11 Declarators [dcl.decl]

11.6 Initializers [dcl.init]

The process of initialization described in this subclause 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.
initializer:
	brace-or-equal-initializer
	( expression-list )
brace-or-equal-initializer:
	= initializer-clause
	braced-init-list
initializer-clause:
	assignment-expression
	braced-init-list
braced-init-list:
	{ initializer-list , }
	{ designated-initializer-list , }
	{ }
initializer-list:
	initializer-clause ...
	initializer-list , initializer-clause ...
designated-initializer-list:
	designated-initializer-clause
	designated-initializer-list , designated-initializer-clause
designated-initializer-clause:
	designator brace-or-equal-initializer
designator:
	. identifier
expr-or-braced-init-list:
	expression
	braced-init-list
[Note
:
The rules in this subclause 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
:
int f(int);
int a = 2;
int b = f(a);
int c(b);
end example
]
[Note
:
Default arguments are more restricted; see [dcl.fct.default].
end note
]
[Note
:
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.
To zero-initialize an object or reference of type T means:
  • if T is a scalar type, the object is initialized to the value obtained by converting the integer literal 0 (zero) to T;104
  • if T is a (possibly cv-qualified) non-union class type, its padding bits ([basic.types]) 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;
  • if T is a (possibly cv-qualified) union type, its padding bits ([basic.types]) are initialized to zero bits and the object's first non-static named data member is zero-initialized;
  • if T is an array type, each element is zero-initialized;
  • if T is a reference type, no initialization is performed.
To default-initialize an object of type T means:
  • If T is a (possibly cv-qualified) class type, constructors are considered.
    The applicable constructors are enumerated ([over.match.ctor]), and the best one for the initializer () is chosen through overload resolution.
    The constructor thus selected is called, with an empty argument list, to initialize the object.
  • If T is 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 T has 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.
To value-initialize an object of type T means:
  • if T is a (possibly cv-qualified) class type with either no default constructor ([class.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;
  • if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, then 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;
  • if T is 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
:
Every object of static storage duration is zero-initialized at program startup before any other initialization takes place.
In some cases, additional initialization is done later.
end note
]
An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.
[Note
:
Since () is not permitted by the syntax for initializer,
X a();
is not the declaration of an object of class X, but the declaration of a function taking no argument and returning an X.
The form () is permitted in certain other initialization contexts ([expr.new], [expr.type.conv], [class.base.init]).
end note
]
If no initializer is specified for an object, the object is default-initialized.
When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced ([expr.ass]).
[Note
:
Objects with static or thread storage duration are zero-initialized, see [basic.start.static].
end note
]
If an indeterminate value is produced by an evaluation, the behavior is undefined except in the following cases:
  • If an indeterminate value of unsigned narrow character type or std​::​byte type ([cstddef.syn]) is produced by the evaluation of: then the result of the operation is an indeterminate value.
  • If an indeterminate value of unsigned narrow character type or std​::​byte type is produced by the evaluation of the right operand of a simple assignment operator whose first operand is an lvalue of unsigned narrow character type or std​::​byte type, an indeterminate value replaces the value of the object referred to by the left operand.
  • If an indeterminate value of unsigned narrow character type is produced by the evaluation of the initialization expression when initializing an object of unsigned narrow character type, that object is initialized to an indeterminate value.
  • If an indeterminate value of unsigned narrow character type or std​::​byte type is produced by the evaluation of the initialization expression when initializing an object of std​::​byte type, that object is initialized to an indeterminate value.
[Example
:
  int f(bool b) {
    unsigned char c;
    unsigned char d = c;        // OK, d has an indeterminate value
    int e = d;                  // undefined behavior
    return b ? d : 0;           // undefined behavior if b is true
  }
end example
]
An initializer for a static member is in the scope of the member's class.
[Example
:
int a;

struct X {
  static int a;
  static int b;
};

