[class.copy.assign] (original) (raw)

11 Classes [class]

11.4 Class members [class.mem]

11.4.6 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 non-object parameter of type X, X&, const X&,volatile X&, or const volatile X&.91

[Note 1:

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

— _end note_]

[Note 2:

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

[Example 1: 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 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 formX& X::operator=(const X&) if

• each direct base class B of Xhas a copy assignment operator whose non-object parameter is of typeconst B&, const volatile B&, or B, and
• for all the non-static data members of Xthat are of a class type M (or array thereof), each such class type has a copy assignment operator whose non-object parameter is of typeconst M&, const volatile M&, or M.92

Otherwise, the implicitly-declared copy assignment operator will have the formX& 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 non-object parameter of type X&&, const X&&, volatile X&&, orconst volatile X&&.

[Note 3:

More than one form of move assignment operator can 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

• X does not have a user-declared copy constructor,
• X does not have a user-declared move constructor,
• X does not have a user-declared copy assignment operator, and
• X does not have a user-declared destructor.

[Example 2:

The class definitionstruct 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 formX& X::operator=(X&&)

The implicitly-declared copy/move assignment operator for classXhas the return typeX&.

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:

• a non-static data member of const non-class type (or possibly multi-dimensional array thereof), or
• a non-static data member of reference type, or
• a direct non-static data member of class type M (or possibly multi-dimensional array thereof) or a direct base class M that cannot be copied/moved because overload resolution ([over.match]), as applied to find M's corresponding assignment operator, either does not result in a usable candidate ([over.match.general]) or, in the case of a variant member, selects a non-trivial function.

[Note 4:

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]).

[Note 5:

A using-declaration in a derived class Cthat names an assignment operator from a base class never suppresses the implicit declaration of an assignment operator of C, even if the base class assignment operator would be a copy or move assignment operator if declared as a member of C.

— _end note_]

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

• classXhas no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
• the assignment operator selected to copy/move each direct base class subobject is trivial, and
• for each non-static data member ofXthat is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial;

otherwise the copy/move assignment operator isnon-trivial.

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 6:

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:

• if the subobject is of class type, as if by a call to operator= with the subobject as the object expression and the corresponding subobject of x as a single function argument (as if by explicit qualification; that is, ignoring any possible virtual overriding functions in more derived classes);
• if the subobject is an array, each element is assigned, in the manner appropriate to the element type;
• if the subobject is of scalar type, the built-in assignment operator is used.

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

[Example 3: 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/move assignment operator for a union X copies the object representation ([basic.types.general]) 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.

The implicitly-defined copy/move assignment operator for a class returns the object for which the assignment operator is invoked, that is, the object assigned to.