { return 5; } }; struct B final : C{};

};

— end example

]

[ Note

:

Complete objects of class type have nonzero size.

Base class subobjects and members declared with the no_­unique_­address attribute ([dcl.attr.nouniqueaddr]) are not so constrained.

— end note

]

[ Note

:

Class objects can be assigned ([over.ass], [class.copy.assign]), passed as arguments to functions ([dcl.init], [class.copy.ctor]), and returned by functions (except objects of classes for which copying or moving has been restricted; see [dcl.fct.def.delete] and [class.access]).

Other plausible operators, such as equality comparison, can be defined by the user; see [over.oper].

— end note

]

11.2 Properties of classes [class.prop]

A trivially copyable class is a class:

A trivial class is a class that is trivially copyable and has one or more eligible default constructors ([class.default.ctor]), all of which are trivial.

[ Note

:

In particular, a trivially copyable or trivial class does not have virtual functions or virtual base classes.

— end note

]

A class S is a standard-layout class if it:

[ Example

:

struct B { int i; };
struct C : B { };
struct D : C { };
struct E : D { char : 4; };

struct Q {}; struct S : Q { }; struct T : Q { }; struct U : S, T { };

— end example

]

A standard-layout struct is a standard-layout class defined with the class-key struct or theclass-key class.

A standard-layout union is a standard-layout class defined with theclass-key union.

[ Note

:

Standard-layout classes are useful for communicating with code written in other programming languages.

Their layout is specified in [class.mem].

— end note

]

[ Example

:

struct N {
int i; int j; virtual ~N(); };

struct T {
int i; private: int j; };

struct SL {
int i; int j; ~SL(); };

struct POD {
int i; int j; };

— end example

]

A class S is an implicit-lifetime classif it is an aggregate or has at least one trivial eligible constructor and a trivial, non-deleted destructor.

11.3 Class names [class.name]

A class definition introduces a new type.

[ Example

:

struct X { int a; }; struct Y { int a; }; X a1; Y a2; int a3;

declares three variables of three different types.

This implies that

a1 = a2;
a1 = a3;

are type mismatches, and that

int f(X); int f(Y);

declare an overloaded function f() and not simply a single function f() twice.

For the same reason,

struct S { int a; }; struct S { int a; };

is ill-formed because it defines S twice.

— end example

]

A class declaration introduces the class name into the scope whereit is declared and hides any class, variable, function, or other declaration of that name in an enclosing scope.

If a class name is declared in a scope where a variable, function, or enumerator of the same name is also declared, then when both declarations are in scope, the class can be referred to only using anelaborated-type-specifier ([basic.lookup.elab]).

[ Example

:

struct stat {

};

stat gstat;

int stat(struct stat*);

void f() { struct stat* ps;
stat(ps);
}

— end example

]

A declaration consisting solely of class-key identifier; is either a redeclaration of the name in the current scope or a forward declaration of the identifier as a class name.

It introduces the class name into the current scope.

[ Example

:

struct s { int a; };

void g() { struct s;
s* p;
struct s { char* p; };
struct s;
}

— end example

]

[ Note

:

Such declarations allow definition of classes that refer to each other.

[ Example

:

class Vector;

class Matrix {

friend Vector operator*(const Matrix&, const Vector&); };

class Vector {

friend Vector operator*(const Matrix&, const Vector&); };

Declaration of friends is described in [class.friend], operator functions in [over.oper].

— end example

]

— end note

]

[ Note

:

It differs from a class declaration in that if a class of the elaborated name is in scope the elaborated name will refer to it.

— end note

]

[ Example

:

struct s { int a; };

void g(int s) { struct s* p = new struct s;
p->a = s;
}

— end example

]

[ Note

:

For example,

class A * A;

first specifies A to be the name of a class and then redefines it as the name of a pointer to an object of that class.

This means that the elaborated form class A must be used to refer to the class.

Such artistry with names can be confusing and is best avoided.

— end note

]

11.4 Class members [class.mem]

member-specification: member-declaration member-specification access-specifier : member-specification

member-declaration: attribute-specifier-seq decl-specifier-seq member-declarator-list ; function-definition using-declaration using-enum-declaration static_assert-declaration template-declaration explicit-specialization deduction-guide alias-declaration opaque-enum-declaration empty-declaration

member-declarator-list: member-declarator member-declarator-list , member-declarator

member-declarator: declarator virt-specifier-seq pure-specifier declarator requires-clause declarator brace-or-equal-initializer identifier attribute-specifier-seq : constant-expression brace-or-equal-initializer

virt-specifier-seq: virt-specifier virt-specifier-seq virt-specifier

virt-specifier: override final

pure-specifier: = 0

The member-specification in a class definition declares the full set of members of the class; no member can be added elsewhere.

A direct member of a class X is a member of Xthat was first declared within the member-specification of X, including anonymous union objects ([class.union.anon]) and direct members thereof.

Members of a class are data members, member functions, nested types, enumerators, and member templates and specializations thereof.

[ Note

:

A specialization of a static data member template is a static data member.

A specialization of a member function template is a member function.

A specialization of a member class template is a nested class.

— end note

]

For any other member-declaration, each declared entity that is not an unnamed bit-fieldis a member of the class, and each such member-declarationshall either declare at least one member name of the class or declare at least one unnamed bit-field.

A data member is a non-function member introduced by amember-declarator.

A member function is a member that is a function.

The enumerators of an unscoped enumeration defined in the class are members of the class.

A data member or member function may be declared static in its member-declaration, in which case it is a static member (see [class.static]) (a static data member ([class.static.data]) orstatic member function ([class.static.mfct]), respectively) of the class.

Any other data member or member function is a non-static member(a non-static data member ornon-static member function ([class.mfct.non-static]), respectively).

[ Note

:

A non-static data member of non-reference type is a member subobject of a class object.

— end note

]

A member shall not be declared twice in themember-specification, except that

[ Note

:

A single name can denote several member functions provided their types are sufficiently different ([over.load]).

— end note

]

[ Note

:

A complete-class context of a nested class is also a complete-class context of any enclosing class, if the nested class is defined within the member-specification of the enclosing class.

— end note

]

A class is considered a completely-defined object type ([basic.types]) (or complete type) at the closing } of the class-specifier.

The class is regarded as complete within its complete-class contexts; otherwise it is regarded as incomplete within its own classmember-specification.

[ Example

:

struct S { using T = void(); T * p = 0;
virtual T f = 0;
};

— end example

]

[ Example

:

int a; const int b = 0; struct S { int x1 : 8 = 42;
int x2 : 8 { 42 };
int y1 : true ? 8 : a = 42;
int y2 : true ? 8 : b = 42;
int y3 : (true ? 8 : b) = 42; int z : 1 || new int { 0 };
};

— end example

]

(For static data members, see [class.static.data]; for non-static data members, see [class.base.init] and [dcl.init.aggr]).

A brace-or-equal-initializer for a non-static data memberspecifies a default member initializer for the member, and shall not directly or indirectly cause the implicit definition of a defaulted default constructor for the enclosing class or the exception specification of that constructor.

Within a class definition, a member shall not be declared with the thread_­local storage-class-specifier unless also declared static.

Apure-specifier shall be used only in the declaration of avirtual functionthat is not a friend declaration.

The optional attribute-specifier-seq in a member-declarationappertains to each of the entities declared by the member-declarators; it shall not appear if the optional member-declarator-list is omitted.

A virt-specifier-seq shall contain at most one of eachvirt-specifier.

A virt-specifier-seqshall appear only in the first declaration of a virtual member function ([class.virtual]).

The type of a non-static data member shall not be an incomplete type ([basic.types]), an abstract class type ([class.abstract]), or a (possibly multi-dimensional) array thereof.

[ Note

:

In particular, a class C cannot contain a non-static member of class C, but it can contain a pointer or reference to an object of class C.

— end note

]

[ Note

:

See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.

— end note

]

[ Note

:

The type of a non-static member function is an ordinary function type, and the type of a non-static data member is an ordinary object type.

There are no special member function types or data member types.

— end note

]

[ Example

:

A simple example of a class definition is

struct tnode { char tword[20]; int count; tnode* left; tnode* right; };

which contains an array of twenty characters, an integer, and two pointers to objects of the same type.

Once this definition has been given, the declaration

tnode s, *sp;

declares s to be a tnode and sp to be a pointer to a tnode.

With these declarations, sp->count refers to the count member of the object to which sp points;s.left refers to the left subtree pointer of the objects; and s.right->tword[0] refers to the initial character of the tword member of the right subtree of s.

— end example

]

[ Note

:

Non-static data members of a (non-union) class with the same access control and non-zero size ([intro.object]) are allocated so that later members have higher addresses within a class object.

The order of allocation of non-static data members with different access control is unspecified.

Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other; so might requirements for space for managing virtual functions ([class.virtual]) and virtual base classes ([class.mi]).

— end note

]

If T is the name of a class, then each of the following shall have a name different from T:

In addition, if class T has a user-declaredconstructor, every non-static data member of classT shall have a name different from T.

The common initial sequence of two standard-layout struct ([class.prop]) types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the structs, such that corresponding entities have layout-compatible types, either both entities are declared with the no_­unique_­address attribute ([dcl.attr.nouniqueaddr]) or neither is, and either both entities are bit-fields with the same width or neither is a bit-field.

[ Example

:

struct A { int a; char b; }; struct B { const int b1; volatile char b2; }; struct C { int c; unsigned : 0; char b; }; struct D { int d; char b : 4; }; struct E { unsigned int e; char b; };

The common initial sequence of A and B comprises all members of either class.

The common initial sequence of A and C and of A and D comprises the first member in each case.

The common initial sequence of A and E is empty.

— end example

]

Two standard-layout struct ([class.prop]) types arelayout-compatible classes if their common initial sequence comprises all members and bit-fields of both classes ([basic.types]).

Two standard-layout unions are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in any order) have layout-compatible types.

In a standard-layout union with an active memberof struct type T1, it is permitted to read a non-static data member m of another union member of struct type T2provided m is part of the common initial sequence of T1 and T2; the behavior is as if the corresponding member of T1 were nominated.

[ Example

:

struct T1 { int a, b; }; struct T2 { int c; double d; }; union U { T1 t1; T2 t2; }; int f() { U u = { { 1, 2 } };
return u.t2.c;
}

— end example

]

[ Note

:

Reading a volatile object through a glvalue of non-volatile type has undefined behavior ([dcl.type.cv]).

— end note

]

If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member if that member is not a bit-field.

Its address is also the same as the address of each of its base class subobjects.

[ Note

:

There might therefore be unnamed padding within a standard-layout struct object inserted by an implementation, but not at its beginning, as necessary to achieve appropriate alignment.

— end note

]

11.4.1 Member functions [class.mfct]

A member function may be defined in its class definition, in which case it is an inline ([dcl.inline]) member function if it is attached to the global module, or it may be defined outside of its class definition if it has already been declared but not defined in its class definition.

[ Note

:

A member function is also inline if it is declaredinline, constexpr, or consteval.

— end note

]

A member function definition that appears outside of the class definition shall appear in a namespace scope enclosing the class definition.

Except for member function definitions that appear outside of a class definition, and except for explicit specializations of member functions of class templates and member function templates ([temp.spec]) appearing outside of the class definition, a member function shall not be redeclared.

[ Note

:

There can be at most one definition of a non-inline member function in a program.

There may be more than one inline member function definition in a program.

— end note

]

[ Note

:

Member functions of a class have the linkage of the name of the class.

— end note

]

If the definition of a member function is lexically outside its class definition, the member function name shall be qualified by its class name using the ​::​ operator.

[ Example

:

struct X { typedef int T; static T count; void f(T); }; void X::f(T t = count) { }

The member function f of class X is defined in global scope; the notation X​::​f specifies that the function fis a member of class X and in the scope of class X.

In the function definition, the parameter type T refers to the typedef member T declared in class X and the default argument count refers to the static data member countdeclared in class X.

— end example

]

[ Note

:

A static local variable or local type in a member function always refers to the same entity, whether or not the member function is inline.

— end note

]

Previously declared member functions may be mentioned in friend declarations.

Member functions of a local class shall be defined inline in their class definition, if they are defined at all.

[ Note

:

A member function can be declared (but not defined) using a typedef for a function type.

The resulting member function has exactly the same type as it would have if the function declarator were provided explicitly, see [dcl.fct].

For example,

typedef void fv(); typedef void fvc() const; struct S { fv memfunc1;
void memfunc2(); fvc memfunc3;
}; fv S::* pmfv1 = &S::memfunc1; fv S::* pmfv2 = &S::memfunc2; fvc S::* pmfv3 = &S::memfunc3;

— end note

]

11.4.2 Non-static member functions [class.mfct.non-static]

A non-static member function may be called for an object of its class type, or for an object of a class derivedfrom its class type, using theclass member access syntax ([over.match.call]).

A non-static member function may also be called directly using the function call syntax ([expr.call], [over.match.call]) from within its class or a class derived from its class, or a member thereof, as described below.

If a non-static member function of a class X is called for an object that is not of type X, or of a type derived fromX, the behavior is undefined.

[ Note

:

If C is not X or a base class of X, the class member access expression is ill-formed.

— end note

]

This transformation does not apply in the template definition context ([temp.dep.type]).