int X::a = 1;
int X::b = a;                   // X​::​b = X​::​a
end example
]
If the entity being initialized does not have class type, the expression-list in a parenthesized initializer shall be a single expression.
The initialization that occurs in the = form of a brace-or-equal-initializer or condition ([stmt.select]), as well as in argument passing, function return, throwing an exception, handling an exception, and aggregate member initialization, is called copy-initialization.
[Note
:
Copy-initialization may invoke a move ([class.copy]).
end note
]
The initialization that occurs in the forms
T x(a);
T x{a};
as well as in new expressions, static_­cast expressions, functional notation type conversions, mem-initializers, and the braced-init-list form of a condition is called direct-initialization.
The semantics of initializers are as follows.
The destination type is the type of the object or reference being initialized and the source type is 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-list or is = braced-init-list, the object or reference is list-initialized.
  • If the destination type is a reference type, see [dcl.init.ref].
  • If the destination type is an array of characters, an array of char16_­t, an array of char32_­t, or an array of wchar_­t, and the initializer is a string literal, see [dcl.init.string].
  • If the initializer is (), the object is value-initialized.
  • Otherwise, if the destination type is an array, the program is ill-formed.
  • 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
      :
      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.
      The constructor so selected is called to initialize the object, with the initializer expression or expression-list as its argument(s).
      If no constructor applies, or the overload resolution is ambiguous, the initialization is ill-formed.
    • Otherwise (i.e., for the remaining copy-initialization cases), user-defined conversion sequences 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.
      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.
    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, the initial value of the object being initialized is the (possibly converted) value of the initializer expression.
    Standard conversions 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 is implementation-defined.
    [Note
    :
    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
    ]
An initializer-clause followed by an ellipsis is a pack expansion.
If the initializer is a parenthesized expression-list, the expressions are evaluated in the order specified for function calls.
The same identifier shall not appear in multiple designators of a designated-initializer-list.
An object whose initialization has completed is deemed to be constructed, even if no constructor of the object's class is invoked for the initialization.
[Note
:
Such an object might have been value-initialized or initialized by aggregate initialization or by an inherited constructor.
end note
]
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
:
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
]
As specified in [conv.ptr], converting an integer literal whose value is 0 to a pointer type results in a null pointer value.

