[dcl.init] (original) (raw)

9 Declarations [dcl.dcl]

9.4 Initializers [dcl.init]

9.4.1 General [dcl.init.general]

The process of initialization described in [dcl.init] applies to all initializations regardless of syntactic context, including the initialization of a function parameter ([expr.call]), the initialization of a return value ([stmt.return]), or when an initializer follows a declarator.

[Note 1:

The rules in [dcl.init] apply even if the grammar permits only the brace-or-equal-initializer form of initializer in a given context.

— _end note_]

Except for objects declared with the constexpr specifier, for which see [dcl.constexpr], an initializer in the definition of a variable can consist of arbitrary expressions involving literals and previously declared variables and functions, regardless of the variable's storage duration.

[Example 1: int f(int);int a = 2;int b = f(a);int c(b); — _end example_]

[Note 3:

The order of initialization of variables with static storage duration is described in [basic.start]and [stmt.dcl].

— _end note_]

A declaration of a block-scope variable with external or internal linkage that has an initializer is ill-formed.

Tozero-initializean object or reference of typeTmeans:

• ifTis a scalar type, the object is initialized to the value obtained by converting the integer literal 0(zero) toT;94
• ifTis a (possibly cv-qualified) non-union class type, its padding bits are initialized to zero bits and each non-static data member, each non-virtual base class subobject, and, if the object is not a base class subobject, each virtual base class subobject is zero-initialized;
• ifTis a (possibly cv-qualified) union type, its padding bits are initialized to zero bits and the object's first non-static named data member is zero-initialized;
• ifTis an array type, each element is zero-initialized;
• ifTis a reference type, no initialization is performed.

Todefault-initializean object of typeTmeans:

• IfTis a (possibly cv-qualified) class type ([class]), constructors are considered.
  The applicable constructors are enumerated ([over.match.ctor]), and the best one for theinitializer () is chosen through overload resolution ([over.match]).
  The constructor thus selected is called, with an empty argument list, to initialize the object.
• IfTis an array type, each element is default-initialized.
• Otherwise, no initialization is performed.

A class type T is const-default-constructible if default-initialization of T would invoke a user-provided constructor of T (not inherited from a base class) or if

• each direct non-variant non-static data member M of Thas a default member initializer or, if M is of class type X (or array thereof),X is const-default-constructible,
• if T is a union with at least one non-static data member, exactly one variant member has a default member initializer,
• if T is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and
• each potentially constructed base class of T is const-default-constructible.

If a program calls for the default-initialization of an object of a const-qualified type T,T shall be a const-default-constructible class type or array thereof.

Tovalue-initializean object of typeTmeans:

• ifTis a (possibly cv-qualified) class type ([class]), then
  • if T has either no default constructor ([class.default.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;
  • otherwise, the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;
• ifTis an array type, then each element is value-initialized;
• otherwise, the object is zero-initialized.

A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed.

[Note 4:

For every object of static storage duration, static initialization ([basic.start.static]) is performed at program startup before any other initialization takes place.

In some cases, additional initialization is done later.

— _end note_]

If no initializer is specified for an object, the object is default-initialized.

An initializer for a static member is in the scope of the member's class.

[Example 2: int a;struct X { static int a;static int b;};int X::a = 1;int X::b = a; — _end example_]

If the entity being initialized does not have class type, theexpression-list in a parenthesized initializer shall be a single expression.

The semantics of initializers are as follows.

Thedestination typeis the type of the object or reference being initialized and thesource typeis the type of the initializer expression.

If the initializer is not a single (possibly parenthesized) expression, the source type is not defined.