[ Example

:

struct tnode { char tword[20]; int count; tnode* left; tnode* right; void set(const char*, tnode* l, tnode* r); };

void tnode::set(const char* w, tnode* l, tnode* r) { count = strlen(w)+1; if (sizeof(tword)<=count) perror("tnode string too long"); strcpy(tword,w); left = l; right = r; }

void f(tnode n1, tnode n2) { n1.set("abc",&n2,0); n2.set("def",0,0); }

In the body of the member function tnode​::​set, the member namestword, count, left, and right refer to members of the object for which the function is called.

Thus, in the call n1.set("abc",&n2,0), tword refers ton1.tword, and in the call n2.set("def",0,0), it refers to n2.tword.

The functions strlen, perror, andstrcpy are not members of the class tnode and should be declared elsewhere.102

— end example

]

A non-static member function may be declared const,volatile, or const volatile.

They also affect the function type of the member function; a member function declaredconst is a const member function, a member function declared volatile is a volatile member function and a member function declared const volatile is aconst volatile member function.

[ Example

:

struct X { void g() const; void h() const volatile; };

X​::​g is a const member function and X​::​h is a const volatile member function.

— end example

]

A non-static member function may be declared virtual ([class.virtual]) or pure virtual ([class.abstract]).

11.4.2.1 The this pointer [class.this]

In the body of a non-static ([class.mfct]) member function, the keyword this is a prvalue whose value is a pointer to the object for which the function is called.

The type of this in a member function whose type has a cv-qualifier-seq cv and whose class is Xis “pointer to cv X”.

[ Note

:

Thus in a const member function, the object for which the function is called is accessed through a const access path.

— end note

]

[ Example

:

struct s { int a; int f() const; int g() { return a++; } int h() const { return a++; } };

int s::f() const { return a; }

The a++ in the body of s​::​h is ill-formed because it tries to modify (a part of) the object for which s​::​h() is called.

This is not allowed in a const member function becausethis is a pointer to const; that is, *this hasconst type.

— end example

]

[ Note

:

Similarly, volatile semantics apply in volatile member functions when accessing the object and its non-static data members.

— end note

]

[ Example

:

void k(s& x, const s& y) { x.f(); x.g(); y.f(); y.g();
}

The call y.g() is ill-formed because y is constand s​::​g() is a non-const member function, that is,s​::​g() is less-qualified than the object expression y.

— end example

]

[ Note

:

Constructors and destructors cannot be declared const, volatile, or const volatile.

However, these functions can be invoked to create and destroy objects with cv-qualified types; see [class.ctor] and [class.dtor].

— end note

]

11.4.3 Special member functions [special]

Default constructors ([class.default.ctor]), copy constructors, move constructors ([class.copy.ctor]), copy assignment operators, move assignment operators ([class.copy.assign]), and prospective destructors ([class.dtor]) arespecial member functions.

[ Note

:

The implementation will implicitly declare these member functions for some class types when the program does not explicitly declare them.

The implementation will implicitly define them if they are odr-used ([basic.def.odr]) or needed for constant evaluation ([expr.const]).

— end note

]

An implicitly-declared special member function is declared at the closing} of the class-specifier.

Programs shall not define implicitly-declared special member functions.

Programs may explicitly refer to implicitly-declared special member functions.

[ Example

:

A program may explicitly call or form a pointer to member to an implicitly-declared special member function.

struct A { };
struct B : A { B& operator=(const B &); }; B& B::operator=(const B& s) { this->A::operator=(s);
return *this; }

— end example

]

[ Note

:

The special member functions affect the way objects of class type are created, copied, moved, and destroyed, and how values can be converted to values of other types.

Often such special member functions are called implicitly.

— end note

]

Special member functions obey the usual access rules ([class.access]).

[ Example

:

Declaring a constructor protected ensures that only derived classes and friends can create objects using it.

— end example

]

Two special member functions are of the same kind if:

An eligible special member function is a special member function for which:

For a class, its non-static data members, its non-virtual direct base classes, and, if the class is not abstract ([class.abstract]), its virtual base classes are called its potentially constructed subobjects.

A defaulted special member function isconstexpr-compatibleif the corresponding implicitly-declared special member function would be a constexpr function.

11.4.4 Constructors [class.ctor]

Constructors do not have names.

[ Example

:

struct S { S();
};

S::S() { }

— end example

]

A constructor is used to initialize objects of its class type.

Because constructors do not have names, they are never found during name lookup; however an explicit type conversion using the functional notation ([expr.type.conv]) will cause a constructor to be called to initialize an object.

[ Note

:

The syntax looks like an explicit call of the constructor.

— end note

]

[ Example

:

complex zz = complex(1,2.3); cprint( complex(7.8,1.2) );

— end example

]

[ Note

:

For initialization of objects of class type see [class.init].

— end note

]

An object created in this way is unnamed.

[ Note

:

Explicit constructor calls do not yield lvalues, see [basic.lval].

— end note

]

[ Note

:

Some language constructs have special semantics when used during construction; see [class.base.init] and [class.cdtor].

— end note

]

A constructor can be invoked for aconst,volatileorconst volatileobject.

constandvolatilesemantics ([dcl.type.cv]) are not applied on an object under construction.

They come into effect when the constructor for the most derived object ([intro.object]) ends.

Areturnstatement in the body of a constructor shall not specify a return value.

The address of a constructor shall not be taken.

A constructor shall not be a coroutine.

11.4.4.1 Default constructors [class.default.ctor]

A default constructor for a class Xis a constructor of class Xfor which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters).

If there is no user-declared constructor for classX, a non-explicit constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]).

An implicitly-declared default constructor is an inline public member of its class.

A defaulted default constructor for class X is defined as deleted if:

A default constructor istrivialif it is not user-provided and if:

Otherwise, the default constructor isnon-trivial.

A default constructor that is defaulted and not defined as deleted isimplicitly definedwhen it is odr-used ([basic.def.odr]) to create an object of its class type ([intro.object]), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.

The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with noctor-initializer and an emptycompound-statement.

If that user-written default constructor would be ill-formed, the program is ill-formed.

If that user-written default constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined default constructor is constexpr.

Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members are implicitly defined.

[ Note

:

An implicitly-declared default constructor has an exception specification ([except.spec]).

An explicitly-defaulted definition might have an implicit exception specification, see [dcl.fct.def].

— end note

]

A program is ill-formed if the default constructor for an object is implicitly used and the constructor is not accessible ([class.access]).

[ Note

:

[class.base.init] describes the order in which constructors for base classes and non-static data members are called and describes how arguments can be specified for the calls to these constructors.

— end note

]

11.4.4.2 Copy/move constructors [class.copy.ctor]

A non-template constructor for classXis a copy constructor if its first parameter is of typeX&,const X&,volatile X&orconst volatile X&, and either there are no other parameters or else all other parameters have default arguments ([dcl.fct.default]).

[ Example

:

X​::​X(const X&)andX​::​X(X&,int=1)are copy constructors.

struct X { X(int); X(const X&, int = 1); }; X a(1);
X b(a, 0);
X c = b;

— end example

]

A non-template constructor for class X is a move constructor if its first parameter is of type X&&, const X&&,volatile X&&, or const volatile X&&, and either there are no other parameters or else all other parameters have default arguments ([dcl.fct.default]).

[ Example

:

Y​::​Y(Y&&) is a move constructor.

struct Y { Y(const Y&); Y(Y&&); }; extern Y f(int); Y d(f(1));
Y e = d;

— end example

]

[ Note

:

All forms of copy/move constructor may be declared for a class.

[ Example

:

struct X { X(const X&); X(X&);
X(X&&); X(const X&&);
};

— end example

]

— end note

]

[ Note

:

If a classXonly has a copy constructor with a parameter of typeX&, an initializer of typeconst Xorvolatile Xcannot initialize an object of typecv X.

[ Example

:

struct X { X();
X(X&);
}; const X cx; X x = cx;

— end example

]

— end note

]

A declaration of a constructor for a classXis ill-formed if its first parameter is of typecv Xand either there are no other parameters or else all other parameters have default arguments.

A member function template is never instantiated to produce such a constructor signature.

[ Example

:

struct S { template S(T); S(); };

S g;

void h() { S a(g);

}

— end example

]

If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly.

If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted ([dcl.fct.def]).

The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor ([depr.impldec]).

The implicitly-declared copy constructor for a classXwill have the form

X::X(const X&)

if each potentially constructed subobject of a class typeM(or array thereof) has a copy constructor whose first parameter is of typeconst M&orconst volatile M&.103

Otherwise, the implicitly-declared copy constructor will have the form

X::X(X&)

If the definition of a class X does not explicitly declare a move constructor, a non-explicit one will be implicitly declared as defaulted if and only if

[ Note

:

When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor may instead invoke a copy constructor.

— end note

]

The implicitly-declared move constructor for class X will have the form

X::X(X&&)

An implicitly-declared copy/move constructor is an inline public member of its class.

A defaulted copy/​move constructor for a classX is defined as deleted ([dcl.fct.def.delete]) if X has:

[ Note

:

A defaulted move constructor that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).

Such a constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead.

— end note

]

A copy/move constructor for classXis trivial if it is not user-provided and if:

otherwise the copy/move constructor isnon-trivial.

A copy/move constructor that is defaulted and not defined as deleted isimplicitly definedwhen it is odr-used ([basic.def.odr]), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.

If the implicitly-defined constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined constructor is constexpr.

Before the defaulted copy/move constructor for a class is implicitly defined, all non-user-provided copy/move constructors for its potentially constructed subobjects are implicitly defined.

[ Note

:

An implicitly-declared copy/move constructor has an implied exception specification ([except.spec]).

— end note

]

The implicitly-defined copy/move constructor for a non-union classXperforms a memberwise copy/move of its bases and members.

[ Note

:

Default member initializers of non-static data members are ignored.

See also the example in [class.base.init].

— end note

]

The order of initialization is the same as the order of initialization of bases and members in a user-defined constructor (see [class.base.init]).

Let x be either the parameter of the constructor or, for the move constructor, an xvalue referring to the parameter.

Each base or non-static data member is copied/moved in the manner appropriate to its type:

Virtual base class subobjects shall be initialized only once by the implicitly-defined copy/move constructor (see [class.base.init]).

The implicitly-defined copy/move constructor for a unionX copies the object representation ([basic.types]) of X.

For each object nested within ([intro.object]) the object that is the source of the copy, a corresponding object o nested within the destination is identified (if the object is a subobject) or created (otherwise), and the lifetime of o begins before the copy is performed.

11.4.5 Copy/move assignment operator [class.copy.assign]

A user-declared copy assignment operator X​::​operator= is a non-static non-template member function of class X with exactly one parameter of type X, X&, const X&,volatile X&, or const volatile X&.104

[ Note

:

An overloaded assignment operator must be declared to have only one parameter; see [over.ass].

— end note

]

[ Note

:

More than one form of copy assignment operator may be declared for a class.

— end note

]

[ Note

:

If a classXonly has a copy assignment operator with a parameter of typeX&, an expression of type constXcannot be assigned to an object of typeX.

[ Example

:

struct X { X(); X& operator=(X&); }; const X cx; X x; void f() { x = cx;
}

— end example

]

— end note

]

If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly.

If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted ([dcl.fct.def]).

The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor ([depr.impldec]).

The implicitly-declared copy assignment operator for a classXwill have the form

X& X::operator=(const X&)

if

Otherwise, the implicitly-declared copy assignment operator will have the form

X& X::operator=(X&)

A user-declared move assignment operator X​::​operator= is a non-static non-template member function of class X with exactly one parameter of type X&&, const X&&, volatile X&&, orconst volatile X&&.

[ Note

:

An overloaded assignment operator must be declared to have only one parameter; see [over.ass].

— end note

]

[ Note

:

More than one form of move assignment operator may be declared for a class.

— end note

]

If the definition of a class X does not explicitly declare a move assignment operator, one will be implicitly declared as defaulted if and only if

[ Example

:

The class definition

struct S { int a; S& operator=(const S&) = default; };

will not have a default move assignment operator implicitly declared because the copy assignment operator has been user-declared.

The move assignment operator may be explicitly defaulted.

struct S { int a; S& operator=(const S&) = default; S& operator=(S&&) = default; };

— end example

]

The implicitly-declared move assignment operator for a class X will have the form

X& X::operator=(X&&)

The implicitly-declared copy/move assignment operator for classXhas the return typeX&; it returns the object for which the assignment operator is invoked, that is, the object assigned to.

An implicitly-declared copy/move assignment operator is an inline public member of its class.

A defaulted copy/move assignment operator for class X is defined as deleted if X has:

[ Note

:

A defaulted move assignment operator that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).

— end note

]

Because a copy/move assignment operator is implicitly declared for a class if not declared by the user, a base class copy/move assignment operator is always hidden by the corresponding assignment operator of a derived class ([over.ass]).

Ausing-declaration that brings in from a base class an assignment operator with a parameter type that could be that of a copy/move assignment operator for the derived class is not considered an explicit declaration of such an operator and does not suppress the implicit declaration of the derived class operator; the operator introduced by theusing-declarationis hidden by the implicitly-declared operator in the derived class.

A copy/move assignment operator for classXis trivial if it is not user-provided and if:

otherwise the copy/move assignment operator isnon-trivial.