11.6.1 Aggregates [dcl.init.aggr]

An aggregate is an array or a class with
[Note
:
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 elements of 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 designator shall 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:
[Example
:
struct A {
  int x;
  struct B {
    int i;
    int j;
  } b;
} a = { 1, { 2, 3 } };
initializes a.x with 1, a.b.i with 2, a.b.j with 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};
initializes d1.b1 with 1, d1.b2 with 2, d1.b3 with 42, d1.d with 4, and d2.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
:
struct S { int a; const char* b; int c; int d = b[a]; };
S ss = { 1, "asdf" };
initializes ss.a with 1, ss.b with "asdf", ss.c with the value of an expression of the form int{} (that is, 0), and ss.d with the value of ss.b[ss.a] (that is, 's'), and in
struct 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].
An array of unknown bound initialized with a brace-enclosed initializer-list containing n initializer-clauses, where n shall be greater than zero, is defined as having n elements ([dcl.array]).
[Example
:
int x[] = { 1, 3, 5 };
declares and initializes x as a one-dimensional array that has three elements since no size was specified and there are three initializers.
end example
]
An empty initializer list {} shall not be used as the initializer-clause for an array of unknown bound.105
[Note
:
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
:
struct S {
  int y[] = { 0 };          // error: non-static data member of incomplete type
};
end example
]
end note
]
[Note
:
Static data members, non-static data members of anonymous union members, and unnamed bit-fields are not considered elements of the aggregate.
[Example
:
struct A {
  int i;
  static int s;
  int j;
  int :17;
  int k;
} a = { 1, 2, 3 };
Here, the second initializer 2 initializes a.j and not the static data member A​::​s, and the third initializer 3 initializes a.k and not the unnamed bit-field before it.
end example
]
end note
]
An initializer-list is ill-formed if the number of initializer-clauses exceeds the number of elements of the aggregate.
[Example
:
char cv[4] = { 'a', 's', 'd', 'f', 0 };     // error
is ill-formed.
end example
]
If a reference member is initialized from its 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
:
  struct A;
  extern A a;
  struct A {
    const A& a1 { A{a,a} };     // OK
    const A& a2 { A{} };        // error
  };
  A a{a,a};                     // OK
end example
]
If an aggregate class C contains a subaggregate element e with no elements, the initializer-clause for e shall not be omitted from an initializer-list for an object of type C unless the initializer-clauses for all elements of C following e are also omitted.
[Example
:
struct S { } s;
struct A {
  S s1;
  int i1;
  S s2;
  int i2;
  S s3;
  int i3;
} a = {
  { },              // Required initialization
  0,
  s,                // Required initialization
  0
};                  // Initialization not required for A​::​s3 because A​::​i3 is also not initialized
end example
]
When initializing a multi-dimensional array, the initializer-clauses initialize the elements with the last (rightmost) index of the array varying the fastest ([dcl.array]).
[Example
:
int x[2][2] = { 3, 1, 4, 2 };
initializes x[0][0] to 3, x[0][1] to 1, x[1][0] to 4, and x[1][1] to 2.
On the other hand,
float y[4][3] = {
  { 1 }, { 2 }, { 3 }, { 4 }
};
initializes the first column of y (regarded as a two-dimensional array) and leaves the rest zero.
end example
]
Braces can be elided in an initializer-list as follows.
If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializer-clauses initializes the elements of a subaggregate; it is erroneous for there to be more initializer-clauses than elements.
If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializer-clauses from the list are taken to initialize the elements of the subaggregate; any remaining initializer-clauses are left to initialize the next element of the aggregate of which the current subaggregate is an element.
[Example
:
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 array y[0], namely y[0][0], y[0][1], and y[0][2].
Likewise the next two lines initialize y[1] and y[2].
The initializer ends early and therefore y[3]s elements are initialized as if explicitly initialized with an expression of the form float(), that is, are initialized with 0.0.
In the following example, braces in the initializer-list are elided; however the initializer-list has the same effect as the completely-braced initializer-list of the above example,
float y[4][3] = {
  1, 3, 5, 2, 4, 6, 3, 5, 7
};
The initializer for y begins with a left brace, but the one for y[0] does not, therefore three elements from the list are used.
Likewise the next three are taken successively for y[1] and y[2].
end example
]
All implicit type conversions ([conv]) are considered when initializing the element with an assignment-expression.
If the assignment-expression can initialize an element, the element is initialized.
Otherwise, if the element is itself a subaggregate, brace elision is assumed and the assignment-expression is considered for the initialization of the first element of the subaggregate.
[Note
:
As specified above, brace elision cannot apply to subaggregates with no elements; an initializer-clause for the entire subobject is required.
end note
]
[Example
:
struct A {
  int i;
  operator int();
};
struct B {
  A a1, a2;
  int z;
};
A a;
B b = { 4, a, a };
Braces are elided around the initializer-clause for b.a1.i.
b.a1.i is initialized with 4, b.a2 is initialized with a, b.z is initialized with whatever a.operator int() returns.
end example
]
[Note
:
An aggregate array or an aggregate class may contain elements of a class type with a user-provided constructor ([class.ctor]).
Initialization of these aggregate objects is described in [class.expl.init].
end note
]
[Note
:
Whether the initialization of aggregates with static storage duration is static or dynamic is specified in [basic.start.static], [basic.start.dynamic], and [stmt.dcl].
end note
]
When a union is initialized with an initializer list, there shall not be more than one explicitly initialized element.
[Example
:
union u { int a; const char* b; };
u a = { 1 };
u b = a;
u c = 1;                        // error
u d = { 0, "asdf" };            // error
u e = { "asdf" };               // error
u f = { .b = "asdf" };
u g = { .a = 1, .b = "asdf" };  // error
end example
]
[Note
:
As described above, the braces around the initializer-clause for a union member can be omitted if the union is a member of another aggregate.
end note
]
The syntax provides for empty initializer-lists, but nonetheless C++ does not have zero length arrays.