• If the initializer is a (non-parenthesized) braced-init-listor is = braced-init-list, the object or reference is list-initialized ([dcl.init.list]).
• If the destination type is a reference type, see [dcl.init.ref].
• If the destination type is an array of characters, an array of char8_­t, an array of char16_­t, an array of char32_­t, or an array ofwchar_­t, and the initializer is a string-literal, see [dcl.init.string].
• If the initializer is (), the object is value-initialized.
  [Note 6:
  Since()is not permitted by the syntax forinitializer,X a();is not the declaration of an object of classX, but the declaration of a function taking no argument and returning anX.
  — _end note_]
• Otherwise, if the destination type is an array, the object is initialized as follows.
  If the destination type is an array of unknown bound, it is defined as having k elements.
  Let n denote the array size after this potential adjustment.
  If k is greater than n, the program is ill-formed.
  Otherwise, the array element is copy-initialized with for each , and value-initialized for each .
  For each , every value computation and side effect associated with the initialization of the element of the array is sequenced before those associated with the initialization of the element.
• Otherwise, if the destination type is a (possibly cv-qualified) class type:
  • If the initializer expression is a prvalue and the cv-unqualified version of the source type is the same class as the class of the destination, the initializer expression is used to initialize the destination object.
    [Example 3:
    T x = T(T(T())); calls the T default constructor to initialize x.
    — _end example_]
  • Otherwise, if the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified version of the source type is the same class as, or a derived class of, the class of the destination, constructors are considered.
    The applicable constructors are enumerated ([over.match.ctor]), and the best one is chosen through overload resolution ([over.match]).
    Then:
    * If overload resolution is successful, the selected constructor is called to initialize the object, with the initializer expression or expression-list as its argument(s).
    * Otherwise, if no constructor is viable, the destination type is an aggregate class, and the initializer is a parenthesized expression-list, the object is initialized as follows.
    Let , …, be the elements of the aggregate ([dcl.init.aggr]).
    If k is greater than n, the program is ill-formed.
    The element is copy-initialized with for .
    The remaining elements are initialized with their default member initializers, if any, and otherwise are value-initialized.
    For each , every value computation and side effect associated with the initialization of is sequenced before those associated with the initialization of .
    [Note 7:
    By contrast with direct-list-initialization, narrowing conversions ([dcl.init.list]) are permitted, designators are not permitted, a temporary object bound to a reference does not have its lifetime extended ([class.temporary]), and there is no brace elision.
    [Example 4: struct A { int a;int&& r;};int f();int n = 10; A a1{1, f()}; A a2(1, f()); A a3{1.0, 1}; A a4(1.0, 1); A a5(1.0, std::move(n)); — _end example_]
    — _end note_]
    * Otherwise, the initialization is ill-formed.
  • Otherwise (i.e., for the remaining copy-initialization cases), user-defined conversions that can convert from the source type to the destination type or (when a conversion function is used) to a derived class thereof are enumerated as described in [over.match.copy], and the best one is chosen through overload resolution ([over.match]).
    If the conversion cannot be done or is ambiguous, the initialization is ill-formed.
    The function selected is called with the initializer expression as its argument; if the function is a constructor, the call is a prvalue of the cv-unqualified version of the destination type whose result object is initialized by the constructor.
    The call is used to direct-initialize, according to the rules above, the object that is the destination of the copy-initialization.
• Otherwise, if the source type is a (possibly cv-qualified) class type, conversion functions are considered.
  The applicable conversion functions are enumerated ([over.match.conv]), and the best one is chosen through overload resolution ([over.match]).
  The user-defined conversion so selected is called to convert the initializer expression into the object being initialized.
  If the conversion cannot be done or is ambiguous, the initialization is ill-formed.
• Otherwise, if the initialization is direct-initialization, the source type is std​::​nullptr_­t, and the destination type is bool, the initial value of the object being initialized is false.
• Otherwise, the initial value of the object being initialized is the (possibly converted) value of the initializer expression.
  A standard conversion sequence ([conv]) will be used, if necessary, to convert the initializer expression to the cv-unqualified version of the destination type; no user-defined conversions are considered.
  If the conversion cannot be done, the initialization is ill-formed.
  When initializing a bit-field with a value that it cannot represent, the resulting value of the bit-field isimplementation-defined.
  [Note 8:
  An expression of type “cv1 T” can initialize an object of type “cv2 T” independently of the cv-qualifiers_cv1_and cv2.
  int a;const int b = a;int c = b; — _end note_]

If the initializer is a parenthesized expression-list, the expressions are evaluated in the order specified for function calls ([expr.call]).

An object whose initialization has completed is deemed to be constructed, even if the object is of non-class type or no constructor of the object's class is invoked for the initialization.

[Note 9:

Such an object might have been value-initialized or initialized by aggregate initialization ([dcl.init.aggr]) or by an inherited constructor ([class.inhctor.init]).

— _end note_]

Destroying an object of class type invokes the destructor of the class.

Destroying a scalar type has no effect other than ending the lifetime of the object ([basic.life]).

Destroying an array destroys each element in reverse subscript order.

A declaration that specifies the initialization of a variable, whether from an explicit initializer or by default-initialization, is called the initializing declaration of that variable.