A copy/move assignment operator for a class Xthat is defaulted and not defined as deleted isimplicitly definedwhen it is odr-used ([basic.def.odr]) (e.g., when it is selected by overload resolution to assign to an object of its class type), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.

The implicitly-defined copy/move assignment operator is constexpr if

Before the defaulted copy/move assignment operator for a class is implicitly defined, all non-user-provided copy/move assignment operators for its direct base classes and its non-static data members are implicitly defined.

[ Note

:

An implicitly-declared copy/move assignment operator has an implied exception specification ([except.spec]).

— end note

]

The implicitly-defined copy/move assignment operator for a non-union class X performs memberwise copy/move assignment of its subobjects.

The direct base classes of X are assigned first, in the order of their declaration in thebase-specifier-list, and then the immediate non-static data members ofX are assigned, in the order in which they were declared in the class definition.

Let x be either the parameter of the function or, for the move operator, an xvalue referring to the parameter.

Each subobject is assigned in the manner appropriate to its type:

It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy/move assignment operator.

[ Example

:

struct V { }; struct A : virtual V { }; struct B : virtual V { }; struct C : B, A { };

It is unspecified whether the virtual base class subobjectVis assigned twice by the implicitly-defined copy/move assignment operator forC.

— end example

]

The implicitly-defined copy assignment operator for a union X copies the object representation ([basic.types]) of X.

If the source and destination of the assignment are not the same object, then for each object nested within ([intro.object]) the object that is the source of the copy, a corresponding object o nested within the destination is created, and the lifetime of o begins before the copy is performed.

11.4.6 Destructors [class.dtor]

A prospective destructor shall take no arguments ([dcl.fct]).

Each decl-specifier of the decl-specifier-seqof a prospective destructor declaration (if any) shall befriend,inline,virtual,constexpr, orconsteval.

If a class has no user-declared prospective destructor, a prospective destructor is implicitly declared as defaulted ([dcl.fct.def]).

An implicitly-declared prospective destructor is an inline public member of its class.

An implicitly-declared prospective destructor for a class X will have the form

~X()

At the end of the definition of a class, overload resolution is performed among the prospective destructors declared in that class with an empty argument list to select the destructor for the class, also known as the selected destructor.

The program is ill-formed if overload resolution fails.

Destructor selection does not constitute a reference to, or odr-use ([basic.def.odr]) of, the selected destructor, and in particular, the selected destructor may be deleted ([dcl.fct.def.delete]).

The address of a destructor shall not be taken.

A destructor can be invoked for aconst,volatileorconst volatileobject.

constandvolatilesemantics ([dcl.type.cv]) are not applied on an object under destruction.

They stop being in effect when the destructor for the most derived object ([intro.object]) starts.

[ Note

:

A declaration of a destructor that does not have a noexcept-specifierhas the same exception specification as if it had been implicitly declared ([except.spec]).

— end note

]

A defaulted destructor for a classX is defined as deleted if:

A destructor is trivial if it is not user-provided and if:

Otherwise, the destructor isnon-trivial.

A defaulted destructor is a constexpr destructor if it satisfies the requirements for a constexpr destructor ([dcl.constexpr]).

A destructor that is defaulted and not defined as deleted isimplicitly definedwhen it is odr-used ([basic.def.odr]) or when it is explicitly defaulted after its first declaration.

Before a defaulted destructor for a class is implicitly defined, all the non-user-provided destructors for its base classes and its non-static data members are implicitly defined.

A prospective destructor can be declaredvirtual ([class.virtual]) or purevirtual ([class.abstract]).

If the destructor of a class is virtual and any objects of that class or any derived class are created in the program, the destructor shall be defined.

If a class has a base class with a virtual destructor, its destructor (whether user- or implicitly-declared) is virtual.

[ Note

:

Some language constructs have special semantics when used during destruction; see [class.cdtor].

— end note

]

After executing the body of the destructor and destroying any objects with automatic storage duration allocated within the body, a destructor for classXcalls the destructors forX's direct non-variant non-static data members, the destructors forX's non-virtual direct base classes and, ifXis the most derived class ([class.base.init]), its destructor calls the destructors forX's virtual base classes.

All destructors are called as if they were referenced with a qualified name, that is, ignoring any possible virtual overriding destructors in more derived classes.

Bases and members are destroyed in the reverse order of the completion of their constructor (see [class.base.init]).

Areturnstatement ([stmt.return]) in a destructor might not directly return to the caller; before transferring control to the caller, the destructors for the members and bases are called.

Destructors for elements of an array are called in reverse order of their construction (see [class.init]).

A destructor is invoked implicitly

In each case, the context of the invocation is the context of the construction of the object.

[ Note

:

An array of class type contains several subobjects for each of which the destructor is invoked.

— end note

]

A destructor can also be invoked explicitly.

A program is ill-formed if a destructor that is potentially invoked is deleted or not accessible from the context of the invocation.

At the point of definition of a virtual destructor (including an implicit definition ([class.dtor])), the non-array deallocation function is determined as if for the expression delete this appearing in a non-virtual destructor of the destructor's class (see [expr.delete]).

If the lookup fails or if the deallocation function has a deleted definition ([dcl.fct.def]), the program is ill-formed.

[ Note

:

This assures that a deallocation function corresponding to the dynamic type of an object is available for thedelete-expression ([class.free]).

— end note

]

In an explicit destructor call, the destructor is specified by a~followed by atype-name or decltype-specifierthat denotes the destructor's class type.

The invocation of a destructor is subject to the usual rules for member functions ([class.mfct]); that is, if the object is not of the destructor's class type and not of a class derived from the destructor's class type (including when the destructor is invoked via a null pointer value), the program has undefined behavior.

[ Note

:

Invoking delete on a null pointer does not call the destructor; see [expr.delete].

— end note

]

[ Example

:

struct B { virtual ~B() { } }; struct D : B { ~D() { } };

D D_object; typedef B B_alias; B* B_ptr = &D_object;

void f() { D_object.B::B();
B_ptr->B();
B_ptr->B_alias();
B_ptr->B_alias::B();
B_ptr->B_alias::~B_alias();
}

— end example

]

[ Note

:

Explicit calls of destructors are rarely needed.

One use of such calls is for objects placed at specific addresses using a placementnew-expression.

Such use of explicit placement and destruction of objects can be necessary to cope with dedicated hardware resources and for writing memory management facilities.

For example,

void* operator new(std::size_t, void* p) { return p; } struct X { X(int); ~X(); }; void f(X* p);

void g() {
char* buf = new char[sizeof(X)]; X* p = new(buf) X(222);
f(p); p->X::~X();
}

— end note

]

Once a destructor is invoked for an object, the object no longer exists; the behavior is undefined if the destructor is invoked for an object whose lifetime has ended ([basic.life]).

[ Example

:

If the destructor for an object with automatic storage duration is explicitly invoked, and the block is subsequently left in a manner that would ordinarily invoke implicit destruction of the object, the behavior is undefined.

— end example

]

[ Note

:

The notation for explicit call of a destructor can be used for any scalar type name ([expr.prim.id.dtor]).

Allowing this makes it possible to write code without having to know if a destructor exists for a given type.

For example:

typedef int I; I* p; p->I::~I();

— end note

]

A destructor shall not be a coroutine.

11.4.7 Conversions [class.conv]

Type conversions of class objects can be specified by constructors and by conversion functions.

User-defined conversions are applied only where they are unambiguous ([class.member.lookup], [class.conv.fct]).

Conversions obey the access control rules ([class.access]).

Access control is applied after ambiguity resolution ([basic.lookup]).

[ Note

:

See [over.match] for a discussion of the use of conversions in function calls as well as examples below.

— end note

]

At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.

[ Example

:

struct X { operator int(); };

struct Y { operator X(); };

Y a; int b = a;
int c = X(a);

— end example

]

User-defined conversions are used implicitly only if they are unambiguous.

A conversion function in a derived class does not hide a conversion function in a base class unless the two functions convert to the same type.

Function overload resolution ([over.match.best]) selects the best conversion function to perform the conversion.

[ Example

:

struct X { operator int(); };

struct Y : X { operator char(); };

void f(Y& a) { if (a) {
} }

— end example

]

11.4.7.1 Conversion by constructor [class.conv.ctor]

A constructor that is not explicit ([dcl.fct.spec]) specifies a conversion from the types of its parameters (if any) to the type of its class.

Such a constructor is called aconverting constructor.

[ Example

:

struct X { X(int); X(const char*, int =0); X(int, int); };

void f(X arg) { X a = 1;
X b = "Jessie";
a = 2;
f(3);
f({1, 2});
}

— end example

]

[ Note

:

An explicit constructor constructs objects just like non-explicit constructors, but does so only where the direct-initialization syntax ([dcl.init]) or where casts ([expr.static.cast], [expr.cast]) are explicitly used; see also [over.match.copy].

A default constructor may be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization ([dcl.init]).

[ Example

:

struct Z { explicit Z(); explicit Z(int); explicit Z(int, int); };

Z a;
Z b{};
Z c = {};
Z a1 = 1;
Z a3 = Z(1);
Z a2(1);
Z* p = new Z(1);
Z a4 = (Z)1;
Z a5 = static_cast(1);
Z a6 = { 3, 4 };

— end example

]

— end note

]

A non-explicit copy/move constructor ([class.copy.ctor]) is a converting constructor.

[ Note

:

An implicitly-declared copy/move constructor is not an explicit constructor; it may be called for implicit type conversions.

— end note

]

11.4.7.2 Conversion functions [class.conv.fct]

A member function of a class X having no parameters with a name of the form

conversion-function-id: operator conversion-type-id

conversion-type-id: type-specifier-seq conversion-declarator

Such functions are called conversion functions.

The type of the conversion function ([dcl.fct]) is “function taking no parameter returningconversion-type-id”.

A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to cv void.106

[ Example

:

struct X { operator int(); operator auto() -> short;
};

void f(X a) { int i = int(a); i = (int)a; i = a; }

In all three cases the value assigned will be converted byX​::​operator int().

— end example

]

A conversion function may be explicit ([dcl.fct.spec]), in which case it is only considered as a user-defined conversion for direct-initialization ([dcl.init]).

Otherwise, user-defined conversions are not restricted to use in assignments and initializations.

[ Example

:

class Y { }; struct Z { explicit operator Y() const; };

void h(Z z) { Y y1(z);
Y y2 = z;
Y y3 = (Y)z;
}

void g(X a, X b) { int i = (a) ? 1+a : 0; int j = (a&&b) ? a+b : i; if (a) { } }

— end example

]

Theconversion-type-idshall not represent a function type nor an array type.

Theconversion-type-idin aconversion-function-idis the longest sequence of tokens that could possibly form a conversion-type-id.

[ Note

:

This prevents ambiguities between the declarator operator * and its expression counterparts.

[ Example

:

&ac.operator int*i;

The * is the pointer declarator and not the multiplication operator.

— end example

]

This rule also prevents ambiguities for attributes.

[ Example

:

operator int [[noreturn]] ();

— end example

]

— end note

]

Conversion functions are inherited.

Conversion functions can be virtual.

A conversion function template shall not have a deduced return type ([dcl.spec.auto]).

[ Example

:

struct S { operator auto() const { return 10; }
template operator auto() const { return 1.2; }
};

— end example

]

11.4.8 Static members [class.static]

A static member s of class X may be referred to using the qualified-id expression X​::​s; it is not necessary to use the class member access syntax ([expr.ref]) to refer to a static member.

A static member may be referred to using the class member access syntax, in which case the object expression is evaluated.

[ Example

:

struct process { static void reschedule(); }; process& g();

void f() { process::reschedule();
g().reschedule();
}

— end example

]

A static member may be referred to directly in the scope of its class or in the scope of a class derived ([class.derived]) from its class; in this case, the static member is referred to as if a qualified-id expression was used, with thenested-name-specifier of the qualified-id naming the class scope from which the static member is referenced.

[ Example

:

int g(); struct X { static int g(); }; struct Y : X { static int i; }; int Y::i = g();

— end example

]

Static members obey the usual class member access rules ([class.access]).

When used in the declaration of a class member, the static specifier shall only be used in the member declarations that appear within the member-specification of the class definition.

[ Note

:

It cannot be specified in member declarations that appear in namespace scope.

— end note

]

11.4.8.1 Static member functions [class.static.mfct]

[ Note

:

The rules described in [class.mfct] apply to static member functions.

— end note

]

[ Note

:

A static member function does not have a thispointer ([class.this]).

— end note

]

A static member function shall not be virtual.

There shall not be a static and a non-static member function with the same name and the same parameter types ([over.load]).

A static member function shall not be declared const,volatile, or const volatile.

11.4.8.2 Static data members [class.static.data]

A static data member is not part of the subobjects of a class.

If a static data member is declared thread_­local there is one copy of the member per thread.

If a static data member is not declaredthread_­local there is one copy of the data member that is shared by all the objects of the class.

A static data member shall not be mutable ([dcl.stc]).

A static data member shall not be a direct member ([class.mem]) of an unnamed ([class.pre]) or local ([class.local]) class or of a (possibly indirectly) nested class ([class.nest]) thereof.

The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other thancv void.

The definition for a static data member that is not defined inline in the class definition shall appear in a namespace scope enclosing the member's class definition.

In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the ​::​operator.