11.6.2 Character arrays [dcl.init.string]

An array of narrow character type, char16_­t array, char32_­t array, or wchar_­t array can be initialized by a narrow string literal, char16_­t string literal, char32_­t 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 literal initialize the elements of the array.
[Example
:
char msg[] = "Syntax error on line %s\n";
shows a character array whose members are initialized with a string-literal.
Note that because '\n' is a single character and because a trailing '\0' is appended, sizeof(msg) is 25.
end example
]
There shall not be more initializers than there are array elements.
[Example
:
char cv[4] = "asdf";            // error
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.

11.6.3 References [dcl.init.ref]

A variable whose declared type is “reference to type T shall be initialized.
[Example
:
int g(int) noexcept;
void f() {
  int i;
  int& r = i;                   // r refers to i
  r = 1;                        // the value of i becomes 1
  int* p = &r;                  // p points to i
  int& rr = r;                  // rr refers to what r refers to, that is, to i
  int (&rg)(int) = g;           // rg refers to the function g
  rg(i);                        // calls function g
  int a[3];
  int (&ra)[3] = a;             // ra refers to the array a
  ra[1] = i;                    // modifies a[1]
}
end example
]
A reference cannot be changed to refer to another object after initialization.
[Note
:
Assignment to a reference assigns to the object referred to by the reference ([expr.ass]).
end note
]
Argument passing and function value 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 the extern specifier is explicitly used.
[Example
:
int& r1;                        // error: initializer missing
extern int& r2;                 // OK
end example
]
Given types “cv1 T1” and “cv2 T2”, “cv1 T1” is reference-related to “cv2 T2” if T1 is the same type as T2, or T1 is a base class of T2.
cv1 T1” is reference-compatible with “cv2 T2” if
  • T1 is reference-related to T2, or
  • T2 is “noexcept function” and T1 is “function”, where the function types are otherwise the same,
and cv1 is the same cv-qualification as, or greater cv-qualification than, cv2.
In all cases where the reference-related or reference-compatible relationship of two types is used to establish the validity of a reference binding, and T1 is a base class of T2, a program that necessitates such a binding is ill-formed if T1 is an inaccessible or ambiguous ([class.member.lookup]) base class of T2.
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., T2 is 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 T3106 (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).
    [Note
    :
    The usual lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are not needed, and therefore are suppressed, when such direct bindings to lvalues are done.
    end note
    ]
    [Example
    :
    double d = 2.0;
    double& rd = d;                 // rd refers to d
    const double& rcd = d;          // rcd refers to d
    
    struct A { };
    struct B : A { operator int&(); } b;
    A& ra = b;                      // ra refers to A subobject in b
    const A& rca = b;               // rca refers to A subobject in b
    int& ir = B();                  // ir refers to the result of B​::​operator int&
    
    end example
    ]
  • Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be const), or the reference shall be an rvalue reference.
    [Example
    :
    double& rd2 = 2.0;              // error: not an lvalue and reference not const
    int  i = 2;
    double& rd3 = i;                // error: type mismatch and reference not const
    
    end example
    ]
    • 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 T1 is 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 is applied.
      In any case, the reference is bound to the resulting glvalue (or to an appropriate base class subobject).
      [Example
      :
      struct A { };
      struct B : A { } b;
      extern B f();
      const A& rca2 = f();                // bound to the A subobject of the B rvalue.
      A&& rra = f();                      // same as above
      struct X {
        operator B();
        operator int&();
      } x;
      const A& r = x;                     // bound to the A subobject of the result of the conversion
      int i2 = 42;
      int&& rri = static_cast<int&&>(i2); // bound directly to i2
      B&& rrb = x;                        // bound directly to the result of operator B
      
      end example
      ]
    • Otherwise:
      • If T1 or T2 is a class type and T1 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.
      If T1 is reference-related to T2:
      • 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
      :
      struct Banana { };
      struct Enigma { operator const Banana(); };
      struct Alaska { operator Banana&(); };
      void enigmatic() {
        typedef const Banana ConstBanana;
        Banana &&banana1 = ConstBanana(); // ill-formed
        Banana &&banana2 = Enigma();      // ill-formed
        Banana &&banana3 = Alaska();      // ill-formed
      }
      