[Note 10:

In most cases this is the defining declaration ([basic.def]) of the variable, but the initializing declaration of a non-inline static data member ([class.static.data]) might be the declaration within the class definition and not the definition at namespace scope.

— _end note_]

9.4.2 Aggregates [dcl.init.aggr]

[Note 1:

Aggregate initialization does not allow accessing protected and private base class' members or constructors.

— _end note_]

The elements of an aggregate are:

• for an array, the array elements in increasing subscript order, or
• for a class, the direct base classes in declaration order, followed by the direct non-static data members ([class.mem]) that are not members of an anonymous union, in declaration order.

When an aggregate is initialized by an initializer list as specified in [dcl.init.list], the elements of the initializer list are taken as initializers for the elements of the aggregate.

The explicitly initialized elementsof the aggregate are determined as follows:

• If the initializer list is a designated-initializer-list, the aggregate shall be of class type, the identifier in each designatorshall name a direct non-static data member of the class, and the explicitly initialized elements of the aggregate are the elements that are, or contain, those members.
• If the initializer list is an initializer-list, the explicitly initialized elements of the aggregate are the first n elements of the aggregate, where n is the number of elements in the initializer list.
• Otherwise, the initializer list must be {}, and there are no explicitly initialized elements.

For each explicitly initialized element:

• If the element is an anonymous union object and the initializer list is a designated-initializer-list, the anonymous union object is initialized by thedesignated-initializer-list { D }, where D is the designated-initializer-clausenaming a member of the anonymous union object.
  There shall be only one such designated-initializer-clause.
  [Example 1:
  struct C { union { int a;const char* p;};int x;} c = { .a = 1, .x = 3 };initializes c.a with 1 and c.x with 3.
  — _end example_]
• Otherwise, the element is copy-initialized from the corresponding initializer-clauseor is initialized with the brace-or-equal-initializerof the corresponding designated-initializer-clause.
  If that initializer is of the formassignment-expression or= assignment-expressionand a narrowing conversion ([dcl.init.list]) is required to convert the expression, the program is ill-formed.
  [Note 2:
  If an initializer is itself an initializer list, the element is list-initialized, which will result in a recursive application of the rules in this subclause if the element is an aggregate.
  — end note_]
  [_Example 2:
  struct A { int x;struct B { int i;int j;} b;} a = { 1, { 2, 3 } };initializesa.xwith 1,a.b.iwith 2,a.b.jwith 3.
  struct base1 { int b1, b2 = 42; };struct base2 { base2() { b3 = 42;} int b3;};struct derived : base1, base2 { int d;}; derived d1{{1, 2}, {}, 4}; derived d2{{}, {}, 4};initializesd1.b1 with 1,d1.b2 with 2,d1.b3 with 42,d1.d with 4, andd2.b1 with 0,d2.b2 with 42,d2.b3 with 42,d2.d with 4.
  — _end example_]

For a non-union aggregate, each element that is not an explicitly initialized element is initialized as follows:

• If the element has a default member initializer ([class.mem]), the element is initialized from that initializer.
• Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).
• Otherwise, the program is ill-formed.

If the aggregate is a union and the initializer list is empty, then

• if any variant member has a default member initializer, that member is initialized from its default member initializer;
• otherwise, the first member of the union (if any) is copy-initialized from an empty initializer list.

[Example 3:

struct S { int a; const char* b; int c; int d = b[a]; }; S ss = { 1, "asdf" };initializesss.awith 1,ss.bwith "asdf",ss.cwith the value of an expression of the formint{}(that is, 0), and ss.d with the value of ss.b[ss.a](that is, 's'), and instruct X { int i, j, k = 42; }; X a[] = { 1, 2, 3, 4, 5, 6 }; X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } }; a and b have the same value

struct A { string a;int b = 42;int c = -1;};

A{.c=21} has the following steps:

• Initialize a with {}
• Initialize b with = 42
• Initialize c with = 21

— _end example_]

The initializations of the elements of the aggregate are evaluated in the element order.

That is, all value computations and side effects associated with a given element are sequenced before those of any element that follows it in order.

An aggregate that is a class can also be initialized with a single expression not enclosed in braces, as described in [dcl.init].

The destructor for each element of class type is potentially invoked ([class.dtor]) from the context where the aggregate initialization occurs.

[Note 3:

This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]).