[ Example

:

class process { static process* run_chain; static process* running; };

process* process::running = get_main(); process* process::run_chain = running;

The static data member run_­chain of classprocess is defined in global scope; the notationprocess​::​run_­chain specifies that the member run_­chainis a member of class process and in the scope of classprocess.

In the static data member definition, theinitializer expression refers to the static data member running of class process.

— end example

]

[ Note

:

Once the static data member has been defined, it exists even if no objects of its class have been created.

[ Example

:

In the example above, run_­chain and running exist even if no objects of class process are created by the program.

— end example

]

— end note

]

The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.

An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer.

If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see [depr.static.constexpr]).

[ Note

:

There is exactly one definition of a static data member that is odr-used ([basic.def.odr]) in a valid program.

— end note

]

[ Note

:

Static data members of a class in namespace scope have the linkage of the name of the class ([basic.link]).

— end note

]

11.4.9 Bit-fields [class.bit]

A bit-field shall not be a static member.

A bit-field shall have integral or enumeration type; the bit-field semantic property is not part of the type of the class member.

The constant-expression shall be an integral constant expression with a value greater than or equal to zero and is called the width of the bit-field.

If the width of a bit-field is larger than the width of the bit-field's type (or, in case of an enumeration type, of its underlying type), the extra bits are padding bits ([basic.types]).

Allocation of bit-fields within a class object isimplementation-defined.

Alignment of bit-fields is implementation-defined.

Bit-fields are packed into some addressable allocation unit.

[ Note

:

Bit-fields straddle allocation units on some machines and not on others.

Bit-fields are assigned right-to-left on some machines, left-to-right on others.

— end note

]

A declaration for a bit-field that omits the identifierdeclares an unnamed bit-field.

Unnamed bit-fields are not members and cannot be initialized.

An unnamed bit-field shall not be declared with a cv-qualified type.

[ Note

:

An unnamed bit-field is useful for padding to conform to externally-imposed layouts.

— end note

]

As a special case, an unnamed bit-field with a width of zero specifies alignment of the next bit-field at an allocation unit boundary.

Only when declaring an unnamed bit-field may the width be zero.

The address-of operator & shall not be applied to a bit-field, so there are no pointers to bit-fields.

A non-const reference shall not be bound to a bit-field ([dcl.init.ref]).

[ Note

:

If the initializer for a reference of type const T& is an lvalue that refers to a bit-field, the reference is bound to a temporary initialized to hold the value of the bit-field; the reference is not bound to the bit-field directly.

— end note

]

If a value of integral type (other than bool) is stored into a bit-field of width N and the value would be representable in a hypothetical signed or unsigned integer type with width N and the same signedness as the bit-field's type, the original value and the value of the bit-field compare equal.

If the value true or false is stored into a bit-field of type bool of any size (including a one bit bit-field), the original bool value and the value of the bit-field compare equal.

If a value of an enumeration type is stored into a bit-field of the same type and the width is large enough to hold all the values of that enumeration type ([dcl.enum]), the original value and the value of the bit-field compare equal.

[ Example

:

enum BOOL { FALSE=0, TRUE=1 }; struct A { BOOL b:1; }; A a; void f() { a.b = TRUE; if (a.b == TRUE)
{ } }

— end example

]

11.4.10 Nested class declarations [class.nest]

A class can be declared within another class.

A class declared within another is called a nested class.

The name of a nested class is local to its enclosing class.

The nested class is in the scope of its enclosing class.

[ Note

:

See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.

— end note

]

[ Example

:

int x; int y;

struct enclose { int x; static int s;

struct inner { void f(int i) { int a = sizeof(x);
x = i;
s = i;
::x = i;
y = i;
} void g(enclose* p, int i) { p->x = i;
} }; };

inner* p = 0;

— end example

]

Member functions and static data members of a nested class can be defined in a namespace scope enclosing the definition of their class.

[ Example

:

struct enclose { struct inner { static int x; void f(int i); }; };

int enclose::inner::x = 1;

void enclose::inner::f(int i) { }

— end example

]

If class X is defined in a namespace scope, a nested classY may be declared in class X and later defined in the definition of class X or be later defined in a namespace scope enclosing the definition of class X.

[ Example

:

class E { class I1;
class I2; class I1 { };
}; class E::I2 { };

— end example

]

Like a member function, a friend function ([class.friend]) defined within a nested class is in the lexical scope of that class; it obeys the same rules for name binding as a static member function of that class ([class.static]), but it has no special access rights to members of an enclosing class.

11.4.11 Nested type names [class.nested.type]

Type names obey exactly the same scope rules as other names.

In particular, type names defined within a class definition cannot be used outside their class without qualification.

[ Example

:

struct X { typedef int I; class Y { }; I a; };

I b;
Y c;
X::Y d;
X::I e;

— end example

]

11.5 Unions [class.union]

A union is a class defined with the class-key union.

In a union, a non-static data member is activeif its name refers to an object whose lifetime has begun and has not ended ([basic.life]).

At most one of the non-static data members of an object of union type can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time.

[ Note

:

One special guarantee is made in order to simplify the use of unions: If a standard-layout union contains several standard-layout structs that share a common initial sequence ([class.mem]), and if a non-static data member of an object of this standard-layout union type is active and is one of the standard-layout structs, it is permitted to inspect the common initial sequence of any of the standard-layout struct members; see [class.mem].

— end note

]

The size of a union is sufficient to contain the largest of its non-static data members.

Each non-static data member is allocated as if it were the sole member of a non-union class.

[ Note

:

As a consequence, all non-static data members of a union object have the same address.

— end note

]

A union can have member functions (including constructors and destructors),but it shall not have virtual ([class.virtual]) functions.

A union shall not have base classes.

A union shall not be used as a base class.

If a union contains a non-static data member of reference type the program is ill-formed.

[ Note

:

Absent default member initializers ([class.mem]), if any non-static data member of a union has a non-trivial default constructor ([class.default.ctor]), copy constructor, move constructor ([class.copy.ctor]), copy assignment operator, move assignment operator ([class.copy.assign]), or destructor ([class.dtor]), the corresponding member function of the union must be user-provided or it will be implicitly deleted ([dcl.fct.def.delete]) for the union.

— end note

]

[ Example

:

Consider the following union:

union U { int i; float f; std::string s; };

Since std​::​string ([string.classes]) declares non-trivial versions of all of the special member functions, U will have an implicitly deleted default constructor, copy/move constructor, copy/move assignment operator, and destructor.

To use U, some or all of these member functions must be user-provided.

— end example

]

When the left operand of an assignment operator involves a member access expression ([expr.ref]) that nominates a union member, it may begin the lifetime of that union member, as described below.

For an expression E, define the set S(E)of subexpressions of Eas follows:

In an assignment expression of the form E1 = E2that uses either the built-in assignment operator ([expr.ass]) or a trivial assignment operator ([class.copy.assign]), for each element X of S(E1), if modification of X would have undefined behavior under [basic.life], an object of the type of X is implicitly created in the nominated storage; no initialization is performed and the beginning of its lifetime is sequenced after the value computation of the left and right operands and before the assignment.

[ Note

:

This ends the lifetime of the previously-active member of the union, if any ([basic.life]).

— end note

]

[ Example

:

union A { int x; int y[4]; }; struct B { A a; }; union C { B b; int k; }; int f() { C c;
c.b.a.y[3] = 4;

return c.b.a.y[3];
}

struct X { const int a; int b; }; union Y { X x; int k; }; void g() { Y y = { { 1, 2 } };
int n = y.x.a; y.k = 4;
y.x.b = n;

}

— end example

]

[ Note

:

In general, one must use explicit destructor calls and placementnew-expression to change the active member of a union.

— end note

]

[ Example

:

Consider an object u of a union type U having non-static data membersm of type M and n of type N.

If M has a non-trivial destructor and N has a non-trivial constructor (for instance, if they declare or inherit virtual functions), the active member of u can be safely switched from m ton using the destructor and placement new-expression as follows:

u.m.~M(); new (&u.n) N;

— end example

]

11.5.1 Anonymous unions [class.union.anon]

A union of the form

union { member-specification } ;

is called an anonymous union; it defines an unnamed type and an unnamed object of that type called an anonymous union object.

Each member-declaration in the member-specificationof an anonymous union shall either define a non-static data member or be astatic_assert-declaration.

Nested types, anonymous unions, and functions shall not be declared within an anonymous union.

The names of the members of an anonymous union shall be distinct from the names of any other entity in the scope in which the anonymous union is declared.

For the purpose of name lookup, after the anonymous union definition, the members of the anonymous union are considered to have been defined in the scope in which the anonymous union is declared.

[ Example

:

void f() { union { int a; const char* p; }; a = 1; p = "Jennifer"; }

Here a and p are used like ordinary (non-member) variables, but since they are union members they have the same address.

— end example

]

Anonymous unions declared in a named namespace or in the global namespace shall be declared static.

Anonymous unions declared at block scope shall be declared with any storage class allowed for a block-scope variable, or with no storage class.

A storage class is not allowed in a declaration of an anonymous union in a class scope.

An anonymous union shall not have private or protected members ([class.access]).

An anonymous union shall not have member functions.

A union for which objects, pointers, or references are declared is not an anonymous union.

[ Example

:

void f() { union { int aa; char* p; } obj, *ptr = &obj; aa = 1;
ptr->aa = 1;
}

The assignment to plain aa is ill-formed since the member name is not visible outside the union, and even if it were visible, it is not associated with any particular object.

— end example

]

[ Note

:

Initialization of unions with no user-declared constructors is described in [dcl.init.aggr].

— end note

]

A union-like class is a union or a class that has an anonymous union as a direct member.

A union-like class X has a set of variant members.

If X is a union, a non-static data member of X that is not an anonymous union is a variant member of X.

In addition, a non-static data member of an anonymous union that is a member of X is also a variant member of X.

At most one variant member of a union may have a default member initializer.

[ Example

:

union U { int x = 0; union { int k; }; union { int z; int y = 1;
}; };

— end example

]

11.6 Local class declarations [class.local]

A class can be declared within a function definition; such a class is called a local class.

The name of a local class is local to its enclosing scope.

The local class is in the scope of the enclosing scope, and has the same access to names outside the function as does the enclosing function.

[ Note

:

A declaration in a local class cannot odr-use ([basic.def.odr]) a local entity from an enclosing scope.

— end note

]

[ Example

:

int x; void f() { static int s; int x; const int N = 5; extern int q(); int arr[2]; auto [y, z] = arr;

struct local { int g() { return x; }
int h() { return s; }
int k() { return ::x; }
int l() { return q(); }
int m() { return N; }
int* n() { return &N; }
int p() { return y; }
}; }

local* p = 0;

— end example

]

An enclosing function has no special access to members of the local class; it obeys the usual access rules ([class.access]).

Member functions of a local class shall be defined within their class definition, if they are defined at all.

If class X is a local class a nested class Y may be declared in class X and later defined in the definition of classX or be later defined in the same scope as the definition of class X.

A class nested within a local class is a local class.

[ Note

:

A local class cannot have static data members ([class.static.data]).

— end note

]

11.7 Derived classes [class.derived]

A class-or-decltype shall denote a (possibly cv-qualified) class type that is not an incompletely defined class ([class.mem]); any cv-qualifiers are ignored.

The class denoted by the class-or-decltype of a base-specifier is called adirect base classfor the class being defined.

During the lookup for a base class name, non-type names are ignored ([basic.scope.hiding]).

A class B is a base class of a class D if it is a direct base class ofD or a direct base class of one of D's base classes.

A class is an indirect base class of another if it is a base class but not a direct base class.

A class is said to be (directly or indirectly) derived from its (direct or indirect) base classes.

[ Note

:

See [class.access] for the meaning ofaccess-specifier.

— end note

]

Unless redeclared in the derived class, members of a base class are also considered to be members of the derived class.

Members of a base class other than constructors are said to beinheritedby the derived class.

Constructors of a base class can also be inherited as described in [namespace.udecl].

Inherited members can be referred to in expressions in the same manner as other members of the derived class, unless their names are hidden or ambiguous ([class.member.lookup]).

[ Note

:

The scope resolution operator ​::​ ([expr.prim.id.qual]) can be used to refer to a direct or indirect base member explicitly.

This allows access to a name that has been redeclared in the derived class.

A derived class can itself serve as a base class subject to access control; see [class.access.base].

A pointer to a derived class can be implicitly converted to a pointer to an accessible unambiguous base class ([conv.ptr]).

An lvalue of a derived class type can be bound to a reference to an accessible unambiguous base class ([dcl.init.ref]).

— end note

]

The base-specifier-list specifies the type of thebase class subobjects contained in an object of the derived class type.

[ Example

:

struct Base { int a, b, c; };

struct Derived : Base { int b; };

struct Derived2 : Derived { int c; };

Here, an object of class Derived2 will have a subobject of classDerived which in turn will have a subobject of classBase.

— end example

]

The order in which the base class subobjects are allocated in the most derived object ([intro.object]) is unspecified.

[ Note

:

A derived class and its base class subobjects can be represented by a directed acyclic graph (DAG) where an arrow means “directly derived from” (see Figure 2).

An arrow need not have a physical representation in memory.

A DAG of subobjects is often referred to as a “subobject lattice”.

dag Base Base Derived1 Derived1 Derived1->Base Derived2 Derived2 Derived2->Derived1

Figure 2: Directed acyclic graph [fig:class.dag]

— end note

]

[ Note

:

Initialization of objects representing base classes can be specified in constructors; see [class.base.init].