      const double& rcd2 = 2;             // rcd2 refers to temporary with value 2.0
      double&& rrd = 2;                   // rrd refers to temporary with value 2.0
      const volatile int cvi = 1;
      const int& r2 = cvi;                // error: cv-qualifier dropped
      struct A { operator volatile int&(); } a;
      const int& r3 = a;                  // error: cv-qualifier dropped
                                          // from result of conversion function
      double d2 = 1.0;
      double&& rrd2 = d2;                 // error: initializer is lvalue of related type
      struct X { operator int&(); };
      int&& rri2 = X();                   // error: result of conversion function is lvalue of related type
      int i3 = 2;
      double&& rrd3 = i3;                 // rrd3 refers to temporary with value 2.0
      
      end example
      ]
In all cases except the last (i.e., implicitly converting the initializer expression to the underlying type of the reference), the reference is said to bind directly to the initializer expression.
[Note
:
[class.temporary] describes the lifetime of temporaries bound to references.
end note
]
This requires a conversion function returning a reference type.

11.6.4 List-initialization [dcl.init.list]

List-initialization is initialization of an object or reference from a braced-init-list.
Such an initializer is called an initializer list, and the comma-separated initializer-clauses of the initializer-list or designated-initializer-clauses of the designated-initializer-list are 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 called direct-list-initialization and list-initialization in a copy-initialization context is called copy-list-initialization.
[Note
:
List-initialization can be used
[Example
:
int a = {1};
std::complex<double> z{1,2};
new std::vector<std::string>{"once", "upon", "a", "time"};  // 4 string elements
f( {"Nicholas","Annemarie"} );  // pass list of two elements
return { "Norah" };             // return list of one element
int* e {};                      // initialization to zero / null pointer
x = double{1};                  // explicitly construct a double
std::map<std::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 to possibly cv-qualified std​::​initializer_­list<E> for some type E, and either there are no other parameters or else all other parameters have default arguments.
[Note
:
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 template std​::​initializer_­list is not predefined; if the header <initializer_­list> is not included prior to a use of std​::​initializer_­list — even an implicit use in which the type is not named — the program is ill-formed.
List-initialization of an object or reference of type T is defined as follows:
  • If the braced-init-list contains a designated-initializer-list, T shall be an aggregate class.
    The ordered identifiers in the designators of the designated-initializer-list shall form a subsequence of the ordered identifiers in the direct non-static data members of T.
    Aggregate initialization is performed ([dcl.init.aggr]).
    [Example
    :
    struct A { int x; int y; int z; };
    A a{.y = 2, .x = 1};                // error: designator order does not match declaration order
    A b{.x = 1, .z = 2};                // OK, b.y initialized to 0
    
    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.
    [Example
    :
    double ad[] = { 1, 2.0 };           // OK
    int ai[] = { 1, 2.0 };              // error: narrowing
    
    struct S2 {
      int m1;
      double m2, m3;
    };
    S2 s21 = { 1, 2, 3.0 };             // OK
    S2 s22 { 1.0, 2, 3 };               // error: narrowing
    S2 s23 { };                         // OK: default to 0,0,0
    
    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
    :
    struct S {
      S(std::initializer_list<double>); // #1
      S(std::initializer_list<int>);    // #2
      S();                              // #3
      // ...
    };
    S s1 = { 1.0, 2.0, 3.0 };           // invoke #1
    S s2 = { 1, 2, 3 };                 // invoke #2
    S s3 = { };                         // invoke #3
    
    end example
    ]
    [Example
    :
    struct Map {
      Map(std::initializer_list<std::pair<std::string,int>>);
    };
    Map ship = {{"Sophie",14}, {"Surprise",28}};
    end example
    ]
    [Example
    :
    struct S {
      // no initializer-list constructors
      S(int, double, double);           // #1
      S();                              // #2
      // ...
    };
    S s1 = { 1, 2, 3.0 };               // OK: invoke #1
    S s2 { 1.0, 2, 3 };                 // error: narrowing
    S s3 { };                           // OK: invoke #2
    