— _end note_]

An array of unknown bound initialized with a brace-enclosedinitializer-listcontainingn initializer-clause_s_is defined as havingnelements ([dcl.array]).

[Example 4:

int x[] = { 1, 3, 5 };declares and initializesxas a one-dimensional array that has three elements since no size was specified and there are three initializers.

— _end example_]

An array of unknown bound shall not be initialized with an empty braced-init-list {}.95

[Note 4:

A default member initializer does not determine the bound for a member array of unknown bound.

Since the default member initializer is ignored if a suitable mem-initializer is present ([class.base.init]), the default member initializer is not considered to initialize the array of unknown bound.

[Example 5: struct S { int y[] = { 0 }; }; — _end example_]

— _end note_]

[Note 5:

Static data members, non-static data members of anonymous union members, and unnamed bit-fields are not considered elements of the aggregate.

[Example 6: struct A { int i;static int s;int j;int :17;int k;} a = { 1, 2, 3 };

Here, the second initializer 2 initializesa.jand not the static data memberA​::​s, and the third initializer 3 initializes a.kand not the unnamed bit-field before it.

— _end example_]

— _end note_]

Aninitializer-listis ill-formed if the number ofinitializer-clause_s_exceeds the number of elements of the aggregate.

[Example 7:

char cv[4] = { 'a', 's', 'd', 'f', 0 }; is ill-formed.

— _end example_]

If a member has a default member initializer and a potentially-evaluated subexpression thereof is an aggregate initialization that would use that default member initializer, the program is ill-formed.

[Example 8: struct A;extern A a;struct A { const A& a1 { A{a,a} }; const A& a2 { A{} }; }; A a{a,a}; struct B { int n = B{}.n; }; — _end example_]

If an aggregate class C contains a subaggregate elemente with no elements, the initializer-clause for e shall not be omitted from an initializer-list for an object of typeC unless the initializer-clauses for all elements of C following e are also omitted.

[Example 9: struct S { } s;struct A { S s1;int i1; S s2;int i2; S s3;int i3;} a = { { }, 0, s, 0 }; — _end example_]

When initializing a multi-dimensional array, theinitializer-clause_s_initialize the elements with the last (rightmost) index of the array varying the fastest ([dcl.array]).

[Example 10:

int x[2][2] = { 3, 1, 4, 2 };initializesx[0][0]to3,x[0][1]to1,x[1][0]to4, andx[1][1]to2.

On the other hand,float y[4][3] = { { 1 }, { 2 }, { 3 }, { 4 } };initializes the first column ofy(regarded as a two-dimensional array) and leaves the rest zero.

— _end example_]

Braces can be elided in aninitializer-listas follows.

If theinitializer-listbegins with a left brace, then the succeeding comma-separated list ofinitializer-clause_s_initializes the elements of a subaggregate; it is erroneous for there to be moreinitializer-clause_s_than elements.

If, however, theinitializer-listfor a subaggregate does not begin with a left brace, then only enoughinitializer-clause_s_from the list are taken to initialize the elements of the subaggregate; any remaininginitializer-clause_s_are left to initialize the next element of the aggregate of which the current subaggregate is an element.

[Example 11:

float y[4][3] = { { 1, 3, 5 },{ 2, 4, 6 },{ 3, 5, 7 },};is a completely-braced initialization: 1, 3, and 5 initialize the first row of the arrayy[0], namelyy[0][0],y[0][1], andy[0][2].

Likewise the next two lines initializey[1]andy[2].

The initializer ends early and thereforey[3]s elements are initialized as if explicitly initialized with an expression of the formfloat(), that is, are initialized with0.0.

In the following example, braces in theinitializer-listare elided; however theinitializer-listhas the same effect as the completely-bracedinitializer-listof the above example,float y[4][3] = { 1, 3, 5, 2, 4, 6, 3, 5, 7 };

The initializer forybegins with a left brace, but the one fory[0]does not, therefore three elements from the list are used.

Likewise the next three are taken successively fory[1]andy[2].

— _end example_]

All implicit type conversions ([conv]) are considered when initializing the element with an assignment-expression.

Otherwise, if the element is itself a subaggregate, brace elision is assumed and theassignment-expressionis considered for the initialization of the first element of the subaggregate.

[Note 6:

As specified above, brace elision cannot apply to subaggregates with no elements; aninitializer-clause for the entire subobject is required.