— end note

]

[ Note

:

A base class subobject might have a layout ([basic.stc]) different from the layout of a most derived object of the same type.

A base class subobject might have a polymorphic behavior ([class.cdtor]) different from the polymorphic behavior of a most derived object of the same type.

A base class subobject may be of zero size ([class]); however, two subobjects that have the same class type and that belong to the same most derived object must not be allocated at the same address ([expr.eq]).

— end note

]

11.7.1 Multiple base classes [class.mi]

A class can be derived from any number of base classes.

[ Note

:

The use of more than one direct base class is often called multiple inheritance.

— end note

]

[ Example

:

class A { }; class B { }; class C { }; class D : public A, public B, public C { };

— end example

]

[ Note

:

The order of derivation is not significant except as specified by the semantics of initialization by constructor ([class.base.init]), cleanup ([class.dtor]), and storage layout ([class.mem], [class.access.spec]).

— end note

]

A class shall not be specified as a direct base class of a derived class more than once.

[ Note

:

A class can be an indirect base class more than once and can be a direct and an indirect base class.

There are limited things that can be done with such a class.

The non-static data members and member functions of the direct base class cannot be referred to in the scope of the derived class.

However, the static members, enumerations and types can be unambiguously referred to.

— end note

]

[ Example

:

class X { }; class Y : public X, public X { };

class L { public: int next; }; class A : public L { }; class B : public L { }; class C : public A, public B { void f(); };
class D : public A, public L { void f(); };

— end example

]

A base class specifier that does not contain the keywordvirtual specifies a non-virtual base class.

A base class specifier that contains the keyword virtual specifies avirtual base class.

For each distinct occurrence of a non-virtual base class in the class lattice of the most derived class, the most derived object ([intro.object]) shall contain a corresponding distinct base class subobject of that type.

For each distinct base class that is specified virtual, the most derived object shall contain a single base class subobject of that type.

[ Note

:

For an object of class type C, each distinct occurrence of a (non-virtual) base class L in the class lattice of Ccorresponds one-to-one with a distinct L subobject within the object of type C.

Given the class C defined above, an object of class C will have two subobjects of class L as shown in Figure 3.

nonvirt L1 L L2 L A A A->L1 B B B->L2 C C C->A C->B

Figure 3: Non-virtual base [fig:class.nonvirt]

In such lattices, explicit qualification can be used to specify which subobject is meant.

The body of function C​::​f could refer to the member next of each L subobject:

void C::f() { A::next = B::next; }

Without the A​::​ or B​::​ qualifiers, the definition ofC​::​f above would be ill-formed because of ambiguity ([class.member.lookup]).

— end note

]

[ Note

:

In contrast, consider the case with a virtual base class:

class V { }; class A : virtual public V { }; class B : virtual public V { }; class C : public A, public B { };

virt V V A A A->V B B B->V C C C->A C->B

Figure 4: Virtual base [fig:class.virt]

For an object c of class type C, a single subobject of type V is shared by every base class subobject of c that has avirtual base class of type V.

Given the class Cdefined above, an object of class C will have one subobject of class V, as shown in Figure 4.

— end note

]

[ Note

:

A class can have both virtual and non-virtual base classes of a given type.

class B { }; class X : virtual public B { }; class Y : virtual public B { }; class Z : public B { }; class AA : public X, public Y, public Z { };

For an object of class AA, all virtual occurrences of base class B in the class lattice of AA correspond to a single B subobject within the object of type AA, and every other occurrence of a (non-virtual) base class B in the class lattice of AA corresponds one-to-one with a distinctB subobject within the object of type AA.

Given the class AA defined above, class AA has two subobjects of class B: Z's B and the virtual B shared by X and Y, as shown in Figure 5.

virtnonvirt B1 B B2 B AA AA X X AA->X Y Y AA->Y Z Z AA->Z X->B1 Y->B1 Z->B2

Figure 5: Virtual and non-virtual base [fig:class.virtnonvirt]

— end note

]

11.7.2 Virtual functions [class.virtual]

A non-static member function is a virtual functionif it is first declared with the keyword virtual or if it overrides a virtual member function declared in a base class (see below).107

[ Note

:

Virtual functions support dynamic binding and object-oriented programming.

— end note

]

A class that declares or inherits a virtual function is called a polymorphic class.108

If a virtual member function vf is declared in a classBase and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list ([dcl.fct]), cv-qualification, and ref-qualifier (or absence of same) as Base​::​vf is declared, then Derived​::​vf overrides109 Base​::​vf.

For convenience we say that any virtual function overrides itself.

A virtual member function C​::​vf of a class object S is a final overrider unless the most derived class ([intro.object]) of which S is a base class subobject (if any) declares or inherits another member function that overridesvf.

In a derived class, if a virtual member function of a base class subobject has more than one final overrider the program is ill-formed.

[ Example

:

struct A { virtual void f(); }; struct B : virtual A { virtual void f(); }; struct C : B , virtual A { using A::f; };

void foo() { C c; c.f();
c.C::f();
}

— end example

]

[ Example

:

struct A { virtual void f(); }; struct B : A { }; struct C : A { void f(); }; struct D : B, C { };

— end example

]

[ Note

:

A virtual member function does not have to be visible to be overridden, for example,

struct B { virtual void f(); }; struct D : B { void f(int); }; struct D2 : D { void f(); };

the function f(int) in class D hides the virtual function f() in its base class B; D​::​f(int) is not a virtual function.

However, f() declared in classD2 has the same name and the same parameter list asB​::​f(), and therefore is a virtual function that overrides the function B​::​f() even though B​::​f() is not visible in class D2.

— end note

]

If a virtual function f in some class B is marked with thevirt-specifier final and in a class D derived from Ba function D​::​f overrides B​::​f, the program is ill-formed.

[ Example

:

struct B { virtual void f() const final; };

struct D : B { void f() const;
};

— end example

]

If a virtual function is marked with the virt-specifier override and does not override a member function of a base class, the program is ill-formed.

[ Example

:

struct B { virtual void f(int); };

struct D : B { virtual void f(long) override;
virtual void f(int) override;
};

— end example

]

[ Example

:

struct A { virtual void f() requires true;
};

— end example

]

Even though destructors are not inherited, a destructor in a derived class overrides a base class destructor declared virtual; see [class.dtor] and [class.free].

The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions.

If a function D​::​f overrides a function B​::​f, the return types of the functions are covariant if they satisfy the following criteria:

If the class type in the covariant return type of D​::​f differs from that ofB​::​f, the class type in the return type of D​::​f shall be complete at the point of declaration of D​::​f or shall be the class type D.

When the overriding function is called as the final overrider of the overridden function, its result is converted to the type returned by the (statically chosen) overridden function ([expr.call]).

[ Example

:

class B { }; class D : private B { friend class Derived; }; struct Base { virtual void vf1(); virtual void vf2(); virtual void vf3(); virtual B* vf4(); virtual B* vf5(); void f(); };

struct No_good : public Base { D* vf4();
};

class A; struct Derived : public Base { void vf1();
void vf2(int);
char vf3();
D* vf4();
A* vf5();
void f(); };

void g() { Derived d; Base* bp = &d;

bp->vf1();
bp->vf2();
bp->f();
B* p = bp->vf4();

Derived* dp = &d; D* q = dp->vf4();

dp->vf2();
}

— end example

]

[ Note

:

The interpretation of the call of a virtual function depends on the type of the object for which it is called (the dynamic type), whereas the interpretation of a call of a non-virtual member function depends only on the type of the pointer or reference denoting that object (the static type) ([expr.call]).

— end note

]

[ Note

:

The virtual specifier implies membership, so a virtual function cannot be a non-member ([dcl.fct.spec]) function.

Nor can a virtual function be a static member, since a virtual function call relies on a specific object for determining which function to invoke.

A virtual function declared in one class can be declared a friend ([class.friend]) in another class.

— end note

]

A virtual function declared in a class shall be defined, or declared pure ([class.abstract]) in that class, or both; no diagnostic is required ([basic.def.odr]).

[ Example

:

Here are some uses of virtual functions with multiple base classes:

struct A { virtual void f(); };

struct B1 : A {
void f(); };

struct B2 : A { void f(); };

struct D : B1, B2 {
};

void foo() { D d;

B1* b1p = &d; A* ap = b1p; D* dp = &d; ap->f();
dp->f();
}

In class D above there are two occurrences of class Aand hence two occurrences of the virtual member function A​::​f.

The final overrider of B1​::​A​::​f is B1​::​f and the final overrider of B2​::​A​::​f is B2​::​f.

— end example

]

[ Example

:

The following example shows a function that does not have a unique final overrider:

struct A { virtual void f(); };

struct VB1 : virtual A {
void f(); };

struct VB2 : virtual A { void f(); };

struct Error : VB1, VB2 {
};

struct Okay : VB1, VB2 { void f(); };

Both VB1​::​f and VB2​::​f override A​::​f but there is no overrider of both of them in class Error.

This example is therefore ill-formed.

Class Okay is well-formed, however, because Okay​::​f is a final overrider.

— end example

]

[ Example

:

The following example uses the well-formed classes from above.

struct VB1a : virtual A {
};

struct Da : VB1a, VB2 { };

void foe() { VB1a* vb1ap = new Da; vb1ap->f();
}

— end example

]

Explicit qualification with the scope operator ([expr.prim.id.qual]) suppresses the virtual call mechanism.

[ Example

:

class B { public: virtual void f(); }; class D : public B { public: void f(); };

void D::f() { B::f(); }

Here, the function call inD​::​freally does callB​::​fand notD​::​f.

— end example

]

A function with a deleted definition ([dcl.fct.def]) shall not override a function that does not have a deleted definition.

Likewise, a function that does not have a deleted definition shall not override a function with a deleted definition.

A consteval virtual function shall not override a virtual function that is not consteval.

A consteval virtual function shall not be overridden by a virtual function that is not consteval.

11.7.3 Abstract classes [class.abstract]

[ Note

:

The abstract class mechanism supports the notion of a general concept, such as a shape, of which only more concrete variants, such ascircle and square, can actually be used.

An abstract class can also be used to define an interface for which derived classes provide a variety of implementations.

— end note

]

A virtual function is specified as a pure virtual function by using apure-specifier in the function declaration in the class definition.

[ Note

:

Such a function might be inherited: see below.

— end note

]

A class is an abstract classif it has at least one pure virtual function.

[ Note

:

An abstract class can be used only as a base class of some other class; no objects of an abstract class can be created except as subobjects of a class derived from it ([basic.def], [class.mem]).

— end note

]

[ Example

:

class point { }; class shape {
point center; public: point where() { return center; } void move(point p) { center=p; draw(); } virtual void rotate(int) = 0; virtual void draw() = 0;
};

— end example

]

[ Note

:

A function declaration cannot provide both a pure-specifierand a definition.

— end note

]

[ Example

:

struct C { virtual void f() = 0 { };
};

— end example

]

[ Note

:

An abstract class type cannot be used as a parameter or return type of a function being defined ([dcl.fct]) or called ([expr.call]), except as specified in [dcl.type.simple].

However, pointers and references to abstract class types can appear in such contexts.

— end note

]

A class is abstract if it contains or inherits at least one pure virtual function for which the final overrider is pure virtual.

[ Example

:

class ab_circle : public shape { int radius; public: void rotate(int) { }

};

Since shape​::​draw() is a pure virtual functionab_­circle​::​draw() is a pure virtual by default.

The alternative declaration,

class circle : public shape { int radius; public: void rotate(int) { } void draw();
};

would make class circle non-abstract and a definition ofcircle​::​draw() must be provided.

— end example

]

[ Note

:

An abstract class can be derived from a class that is not abstract, and a pure virtual function may override a virtual function which is not pure.

— end note

]

Member functions can be called from a constructor (or destructor) of an abstract class;the effect of making a virtual call ([class.virtual]) to a pure virtual function directly or indirectly for the object being created (or destroyed) from such a constructor (or destructor) is undefined.

11.8 Member name lookup [class.member.lookup]

Name lookup can result in an ambiguity, in which case the program is ill-formed.

Name lookup takes place before access control ([basic.lookup], [class.access]).

The following steps define the result of name lookup for a member namef in a class scope C.

The lookup set for f in C, called , consists of two component sets: the declaration set, a set of members named f; and the subobject set, a set of subobjects where declarations of these members (possibly includingusing-declarations) were found.

In the declaration set,using-declarations are replaced by the set of designated members that are not hidden or overridden by members of the derived class ([namespace.udecl]), and type declarations (including injected-class-names) are replaced by the types they designate.

is calculated as follows:

If C contains a declaration of the name f, the declaration set contains every declaration of f declared inC that satisfies the requirements of the language construct in which the lookup occurs.

[ Note

:

As another example, looking up a name in ausing-declaration includes the declaration of a class or enumeration that would ordinarily be hidden by another declaration of that name in the same scope.

— end note

]

If the resulting declaration set is not empty, the subobject set contains C itself, and calculation is complete.

Otherwise (i.e., C does not contain a declaration of for the resulting declaration set is empty), is initially empty.

If C has base classes, calculate the lookup set for f in each direct base class subobject , and merge each such lookup set in turn into .

The following steps define the result of merging lookup set into the intermediate :

The result of name lookup for f in C is the declaration set of .