    end example
    ]
  • Otherwise, if T is an enumeration with a fixed underlying type ([dcl.enum]), the initializer-list has a single element v, 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 v to the underlying type of T, the program is ill-formed.
    [Example
    :
    enum byte : unsigned char { };
    byte b { 42 };                      // OK
    byte c = { 42 };                    // error
    byte d = byte{ 42 };                // OK; same value as b
    byte e { -1 };                      // error
    
    struct A { byte b; };
    A a1 = { { 42 } };                  // error
    A a2 = { byte{ 42 } };              // OK
    
    void f(byte);
    f({ 42 });                          // error
    
    enum class Handle : uint32_t { Invalid = 0 };
    Handle h { 42 };                    // OK
    
    end example
    ]
  • Otherwise, if the initializer list has a single element of type E and either T 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
    :
    int x1 {2};                         // OK
    int x2 {2.0};                       // error: narrowing
    
    end example
    ]
  • Otherwise, if T is a reference type, a prvalue of the type referenced by T is generated.
    The prvalue initializes its result object by copy-list-initialization or direct-list-initialization, depending on the kind of initialization for the reference.
    The prvalue is then used to direct-initialize the reference.
    [Note
    :
    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
    :
    struct S {
      S(std::initializer_list<double>); // #1
      S(const std::string&);            // #2
      // ...
    };
    const S& r1 = { 1, 2, 3.0 };        // OK: invoke #1
    const S& r2 { "Spinach" };          // OK: invoke #2
    S& r3 = { 1, 2, 3 };                // error: initializer is not an lvalue
    const int& i1 = { 1 };              // OK
    const int& i2 = { 1.1 };            // error: narrowing
    const int (&iar)[2] = { 1, 2 };     // OK: iar is bound to temporary array
    
    end example
    ]
  • Otherwise, if the initializer list has no elements, the object is value-initialized.
    [Example
    :
    int** pp {};                        // initialized to null pointer
    
    end example
    ]
  • Otherwise, the program is ill-formed.
    [Example
    :
    struct A { int i; int j; };
    A a1 { 1, 2 };                      // aggregate initialization
    A a2 { 1.2 };                       // error: narrowing
    struct B {
      B(std::initializer_list<int>);
    };
    B b1 { 1, 2 };                      // creates initializer_­list<int> and calls constructor
    B b2 { 1, 2.0 };                    // error: narrowing
    struct C {
      C(int i, double j);
    };
    C c1 = { 1, 2.2 };                  // calls constructor with arguments (1, 2.2)
    C c2 = { 1.1, 2 };                  // error: narrowing
    
    int j { 1 };                        // initialize to 1
    int k { };                          // initialize to 0
    
    end example
    ]
Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions, 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-clause that follows it in the comma-separated list of the initializer-list.
[Note
:
This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of the initializer-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 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 the std​::​initializer_­list<E> object is constructed to refer to that array.
[Note
:
A constructor or conversion function selected for the copy shall be accessible 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
:
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, except that initializing an initializer_­list object from the array extends the lifetime of the array exactly like binding a reference to a temporary.
[Example
:
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 } {}            // ill-formed, would create a dangling reference
};
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
:
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 from double 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.
[Note
:
As indicated above, such conversions are not allowed at the top level in list-initializations.
end note
]
[Example
:
int x = 999;                    // x is not a constant expression
const int y = 999;
const int z = 99;
char c1 = x;                    // OK, though it might narrow (in this case, it does narrow)
char c2{x};                     // error: might narrow
char c3{y};                     // error: narrows (assuming char is 8 bits)
char c4{z};                     // OK: no narrowing needed
unsigned char uc1 = {5};        // OK: no narrowing needed
unsigned char uc2 = {-1};       // error: narrows
unsigned int ui1 = {-1};        // error: narrows
signed int si1 =
  { (unsigned int)-1 };         // error: narrows
int ii = {2.0};                 // error: narrows
float f1 { x };                 // error: might narrow
float f2 { 7 };                 // OK: 7 can be exactly represented as a float
int f(int);
int a[] = { 2, f(2), f(2.0) };  // OK: the double-to-int conversion is not at the top level
end example
]