— _end note_]

[Example 12: struct A { int i;operator int();};struct B { A a1, a2;int z;}; A a; B b = { 4, a, a };

Braces are elided around theinitializer-clauseforb.a1.i.

b.a1.iis initialized with 4,b.a2is initialized witha,b.zis initialized with whatevera.operator int()returns.

— _end example_]

[Note 7:

An aggregate array or an aggregate class can contain elements of a class type with a user-declared constructor ([class.ctor]).

Initialization of these aggregate objects is described in [class.expl.init].

— _end note_]

When a union is initialized with an initializer list, there shall not be more than one explicitly initialized element.

[Example 13: union u { int a; const char* b; }; u a = { 1 }; u b = a; u c = 1; u d = { 0, "asdf" }; u e = { "asdf" }; u f = { .b = "asdf" }; u g = { .a = 1, .b = "asdf" }; — _end example_]

[Note 9:

As described above, the braces around theinitializer-clausefor a union member can be omitted if the union is a member of another aggregate.

— _end note_]

9.4.3 Character arrays [dcl.init.string]

An array of ordinary character type ([basic.fundamental]),char8_­t array,char16_­t array,char32_­t array, or wchar_­t array can be initialized by an ordinary string literal, UTF-8 string literal, UTF-16 string literal, UTF-32 string literal, or wide string literal, respectively, or by an appropriately-typed string-literal enclosed in braces ([lex.string]).

Successive characters of the value of the string-literalinitialize the elements of the array.

[Example 1:

char msg[] = "Syntax error on line %s\n";shows a character array whose members are initialized with astring-literal.

Note that because'\n'is a single character and because a trailing'\0'is appended,sizeof(msg)is25.

— _end example_]

There shall not be more initializers than there are array elements.

[Example 2:

char cv[4] = "asdf"; is ill-formed since there is no space for the implied trailing'\0'.

— _end example_]

If there are fewer initializers than there are array elements, each element not explicitly initialized shall be zero-initialized ([dcl.init]).

9.4.4 References [dcl.init.ref]

A variable whose declared type is “reference to T” ([dcl.ref]) shall be initialized.

[Example 1: int g(int) noexcept;void f() { int i;int& r = i; r = 1; int* p = &r; int& rr = r; int (&rg)(int) = g; rg(i); int a[3];int (&ra)[3] = a; ra[1] = i; } — _end example_]

A reference cannot be changed to refer to another object after initialization.

[Note 1:

Assignment to a reference assigns to the object referred to by the reference ([expr.ass]).

— _end note_]

Argument passing ([expr.call])and function value return ([stmt.return]) are initializations.

The initializer can be omitted for a reference only in a parameter declaration ([dcl.fct]), in the declaration of a function return type, in the declaration of a class member within its class definition ([class.mem]), and where theexternspecifier is explicitly used.

[Example 2: int& r1; extern int& r2; — _end example_]

Given types “cv1 T1” and “cv2 T2”, “cv1 T1” is to “cv2 T2” ifT1 is similar ([conv.qual]) to T2, orT1 is a base class of T2.

“cv1 T1” is reference-compatiblewith “cv2 T2” if a prvalue of type “pointer to cv2 T2” can be converted to the type “pointer to cv1 T1” via a standard conversion sequence ([conv]).

In all cases where the reference-compatible relationship of two types is used to establish the validity of a reference binding and the standard conversion sequence would be ill-formed, a program that necessitates such a binding is ill-formed.

A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:

• If the reference is an lvalue reference and the initializer expression
  • is an lvalue (but is not a bit-field), and “cv1 T1” is reference-compatible with “cv2 T2”, or
  • has a class type (i.e.,T2is a class type), where T1 is not reference-related to T2, and can be converted to an lvalue of type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3”96(this conversion is selected by enumerating the applicable conversion functions ([over.match.ref]) and choosing the best one through overload resolution),
    then the reference is bound to the initializer expression lvalue in the first case and to the lvalue result of the conversion in the second case (or, in either case, to the appropriate base class subobject of the object).
    [Example 3: double d = 2.0;double& rd = d; const double& rcd = d; struct A { };struct B : A { operator int&(); } b; A& ra = b; const A& rca = b; int& ir = B(); — _end example_]
• Otherwise, if the reference is an lvalue reference to a type that is not const-qualified or is volatile-qualified, the program is ill-formed.
  [Example 4: double& rd2 = 2.0; int i = 2;double& rd3 = i; — _end example_]
• Otherwise, if the initializer expression
  • is an rvalue (but not a bit-field) or function lvalue and “cv1 T1” is reference-compatible with “cv2 T2”, or
  • has a class type (i.e., T2 is a class type), where T1is not reference-related to T2, and can be converted to an rvalue or function lvalue of type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3” (see [over.match.ref]),
    then the value of the initializer expression in the first case and the result of the conversion in the second case is called the converted initializer.
    If the converted initializer is a prvalue, its type T4 is adjusted to type “cv1 T4” ([conv.qual]) and the temporary materialization conversion ([conv.rval]) is applied.
    In any case, the reference is bound to the resulting glvalue (or to an appropriate base class subobject).
    [Example 5: struct A { };struct B : A { } b;extern B f();const A& rca2 = f(); A&& rra = f(); struct X { operator B();operator int&();} x;const A& r = x; int i2 = 42;int&& rri = static_cast<int&&>(i2); B&& rrb = x; — _end example_]
• Otherwise:
  • If T1 or T2 is a class type andT1 is not reference-related to T2, user-defined conversions are considered using the rules for copy-initialization of an object of type “cv1 T1” by user-defined conversion ([dcl.init], [over.match.copy], [over.match.conv]); the program is ill-formed if the corresponding non-reference copy-initialization would be ill-formed.
    The result of the call to the conversion function, as described for the non-reference copy-initialization, is then used to direct-initialize the reference.
    For this direct-initialization, user-defined conversions are not considered.
  • Otherwise, the initializer expression is implicitly converted to a prvalue of type “cv1 T1”.
    The temporary materialization conversion is applied and the reference is bound to the result.
    IfT1is reference-related toT2:
  • _cv1_shall be the same cv-qualification as, or greater cv-qualification than,cv2; and
  • if the reference is an rvalue reference, the initializer expression shall not be an lvalue.
    [Example 6: struct Banana { };struct Enigma { operator const Banana(); };struct Alaska { operator Banana&(); };void enigmatic() { typedef const Banana ConstBanana; Banana &&banana1 = ConstBanana(); Banana &&banana2 = Enigma(); Banana &&banana3 = Alaska(); } const double& rcd2 = 2; double&& rrd = 2; const volatile int cvi = 1;const int& r2 = cvi; struct A { operator volatile int&(); } a;const int& r3 = a; double d2 = 1.0;double&& rrd2 = d2; struct X { operator int&(); };int&& rri2 = X(); int i3 = 2;double&& rrd3 = i3; — _end example_]

In all cases except the last (i.e., implicitly converting the initializer expression to the referenced type), the reference is said to bind directly to the initializer expression.

[Note 3:

[class.temporary] describes the lifetime of temporaries bound to references.

— _end note_]

9.4.5 List-initialization [dcl.init.list]

List-initialization is initialization of an object or reference from abraced-init-list.

Such an initializer is called an initializer list, and the comma-separatedinitializer-clause_s_of the initializer-listordesignated-initializer-clause_s_of the designated-initializer-listare called the elements of the initializer list.

An initializer list may be empty.

List-initialization can occur in direct-initialization or copy-initialization contexts; list-initialization in a direct-initialization context is calleddirect-list-initialization and list-initialization in a copy-initialization context is called copy-list-initialization.

[Note 1:

List-initialization can be used

• as the initializer in a variable definition ([dcl.init])
• as the initializer in a new-expression ([expr.new])
• in a return statement ([stmt.return])
• as a for-range-initializer ([stmt.iter])
• as a function argument ([expr.call])
• as a subscript ([expr.sub])
• as an argument to a constructor invocation ([dcl.init], [expr.type.conv])
• as an initializer for a non-static data member ([class.mem])
• in a mem-initializer ([class.base.init])
• on the right-hand side of an assignment ([expr.ass])

[Example 1: int a = {1}; std::complex<double> z{1,2};new std::vectorstd::string\{"once", "upon", "a", "time"}; f( {"Nicholas","Annemarie"} ); return { "Norah" }; int* e {}; x = double{1}; std::mapstd::string,int\ anim = { {"bear",4}, {"cassowary",2}, {"tiger",7} }; — _end example_]

— _end note_]

A constructor is an initializer-list constructor if its first parameter is of type std​::​initializer_­list<E> or reference tocv std​::​initializer_­list<E> for some type E, and either there are no other parameters or else all other parameters have default arguments ([dcl.fct.default]).