If it is an invalid set, the program is ill-formed.

[ Example

:

struct A { int x; };
struct B { float x; };
struct C: public A, public B { };
struct D: public virtual C { };
struct E: public virtual C { char x; }; struct F: public D, public E { };
int main() { F f; f.x = 0;
}

is unambiguous because the A and B base class subobjects of D are also base class subobjects of E, so is discarded in the first merge step.

— end example

]

If the name of an overloaded function is unambiguously found, overload resolution ([over.match]) also takes place before access control.

Ambiguities can often be resolved by qualifying a name with its class name.

[ Example

:

struct A { int f(); };

struct B { int f(); };

struct C : A, B { int f() { return A::f() + B::f(); } };

— end example

]

[ Note

:

A static member, a nested type or an enumerator defined in a base classT can unambiguously be found even if an object has more than one base class subobject of type T.

Two base class subobjects share the non-static member subobjects of their common virtual base classes.

— end note

]

[ Example

:

struct V { int v; }; struct A { int a; static int s; enum { e }; }; struct B : A, virtual V { }; struct C : A, virtual V { }; struct D : B, C { };

void f(D* pd) { pd->v++;
pd->s++;
int i = pd->e;
pd->a++;
}

— end example

]

[ Note

:

When virtual base classes are used, a hidden declaration can be reached along a path through the subobject lattice that does not pass through the hiding declaration.

This is not an ambiguity.

The identical use with non-virtual base classes is an ambiguity; in that case there is no unique instance of the name that hides all the others.

— end note

]

[ Example

:

struct V { int f(); int x; }; struct W { int g(); int y; }; struct B : virtual V, W { int f(); int x; int g(); int y; }; struct C : virtual V, W { };

struct D : B, C { void glorp(); };

virt W1 W V V W2 W B B B->W1 B->V C C C->V C->W2 D D D->B D->C

Figure 6: Name lookup [fig:class.lookup]

As illustrated in Figure 6, the names declared in V and the left-hand instance of Ware hidden by those in B, but the names declared in the right-hand instance of W are not hidden at all.

void D::glorp() { x++;
f();
y++;
g();
}

— end example

]

An explicit or implicit conversion from a pointer to or an expression designating an object of a derived class to a pointer or reference to one of its base classes shall unambiguously refer to a unique object representing the base class.

[ Example

:

struct V { }; struct A { }; struct B : A, virtual V { }; struct C : A, virtual V { }; struct D : B, C { };

void g() { D d; B* pb = &d; A* pa = &d;
V* pv = &d;
}

— end example

]

[ Note

:

Even if the result of name lookup is unambiguous, use of a name found in multiple subobjects might still be ambiguous ([conv.mem], [expr.ref], [class.access.base]).

— end note

]

[ Example

:

struct B1 { void f(); static void f(int); int i; }; struct B2 { void f(double); }; struct I1: B1 { }; struct I2: B1 { };

struct D: I1, I2, B2 { using B1::f; using B2::f; void g() { f();
f(0);
f(0.0);
int B1::* mpB1 = &D::i;
int D::* mpD = &D::i;
} };

— end example

]

11.9 Member access control [class.access]

A member of a class can be

A member of a class can also access all the names to which the class has access.

A local class of a member function may access the same names that the member function itself may access.111

Members of a class defined with the keywordclassareprivateby default.

Members of a class defined with the keywordsstruct or unionare public by default.

[ Example

:

class X { int a;
};

struct S { int a;
};

— end example

]

Access control is applied uniformly to all names, whether the names are referred to from declarations or expressions.

[ Note

:

Access control applies to names nominated by friend declarations ([class.friend]) andusing-declarations.

— end note

]

In the case of overloaded function names, access control is applied to the function selected by overload resolution.

[ Note

:

Because access control applies to names, if access control is applied to a typedef name, only the accessibility of the typedef name itself is considered.

The accessibility of the entity referred to by the typedef is not considered.

For example,

class A { class B { }; public: typedef B BB; };

void f() { A::BB x;
A::B y;
}

— end note

]

[ Note

:

Access to members and base classes is controlled, not their visibility ([basic.scope.hiding]).

Names of members are still visible, and implicit conversions to base classes are still considered, when those members and base classes are inaccessible.

— end note

]

The interpretation of a given construct is established without regard to access control.

If the interpretation established makes use of inaccessible member names or base classes, the construct is ill-formed.

All access controls in [class.access] affect the ability to access a class member name from the declaration of a particular entity, including parts of the declaration preceding the name of the entity being declared and, if the entity is a class, the definitions of members of the class appearing outside the class's member-specification.

[ Note

: This access also applies to implicit references to constructors, conversion functions, and destructors. — end note

]

[ Example

:

class A { typedef int I;
I f(); friend I g(I); static I x; template struct Q; template friend struct R; protected: struct B { }; };

A::I A::f() { return 0; } A::I g(A::I p = A::x); A::I g(A::I p) { return 0; } A::I A::x = 0; template<A::I> struct A::Q { }; template<A::I> struct R { };

struct D: A::B, A { };

Here, all the uses ofA​::​Iare well-formed becauseA​::​f,A​::​x, and A​::​Qare members of classAandgand R are friends of classA.

This implies, for example, that access checking on the first use ofA​::​Imust be deferred until it is determined that this use ofA​::​Iis as the return type of a member of classA.

Similarly, the use of A​::​B as abase-specifier is well-formed because Dis derived from A, so checking of base-specifiersmust be deferred until the entire base-specifier-list has been seen.

— end example

]

The names in a default argument ([dcl.fct.default]) are bound at the point of declaration, and access is checked at that point rather than at any points of use of the default argument.

Access checking for default arguments in function templates and in member functions of class templates is performed as described in [temp.inst].

[ Example

:

class B { }; template class C { protected: typedef T TT; };

template <class U, class V = typename U::TT> class D : public U { };

D <C >* d;

— end example

]

11.9.1 Access specifiers [class.access.spec]

Member declarations can be labeled by anaccess-specifier([class.derived]):

access-specifier : member-specification

Anaccess-specifierspecifies the access rules for members following it until the end of the class or until anotheraccess-specifieris encountered.

[ Example

:

class X { int a;
public: int b;
int c;
};

— end example

]

Any number of access specifiers is allowed and no particular order is required.

[ Example

:

struct S { int a;
protected: int b;
private: int c;
public: int d;
};

— end example

]

[ Note

:

The effect of access control on the order of allocation of data members is specified in [expr.rel].

— end note

]

When a member is redeclared within its class definition, the access specified at its redeclaration shall be the same as at its initial declaration.

[ Example

:

struct S { class A; enum E : int; private: class A { };
enum E: int { e0 };
};

— end example

]

[ Note

:

In a derived class, the lookup of a base class name will find the injected-class-name instead of the name of the base class in the scope in which it was declared.

The injected-class-name might be less accessible than the name of the base class in the scope in which it was declared.

— end note

]

[ Example

:

class A { }; class B : private A { }; class C : public B { A* p;
::A* q;
};

— end example

]

11.9.2 Accessibility of base classes and base class members [class.access.base]

If a class is declared to be a base class ([class.derived]) for another class using thepublicaccess specifier, the public members of the base class are accessible as public members of the derived class and protected members of the base class are accessible as protected members of the derived class.

If a class is declared to be a base class for another class using theprotectedaccess specifier, the public and protected members of the base class are accessible as protected members of the derived class.

If a class is declared to be a base class for another class using theprivateaccess specifier, the public and protected members of the base class are accessible as private members of the derived class.112

In the absence of anaccess-specifierfor a base class,publicis assumed when the derived class is defined with the class-key structandprivateis assumed when the class is defined with the class-key class.

[ Example

:

class B { }; class D1 : private B { }; class D2 : public B { }; class D3 : B { };
struct D4 : public B { }; struct D5 : private B { }; struct D6 : B { };
class D7 : protected B { }; struct D8 : protected B { };

HereBis a public base ofD2,D4, andD6, a private base ofD1,D3, andD5, and a protected base ofD7andD8.

— end example

]

[ Note

:

A member of a private base class might be inaccessible as an inherited member name, but accessible directly.

Because of the rules on pointer conversions ([conv.ptr]) and explicit casts ([expr.type.conv], [expr.static.cast], [expr.cast]), a conversion from a pointer to a derived class to a pointer to an inaccessible base class might be ill-formed if an implicit conversion is used, but well-formed if an explicit cast is used.

For example,

class B { public: int mi;
static int si;
}; class D : private B { }; class DD : public D { void f(); };

void DD::f() { mi = 3;
si = 3;
::B b; b.mi = 3;
b.si = 3;
::B::si = 3;
::B* bp1 = this;
::B* bp2 = (::B*)this;
bp2->mi = 3;
}

— end note

]

A base classBofNisaccessibleatR, if

[ Example

:

class B { public: int m; };

class S: private B { friend class N; };

class N: private S { void f() { B* p = this;

} };

— end example

]

If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class ([conv.ptr], [conv.mem]).

[ Note

:

It follows that members and friends of a classXcan implicitly convert anX*to a pointer to a private or protected immediate base class ofX.

— end note

]

The access to a member is affected by the class in which the member is named.

This naming class is the class in which the member name was looked up and found.

[ Note

:

This class can be explicit, e.g., when aqualified-idis used, or implicit, e.g., when a class member access operator ([expr.ref]) is used (including cases where an implicit “this->” is added).

If both a class member access operator and aqualified-idare used to name the member (as inp->T​::​m), the class naming the member is the class denoted by thenested-name-specifierof thequalified-id(that is,T).

— end note

]

A membermis accessible at the pointRwhen named in classNif

If a class member access operator, including an implicit “this->”, is used to access a non-static data member or non-static member function, the reference is ill-formed if the left operand (considered as a pointer in the “.” operator case) cannot be implicitly converted to a pointer to the naming class of the right operand.

[ Note

:

This requirement is in addition to the requirement that the member be accessible as named.

— end note

]

11.9.3 Friends [class.friend]

A friend of a class is a function or class that is given permission to use the private and protected member names from the class.

A class specifies its friends, if any, by way of friend declarations.

Such declarations give special access rights to the friends, but they do not make the nominated friends members of the befriending class.

[ Example

:

The following example illustrates the differences between members and friends:

class X { int a; friend void friend_set(X*, int); public: void member_set(int); };

void friend_set(X* p, int i) { p->a = i; } void X::member_set(int i) { a = i; }

void f() { X obj; friend_set(&obj,10); obj.member_set(10); }

— end example

]

Declaring a class to be a friend implies that the names of private and protected members from the class granting friendship can be accessed in thebase-specifiers and member declarations of the befriended class.

[ Example

:

class A { class B { }; friend class X; };

struct X : A::B {
A::B mx;
class Y { A::B my;
}; };

— end example

]

[ Example

:

class X { enum { a=100 }; friend class Y; };

class Y { int v[X::a];
};

class Z { int v[X::a];
};

— end example

]

A class shall not be defined in a friend declaration.

[ Example

:

class A { friend class B { };
};

— end example

]

A friend declaration that does not declare a function shall have one of the following forms:

friend elaborated-type-specifier ; friend simple-type-specifier ; friend typename-specifier ;

If the type specifier in a friend declaration designates a (possibly cv-qualified) class type, that class is declared as a friend; otherwise, the friend declaration is ignored.

[ Example

:

class C; typedef C Ct;

class X1 { friend C;
};

class X2 { friend Ct;
friend D;
friend class D;
};

template class R { friend T; };

R rc;
R Ri;

— end example

]

A function first declared in a friend declaration has the linkage of the namespace of which it is a member ([basic.link],[namespace.memdef]).

Otherwise, the function retains its previous linkage ([dcl.stc]).

When a friend declaration refers to an overloaded name or operator, only the function specified by the parameter types becomes a friend.

A member function of a classXcan be a friend of a classY.

[ Example

:

class Y { friend char* X::foo(int); friend X::X(char);
friend X::~X();
};

— end example

]

A function can be defined in a friend declaration of a class if and only if the class is a non-local class ([class.local]), the function name is unqualified, and the function has namespace scope.

[ Example

:

class M { friend void f() { }

};

— end example

]

Such a function is implicitly an inline ([dcl.inline]) function if it is attached to the global module.

A friend function defined in a class is in the (lexical) scope of the class in which it is defined.

A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration.

The meaning of the friend declaration is the same whether the friend declaration appears in the private, protected, or public ([class.mem]) portion of the classmember-specification.

Friendship is neither inherited nor transitive.

[ Example

:

class A { friend class B; int a; };

class B { friend class C; };

class C { void f(A* p) { p->a++;
} };

class D : public B { void f(A* p) { p->a++;
} };

— end example

]

If a friend declaration appears in a local class ([class.local]) and the name specified is an unqualified name, a prior declaration is looked up without considering scopes that are outside the innermost enclosing non-class scope.

For a friend function declaration, if there is no prior declaration, the program is ill-formed.

For a friend class declaration, if there is no prior declaration, the class that is specified belongs to the innermost enclosing non-class scope, but if it is subsequently referenced, its name is not found by name lookup until a matching declaration is provided in the innermost enclosing non-class scope.

[ Example

:

class X; void a(); void f() { class Y; extern void b(); class A { friend class X;
friend class Y;
friend class Z;
friend void a();
friend void b();
friend void c();
}; X* px;
Z* pz;
}

— end example

]

11.9.4 Protected member access [class.protected]

An additional access check beyond those described earlier in [class.access]is applied when a non-static data member or non-static member function is a protected member of its naming class ([class.access.base]).113

As described earlier, access to a protected member is granted because the reference occurs in a friend or member of some class C.

All other accesses involve a (possibly implicit) object expression ([expr.ref]).

In this case, the class of the object expression shall beC or a class derived from C.

[ Example

:

class B { protected: int i; static int j; };

class D1 : public B { };

class D2 : public B { friend void fr(B*,D1*,D2*); void mem(B*,D1*); };

void fr(B* pb, D1* p1, D2* p2) { pb->i = 1;
p1->i = 2;
p2->i = 3;
p2->B::i = 4;
int B::* pmi_B = &B::i;
int B::* pmi_B2 = &D2::i;
B::j = 5;
D2::j = 6;
}

void D2::mem(B* pb, D1* p1) { pb->i = 1;
p1->i = 2;
i = 3;
B::i = 4;
int B::* pmi_B = &B::i;
int B::* pmi_B2 = &D2::i;
j = 5;
B::j = 6;
}

void g(B* pb, D1* p1, D2* p2) { pb->i = 1;
p1->i = 2;
p2->i = 3;
}

— end example

]

11.9.5 Access to virtual functions [class.access.virt]

The access rules ([class.access]) for a virtual function are determined by its declaration and are not affected by the rules for a function that later overrides it.

[ Example

:

class B { public: virtual int f(); };

class D : public B { private: int f(); };

void f() { D d; B* pb = &d; D* pd = &d;

pb->f();
pd->f();
}

— end example

]

Access is checked at the call point using the type of the expression used to denote the object for which the member function is called (B*in the example above).

The access of the member function in the class in which it was defined (Din the example above) is in general not known.

11.9.6 Multiple access [class.paths]

If a name can be reached by several paths through a multiple inheritance graph, the access is that of the path that gives most access.

[ Example

:

class W { public: void f(); }; class A : private virtual W { }; class B : public virtual W { }; class C : public A, public B { void f() { W::f(); }
};

SinceW​::​f()is available toC​::​f()along the public path throughB, access is allowed.

— end example

]

11.9.7 Nested classes [class.access.nest]

A nested class is a member and as such has the same access rights as any other member.

The members of an enclosing class have no special access to members of a nested class; the usual access rules ([class.access]) shall be obeyed.

[ Example

:

class E { int x; class B { };

class I { B b;
int y; void f(E* p, int i) { p->x = i;
} };

int g(I* p) { return p->y;
} };

— end example

]

11.10 Initialization [class.init]

When no initializer is specified for an object of (possibly cv-qualified) class type (or array thereof), or the initializer has the form(), the object is initialized as specified in [dcl.init].

An object of class type (or array thereof) can be explicitly initialized; see [class.expl.init] and [class.base.init].

When an array of class objects is initialized (either explicitly or implicitly) and the elements are initialized by constructor, the constructor shall be called for each element of the array, following the subscript order; see [dcl.array].

[ Note

:

Destructors for the array elements are called in reverse order of their construction.

— end note

]

11.10.1 Explicit initialization [class.expl.init]

An object of class type can be initialized with a parenthesizedexpression-list, where theexpression-listis construed as an argument list for a constructor that is called to initialize the object.

Either direct-initialization semantics or copy-initialization semantics apply; see [dcl.init].

[ Example

:

struct complex { complex(); complex(double); complex(double,double); };

complex sqrt(complex,complex);

complex a(1);
complex b = a;
complex c = complex(1,2);
complex d = sqrt(b,c);
complex e;
complex f = 3;
complex g = { 1, 2 };

— end example

]

[ Note

:

Overloading of the assignment operator ([over.ass]) has no effect on initialization.

— end note

]

An object of class type can also be initialized by abraced-init-list.

[ Example

:

complex v[6] = { 1, complex(1,2), complex(), 2 };

Here,complex​::​complex(double)is called for the initialization ofv[0]andv[3],complex​::​complex(​double, double)is called for the initialization ofv[1],complex​::​complex()is called for the initializationv[2],v[4], andv[5].

For another example,

struct X { int i; float f; complex c; } x = { 99, 88.8, 77.7 };

Here,x.iis initialized with 99,x.fis initialized with 88.8, andcomplex​::​complex(double)is called for the initialization ofx.c.

— end example

]

[ Note

:

Braces can be elided in theinitializer-listfor any aggregate, even if the aggregate has members of a class type with user-defined type conversions; see [dcl.init.aggr].

— end note

]

[ Note

:

IfTis a class type with no default constructor, any declaration of an object of typeT(or array thereof) is ill-formed if noinitializeris explicitly specified (see [class.init] and [dcl.init]).

— end note

]

11.10.2 Initializing bases and members [class.base.init]

In a mem-initializer-id an initial unqualifiedidentifier is looked up in the scope of the constructor's class and, if not found in that scope, it is looked up in the scope containing the constructor's definition.

[ Note

:

If the constructor's class contains a member with the same name as a direct or virtual base class of the class, amem-initializer-idnaming the member or base class and composed of a single identifier refers to the class member.

Amem-initializer-idfor the hidden base class may be specified using a qualified name.

— end note

]

Unless themem-initializer-idnames the constructor's class, a non-static data member of the constructor's class, or a direct or virtual base of that class, themem-initializeris ill-formed.

Amem-initializer-listcan initialize a base class using any class-or-decltype that denotes that base class type.

[ Example

:

struct A { A(); }; typedef A global_A; struct B { }; struct C: public A, public B { C(); }; C::C(): global_A() { }

— end example

]

If amem-initializer-idis ambiguous because it designates both a direct non-virtual base class and an inherited virtual base class, themem-initializeris ill-formed.

[ Example

:

struct A { A(); }; struct B: public virtual A { }; struct C: public A, public B { C(); }; C::C(): A() { }

— end example

]

Actor-initializermay initialize a variant member of the constructor's class.

If actor-initializerspecifies more than onemem-initializerfor the same member or for the same base class, thector-initializeris ill-formed.

A mem-initializer-list can delegate to another constructor of the constructor's class using anyclass-or-decltype that denotes the constructor's class itself.

If amem-initializer-id designates the constructor's class, it shall be the only mem-initializer; the constructor is a delegating constructor, and the constructor selected by themem-initializer is the target constructor.

The target constructor is selected by overload resolution.

Once the target constructor returns, the body of the delegating constructor is executed.

If a constructor delegates to itself directly or indirectly, the program is ill-formed, no diagnostic required.

[ Example

:

struct C { C( int ) { }
C(): C(42) { }
C( char c ) : C(42.0) { }
C( double d ) : C('a') { }
};

— end example

]

Theexpression-listor braced-init-listin amem-initializeris used to initialize the designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of [dcl.init] for direct-initialization.

[ Example

:

struct B1 { B1(int); }; struct B2 { B2(int); }; struct D : B1, B2 { D(int); B1 b; const int c; };

D::D(int a) : B2(a+1), B1(a+2), c(a+3), b(a+4) { } D d(10);

— end example

]

[ Note

:

The initialization performed by each mem-initializerconstitutes a full-expression ([intro.execution]).

Any expression in amem-initializeris evaluated as part of the full-expression that performs the initialization.

— end note

]

A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.

A temporary expression bound to a reference member in a mem-initializeris ill-formed.

[ Example

:

struct A { A() : v(42) { }
const int& v; };

— end example

]

In a non-delegating constructor, if a given potentially constructed subobject is not designated by amem-initializer-id(including the case where there is nomem-initializer-listbecause the constructor has noctor-initializer), then

[ Note

:

An abstract class ([class.abstract]) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted.

— end note

]

An attempt to initialize more than one non-static data member of a union renders the program ill-formed.

[ Note

:

After the call to a constructor for classXfor an object with automatic or dynamic storage duration has completed, if the constructor was not invoked as part of value-initialization and a member ofXis neither initialized nor given a value during execution of the compound-statement of the body of the constructor, the member has an indeterminate value.

— end note

]

[ Example

:

struct A { A(); };

struct B { B(int); };

struct C { C() { }
A a;
const B b;
int i;
int j = 5;
};

— end example

]

If a given non-static data member has both a default member initializer and a mem-initializer, the initialization specified by themem-initializer is performed, and the non-static data member's default member initializer is ignored.

[ Example

:

Given

struct A { int i = ; A(int arg) : i(arg) { }

};

the A(int) constructor will simply initialize i to the value ofarg, and theside effects in i's default member initializer will not take place.

— end example

]

A temporary expression bound to a reference member from a default member initializer is ill-formed.

[ Example

:

struct A { A() = default;
A(int v) : v(v) { }
const int& v = 42;
}; A a1;
A a2(1);

— end example

]

In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked ([class.dtor]).

[ Note

:

This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]).

— end note

]

In a non-delegating constructor, initialization proceeds in the following order:

[ Note

:

The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization.

— end note

]

[ Example

:

struct V { V(); V(int); };

struct A : virtual V { A(); A(int); };

struct B : virtual V { B(); B(int); };

struct C : A, B, virtual V { C(); C(int); };

A::A(int i) : V(i) { } B::B(int i) { } C::C(int i) { }

V v(1);
A a(2);
B b(3);
C c(4);

— end example

]

[ Example

:

class X { int a; int b; int i; int j; public: const int& r; X(int i): r(a), b(i), i(i), j(this->i) { } };

initializesX​::​rto refer toX​::​a, initializesX​::​bwith the value of the constructor parameteri, initializesX​::​iwith the value of the constructor parameteri, and initializesX​::​jwith the value ofX​::​i; this takes place each time an object of classXis created.

— end example

]

[ Note

:

Because themem-initializerare evaluated in the scope of the constructor, thethispointer can be used in theexpression-listof amem-initializerto refer to the object being initialized.

— end note

]

Member functions (including virtual member functions, [class.virtual]) can be called for an object under construction.

Similarly, an object under construction can be the operand of thetypeidoperator ([expr.typeid]) or of adynamic_­cast ([expr.dynamic.cast]).

However, if these operations are performed in actor-initializer(or in a function called directly or indirectly from actor-initializer) before all themem-initializersfor base classes have completed, the program has undefined behavior.

[ Example

:

class A { public: A(int); };

class B : public A { int j; public: int f(); B() : A(f()),
j(f()) { }
};

class C { public: C(int); };

class D : public B, C { int i; public: D() : C(f()),
i(f()) { }
};

— end example

]

[ Note

:

[class.cdtor] describes the result of virtual function calls,typeidanddynamic_­casts during construction for the well-defined cases; that is, describes the polymorphic behavior of an object under construction.

— end note

]

A mem-initializer followed by an ellipsis is a pack expansion ([temp.variadic]) that initializes the base classes specified by a pack expansion in the base-specifier-listfor the class.

[ Example

:

template<class... Mixins> class X : public Mixins... { public: X(const Mixins&... mixins) : Mixins(mixins)... { } };

— end example

]

11.10.3 Initialization by inherited constructor [class.inhctor.init]

When a constructor for type B is invoked to initialize an object of a different type D(that is, when the constructor was inherited ([namespace.udecl])), initialization proceeds as if a defaulted default constructor were used to initialize the D object and each base class subobject from which the constructor was inherited, except that the B subobject is initialized by the invocation of the inherited constructor.

The complete initialization is considered to be a single function call; in particular, the initialization of the inherited constructor's parameters is sequenced before the initialization of any part of the D object.

[ Example

:

struct B1 { B1(int, ...) { } };

struct B2 { B2(double) { } };

int get();

struct D1 : B1 { using B1::B1;
int x; int y = get(); };

void test() { D1 d(2, 3, 4);

D1 e;
}

struct D2 : B2 { using B2::B2; B1 b; };

D2 f(1.0);

struct W { W(int); }; struct X : virtual W { using W::W; X() = delete; }; struct Y : X { using X::X; }; struct Z : Y, virtual W { using Y::Y; }; Z z(0);

template struct Log : T { using T::T;
~Log() { std::clog << "Destroying wrapper" << std::endl; } };

Class template Log wraps any class and forwards all of its constructors, while writing a message to the standard log whenever an object of class Log is destroyed.

— end example

]

If the constructor was inherited from multiple base class subobjects of type B, the program is ill-formed.

[ Example

:

struct A { A(int); }; struct B : A { using A::A; };

struct C1 : B { using B::B; }; struct C2 : B { using B::B; };

struct D1 : C1, C2 { using C1::C1; using C2::C2; };

struct V1 : virtual B { using B::B; }; struct V2 : virtual B { using B::B; };

struct D2 : V1, V2 { using V1::V1; using V2::V2; };

D1 d1(0);
D2 d2(0);

struct M { M(); M(int); }; struct N : M { using M::M; }; struct O : M {}; struct P : N, O { using N::N; using O::O; }; P p(0);

— end example

]

When an object is initialized by an inherited constructor, initialization of the object is complete when the initialization of all subobjects is complete.

11.10.4 Construction and destruction [class.cdtor]

For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior.

For an object with a non-trivial destructor, referring to any non-static member or base class of the object after the destructor finishes execution results in undefined behavior.