[Note 2:

Initializer-list constructors are favored over other constructors in list-initialization ([over.match.list]).

Passing an initializer list as the argument to the constructor template template<class T> C(T) of a class C does not create an initializer-list constructor, because an initializer list argument causes the corresponding parameter to be a non-deduced context ([temp.deduct.call]).

— _end note_]

The templatestd​::​initializer_­list is not predefined; if the header<initializer_­list> is not imported or included prior to a use ofstd​::​initializer_­list — even an implicit use in which the type is not named ([dcl.spec.auto]) — the program is ill-formed.

List-initialization of an object or reference of type T is defined as follows:

• If the braced-init-listcontains a designated-initializer-list,T shall be an aggregate class.
  The ordered identifier_s_in the designator_s_of the designated-initializer-listshall form a subsequence of the ordered identifiers_in the direct non-static data members of T.
  Aggregate initialization is performed ([dcl.init.aggr]).
  [_Example 2: struct A { int x; int y; int z; }; A a{.y = 2, .x = 1}; A b{.x = 1, .z = 2}; — _end example_]
• If T is an aggregate class and the initializer list has a single element of type cv U, where U is T or a class derived from T, the object is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization).
• Otherwise, if T is a character array and the initializer list has a single element that is an appropriately-typed string-literal ([dcl.init.string]), initialization is performed as described in that subclause.
• Otherwise, if T is an aggregate, aggregate initialization is performed ([dcl.init.aggr]).
  [Example 3: double ad[] = { 1, 2.0 }; int ai[] = { 1, 2.0 }; struct S2 { int m1;double m2, m3;}; S2 s21 = { 1, 2, 3.0 }; S2 s22 { 1.0, 2, 3 }; S2 s23 { }; — _end example_]
• Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.
• Otherwise, if T is a specialization of std​::​initializer_­list<E>, the object is constructed as described below.
• Otherwise, if T is a class type, constructors are considered.
  The applicable constructors are enumerated and the best one is chosen through overload resolution ([over.match], [over.match.list]).
  If a narrowing conversion (see below) is required to convert any of the arguments, the program is ill-formed.
  [Example 4: struct S { S(std::initializer_list<double>); S(std::initializer_list<int>); S(); }; S s1 = { 1.0, 2.0, 3.0 }; S s2 = { 1, 2, 3 }; S s3 = { }; — end example_]
  [_Example 5: struct Map { Map(std::initializer_list<std::pairstd::string,int\>);}; Map ship = {{"Sophie",14}, {"Surprise",28}}; — end example_]
  [_Example 6: struct S { S(int, double, double); S(); }; S s1 = { 1, 2, 3.0 }; S s2 { 1.0, 2, 3 }; S s3 { }; — _end example_]
• Otherwise, if T is an enumeration with a fixed underlying type ([dcl.enum]) U, the initializer-list has a single element v,v can be implicitly converted to U, and the initialization is direct-list-initialization, the object is initialized with the value T(v) ([expr.type.conv]); if a narrowing conversion is required to convert vto U, the program is ill-formed.
  [Example 7: enum byte : unsigned char { }; byte b { 42 }; byte c = { 42 }; byte d = byte{ 42 }; byte e { -1 }; struct A { byte b; }; A a1 = { { 42 } }; A a2 = { byte{ 42 } }; void f(byte); f({ 42 }); enum class Handle : uint32_t { Invalid = 0 }; Handle h { 42 }; — _end example_]
• Otherwise, if the initializer list has a single element of type E and eitherT is not a reference type or its referenced type is reference-related to E, the object or reference is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization); if a narrowing conversion (see below) is required to convert the element to T, the program is ill-formed.
  [Example 8: int x1 {2}; int x2 {2.0}; — _end example_]
• Otherwise, if T is a reference type, a prvalue is generated.
  The prvalue initializes its result object by copy-list-initialization.
  The prvalue is then used to direct-initialize the reference.
  The type of the temporary is the type referenced by T, unless T is “reference to array of unknown bound of U”, in which case the type of the temporary is the type of x in the declaration U x[] H, where H is the initializer list.
  [Note 3:
  As usual, the binding will fail and the program is ill-formed if the reference type is an lvalue reference to a non-const type.
  — end note_]
  [_Example 9: struct S { S(std::initializer_list<double>); S(const std::string&); };const S& r1 = { 1, 2, 3.0 }; const S& r2 { "Spinach" }; S& r3 = { 1, 2, 3 }; const int& i1 = { 1 }; const int& i2 = { 1.1 }; const int (&iar)[2] = { 1, 2 }; struct A { } a;struct B { explicit B(const A&); };const B& b2{a}; — _end example_]
• Otherwise, if the initializer list has no elements, the object is value-initialized.
  [Example 10: int** pp {}; — _end example_]
• Otherwise, the program is ill-formed.
  [Example 11: struct A { int i; int j; }; A a1 { 1, 2 }; A a2 { 1.2 }; struct B { B(std::initializer_list<int>);}; B b1 { 1, 2 }; B b2 { 1, 2.0 }; struct C { C(int i, double j);}; C c1 = { 1, 2.2 }; C c2 = { 1.1, 2 }; int j { 1 }; int k { }; — _end example_]

Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions ([temp.variadic]), are evaluated in the order in which they appear.

That is, every value computation and side effect associated with a given initializer-clause is sequenced before every value computation and side effect associated with any initializer-clausethat follows it in the comma-separated list of the initializer-list.

[Note 4:

This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of theinitializer-list are interpreted as arguments of a constructor call, even though ordinarily there are no sequencing constraints on the arguments of a call.

— _end note_]

An object of type std​::​initializer_­list<E> is constructed from an initializer list as if the implementation generated and materialized ([conv.rval]) a prvalue of type “array of N const E”, where N is the number of elements in the initializer list.

Each element of that array is copy-initialized with the corresponding element of the initializer list, and thestd​::​initializer_­list<E> object is constructed to refer to that array.

[Note 5:

A constructor or conversion function selected for the copy is required to be accessible ([class.access]) in the context of the initializer list.

— _end note_]

If a narrowing conversion is required to initialize any of the elements, the program is ill-formed.

[Example 12: struct X { X(std::initializer_list<double> v);}; X x{ 1,2,3 };

The initialization will be implemented in a way roughly equivalent to this:const double __a[3] = {double{1}, double{2}, double{3}}; X x(std::initializer_list<double>(__a, __a+3));assuming that the implementation can construct an initializer_­list object with a pair of pointers.

— _end example_]

The array has the same lifetime as any other temporary object ([class.temporary]), except that initializing aninitializer_­list object from the array extends the lifetime of the array exactly like binding a reference to a temporary.

[Example 13: typedef std::complex<double> cmplx; std::vector<cmplx> v1 = { 1, 2, 3 };void f() { std::vector<cmplx> v2{ 1, 2, 3 }; std::initializer_list<int> i3 = { 1, 2, 3 };} struct A { std::initializer_list<int> i4; A() : i4{ 1, 2, 3 } {} };

For v1 and v2, the initializer_­list object is a parameter in a function call, so the array created for{ 1, 2, 3 } has full-expression lifetime.

For i3, the initializer_­list object is a variable, so the array persists for the lifetime of the variable.

For i4, the initializer_­list object is initialized in the constructor's ctor-initializer as if by binding a temporary array to a reference member, so the program is ill-formed ([class.base.init]).

— _end example_]

[Note 6:

The implementation is free to allocate the array in read-only memory if an explicit array with the same initializer could be so allocated.

— _end note_]

A narrowing conversion is an implicit conversion

• from a floating-point type to an integer type, or
• from long double to double or float, or fromdouble to float, except where the source is a constant expression and the actual value after conversion is within the range of values that can be represented (even if it cannot be represented exactly), or
• from an integer type or unscoped enumeration type to a floating-point type, except where the source is a constant expression and the actual value after conversion will fit into the target type and will produce the original value when converted back to the original type, or
• from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where the source is a constant expression whose value after integral promotions will fit into the target type, or
• from a pointer type or a pointer-to-member type to bool.

[Note 7:

As indicated above, such conversions are not allowed at the top level in list-initializations.

— _end note_]

[Example 14: int x = 999; const int y = 999;const int z = 99;char c1 = x; char c2{x}; char c3{y}; char c4{z}; unsigned char uc1 = {5}; unsigned char uc2 = {-1}; unsigned int ui1 = {-1}; signed int si1 = { (unsigned int)-1 }; int ii = {2.0}; float f1 { x }; float f2 { 7 }; bool b = {"meow"}; int f(int);int a[] = { 2, f(2), f(2.0) }; — _end example_]