[ Example

:

struct X { int i; }; struct Y : X { Y(); };
struct A { int a; }; struct B : public A { int j; Y y; };

extern B bobj; B* pb = &bobj;
int* p1 = &bobj.a;
int* p2 = &bobj.y.i;

A* pa = &bobj;
B bobj;

extern X xobj; int* p3 = &xobj.i;
X xobj;

For another example,

struct W { int j; }; struct X : public virtual W { }; struct Y { int* p; X x; Y() : p(&x.j) {
} };

— end example

]

During the construction of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor'sthispointer, the value of the object or subobject thus obtained is unspecified.

[ Example

:

struct C; void no_opt(C*);

struct C { int c; C() : c(0) { no_opt(this); } };

const C cobj;

void no_opt(C* cptr) { int i = cobj.c * 100;
cptr->c = 1; cout << cobj.c * 100
<< '\n'; }

extern struct D d; struct D { D(int a) : a(a), b(d.a) {} int a, b; }; D d = D(1);

— end example

]

To explicitly or implicitly convert a pointer (a glvalue) referring to an object of classXto a pointer (reference) to a direct or indirect base classBofX, the construction ofXand the construction of all of its direct or indirect bases that directly or indirectly derive fromBshall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior.

To form a pointer to (or access the value of) a direct non-static member of an objectobj, the construction ofobjshall have started and its destruction shall not have completed, otherwise the computation of the pointer value (or accessing the member value) results in undefined behavior.

[ Example

:

struct A { }; struct B : virtual A { }; struct C : B { }; struct D : virtual A { D(A*); }; struct X { X(A*); };

struct E : C, D, X { E() : D(this),

X(this) {}
};

— end example

]

Member functions, including virtual functions ([class.virtual]), can be called during construction or destruction ([class.base.init]).

When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class.

If the virtual function call uses an explicit class member access ([expr.ref]) and the object expression refers to the complete object of x or one of that object's base class subobjects but not x or one of its base class subobjects, the behavior is undefined.

[ Example

:

struct V { virtual void f(); virtual void g(); };

struct A : virtual V { virtual void f(); };

struct B : virtual V { virtual void g(); B(V*, A*); };

struct D : A, B { virtual void f(); virtual void g(); D() : B((A*)this, this) { } };

B::B(V* v, A* a) { f();
g();
v->g();
a->f();
}

— end example

]

Thetypeidoperator ([expr.typeid]) can be used during construction or destruction ([class.base.init]).

Whentypeidis used in a constructor (including themem-initializer or default member initializer ([class.mem]) for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand oftypeidrefers to the object under construction or destruction,typeidyields thestd​::​type_­infoobject representing the constructor or destructor's class.

If the operand oftypeidrefers to the object under construction or destruction and the static type of the operand is neither the constructor or destructor's class nor one of its bases, the behavior is undefined.

dynamic_­casts ([expr.dynamic.cast]) can be used during construction or destruction ([class.base.init]).

When adynamic_­castis used in a constructor (including themem-initializer or default member initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of thedynamic_­castrefers to the object under construction or destruction, this object is considered to be a most derived object that has the type of the constructor or destructor's class.

If the operand of thedynamic_­castrefers to the object under construction or destruction and the static type of the operand is not a pointer to or object of the constructor or destructor's own class or one of its bases, thedynamic_­castresults in undefined behavior.

[ Example

:

struct V { virtual void f(); };

struct A : virtual V { };

struct B : virtual V { B(V*, A*); };

struct D : A, B { D() : B((A*)this, this) { } };

B::B(V* v, A* a) { typeid(*this);
typeid(*v);
typeid(*a);
dynamic_cast<B*>(v);
dynamic_cast<B*>(a);
}

— end example

]

11.10.5 Copy/move elision [class.copy.elision]

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object haveside effects.

In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object.

If the first parameter of the selected constructor is an rvalue reference to the object's type, the destruction of that object occurs when the target would have been destroyed; otherwise, the destruction occurs at the later of the times when the two objects would have been destroyed without the optimization.114

This elision of copy/move operations, calledcopy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):

Copy elision is not permitted where an expression is evaluated in a context requiring a constant expression ([expr.const]) and in constant initialization ([basic.start.static]).

[ Note

:

Copy elision might be performed if the same expression is evaluated in another context.

— end note

]

[ Example

:

class Thing { public: Thing(); ~Thing(); Thing(const Thing&); };

Thing f() { Thing t; return t; }

Thing t2 = f();

struct A { void *p; constexpr A(): p(this) {} };

constexpr A g() { A loc; return loc; }

constexpr A a;
constexpr A b = g();

void h() { A c = g();
}

Here the criteria for elision can eliminate the copying of the object t with automatic storage duration into the result object for the function call f(), which is the global object t2.

Effectively, the construction of the local object tcan be viewed as directly initializing the global object t2, and that object's destruction will occur at program exit.

Adding a move constructor to Thing has the same effect, but it is the move construction from the object with automatic storage duration to t2 that is elided.

— end example

]

An implicitly movable entity is a variable of automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type.

If the first overload resolution fails or was not performed, overload resolution is performed again, considering the expression or operand as an lvalue.

[ Note

:

This two-stage overload resolution must be performed regardless of whether copy elision will occur.

It determines the constructor or the return_­value overload to be called if elision is not performed, and the selected constructor or return_­value overload must be accessible even if the call is elided.

— end note

]

[ Example

:

class Thing { public: Thing(); ~Thing(); Thing(Thing&&); private: Thing(const Thing&); };

Thing f(bool b) { Thing t; if (b) throw t;
return t;
}

Thing t2 = f(false);

struct Weird { Weird(); Weird(Weird&); };

Weird g() { Weird w; return w;
}

— end example

]

[ Example

:

template void g(const T&);

template void f() { T x; try { T y; try { g(x); } catch (...) { if () throw x;
throw y;
} g(y); } catch(...) { g(x); g(y);
} }

— end example

]

11.11 Comparisons [class.compare]

11.11.1 Defaulted comparison operator functions [class.compare.default]

A defaulted comparison operator function ([over.binary]) for some class Cshall be a non-template function that is

A comparison operator function for class C that is defaulted on its first declaration and is not defined as deleted isimplicitly definedwhen it is odr-used or needed for constant evaluation.

Name lookups in the defaulted definition of a comparison operator function are performed from a context equivalent to its function-body.

A definition of a comparison operator as defaulted that appears in a class shall be the first declaration of that function.

A defaulted <=> or == operator function for class C is defined as deleted if any non-static data member of C is of reference type orC has variant members ([class.union.anon]).

A binary operator expression a @ b isusableif either

A defaulted comparison function isconstexpr-compatibleif it satisfies the requirements for a constexpr function ([dcl.constexpr]) and no overload resolution performed when determining whether to delete the function results in a usable candidate that is a non-constexpr function.

[ Note

:

This includes the overload resolutions performed:

— end note

]

If the member-specificationdoes not explicitly declare any member or friend named operator==, an == operator function is declared implicitly for each three-way comparison operator function defined as defaulted in the member-specification, with the same access and function-definition and in the same class scope as the respective three-way comparison operator function, except that the return type is replaced with bool and the declarator-id is replaced with operator==.

[ Note

:

Such an implicitly-declared == operator for a class Xis defined as defaulted in the definition of X and has the same parameter-declaration-clause and trailing requires-clause as the respective three-way comparison operator.

It is declared with friend, virtual, constexpr, or consteval if the three-way comparison operator function is so declared.

If the three-way comparison operator function has no noexcept-specifier, the implicitly-declared == operator function has an implicit exception specification ([except.spec]) that may differ from the implicit exception specification of the three-way comparison operator function.

— end note

]

[ Example

:

template struct X { friend constexpr std::partial_ordering operator<=>(X, X) requires (sizeof(T) != 1) = default;

[[nodiscard]] virtual std::strong_ordering operator<=>(const X&) const = default;

};

— end example

]

[ Note

:

The == operator function is declared implicitly even if the defaulted three-way comparison operator function is defined as deleted.

— end note

]

The direct base class subobjects of C, in the order of their declaration in the base-specifier-list of C, followed by the non-static data members of C, in the order of their declaration in the member-specification of C, form a list of subobjects.

In that list, any subobject of array type is recursively expanded to the sequence of its elements, in the order of increasing subscript.

Let be an lvalue denoting the element in the expanded list of subobjects for an object x(of length n), where is formed by a sequence of derived-to-base conversions ([over.best.ics]), class member access expressions ([expr.ref]), and array subscript expressions ([expr.sub]) applied to x.

11.11.2 Equality operator [class.eq]

A defaulted equality operator function ([over.binary]) shall have a declared return type bool.

A defaulted == operator function for a class Cis defined as deleted unless, for each in the expanded list of subobjects for an object x of type C,is usable ([class.compare.default]).

The return value V of a defaulted == operator function with parameters x and y is determined by comparing corresponding elements and in the expanded lists of subobjects for x and y(in increasing index order) until the first index iwhere yields a result value which, when contextually converted to bool, yields false.

If no such index exists, V is true.

Otherwise, V is false.

[ Example

:

struct D { int i; friend bool operator==(const D& x, const D& y) = default;

};

— end example

]

11.11.3 Three-way comparison [class.spaceship]

The synthesized three-way comparisonof type R ([cmp.categories]) of glvalues a and b of the same type is defined as follows:

[ Note

:

A synthesized three-way comparison may be ill-formed if overload resolution finds usable candidates that do not otherwise meet the requirements implied by the defined expression.

— end note

]

Let R be the declared return type of a defaulted three-way comparison operator function, and let be the elements of the expanded list of subobjects for an object x of type C.

The return value V of type Rof the defaulted three-way comparison operator function with parameters x and y of the same type is determined by comparing corresponding elements and in the expanded lists of subobjects for x and y(in increasing index order) until the first index i where the synthesized three-way comparison of type Rbetween and yields a result value where , contextually converted to bool, yields true;V is a copy of .

If no such index exists, V isstatic_­cast<R>(std​::​strong_­ordering​::​equal).

The common comparison type Uof a possibly-empty list of n comparison category types, , …, is defined as follows:

11.11.4 Secondary comparison operators [class.compare.secondary]

A secondary comparison operator is a relational operator ([expr.rel]) or the != operator.

A defaulted operator function ([over.binary]) for a secondary comparison operator @shall have a declared return type bool.

The operator function with parameters x and yis defined as deleted if

Otherwise, the operator function yields x @ y.

The defaulted operator function is not considered as a candidate in the overload resolution for the @ operator.

[ Example

:

struct HasNoLessThan { };

struct C { friend HasNoLessThan operator<=>(const C&, const C&); bool operator<(const C&) const = default;
};

— end example

]

11.12 Free store [class.free]

Any allocation function for a classTis a static member (even if not explicitly declaredstatic).

[ Example

:

class Arena; struct B { void* operator new(std::size_t, Arena*); }; struct D1 : B { };

Arena* ap; void foo(int i) { new (ap) D1;
new D1[i];
new D1;
}

— end example

]

When an object is deleted with adelete-expression, a deallocation function(operator delete() for non-array objects oroperator delete[]() for arrays) is (implicitly) called to reclaim the storage occupied by the object ([basic.stc.dynamic.deallocation]).

Class-specific deallocation function lookup is a part of general deallocation function lookup ([expr.delete]) and occurs as follows.

If the delete-expressionis used to deallocate a class object whose static type has a virtual destructor, the deallocation function is the one selected at the point of definition of the dynamic type's virtual destructor ([class.dtor]).115

Otherwise, if thedelete-expressionis used to deallocate an object of classTor array thereof, the deallocation function's name is looked up in the scope ofT.

If this lookup fails to find the name, general deallocation function lookup ([expr.delete]) continues.

If the result of the lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function, the program is ill-formed.

Any deallocation function for a classXis a static member (even if not explicitly declaredstatic).

[ Example

:

class X { void operator delete(void*); void operator delete[](void*, std::size_t); };

class Y { void operator delete(void*, std::size_t); void operator delete; };

— end example

]

Since member allocation and deallocation functions arestaticthey cannot be virtual.

[ Note

:

However, when thecast-expressionof adelete-expressionrefers to an object of class type, because the deallocation function actually called is looked up in the scope of the class that is the dynamic type of the object if the destructor is virtual, the effect is the same in that case.

For example,

struct B { virtual ~B(); void operator delete(void*, std::size_t); };

struct D : B { void operator delete(void*); };

struct E : B { void log_deletion(); void operator delete(E *p, std::destroying_delete_t) { p->log_deletion(); p->~E(); ::operator delete(p); } };

void f() { B* bp = new D; delete bp;
bp = new E; delete bp;
}

Here, storage for the object of classDis deallocated byD​::​operator delete(), and the object of class E is destroyed and its storage is deallocated by E​::​operator delete(), due to the virtual destructor.

— end note

]

[ Note

:

Virtual destructors have no effect on the deallocation function actually called when thecast-expressionof adelete-expressionrefers to an array of objects of class type.

For example,

struct B { virtual ~B(); void operator delete[](void*, std::size_t); };

struct D : B { void operator delete[](void*, std::size_t); };

void f(int i) { D* dp = new D[i]; delete [] dp;
B* bp = new D[i]; delete[] bp;
}

— end note

]

Access to the deallocation function is checked statically.

Hence, even though a different one might actually be executed, the statically visible deallocation function is required to be accessible.

[ Example

:

For the call on line “// 1” above, ifB​::​operator delete()had been private, the delete expression would have been ill-formed.

— end example

]