9 Declarations [dcl.dcl]

9.4 Initializers [dcl.init]

9.4.1 General [dcl.init.general]

1
#
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]
2
#
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]
3
#
[Note 2: 
Default arguments are more restricted; see [dcl.fct.default].
— end note]
4
#
[Note 3: 
The order of initialization of variables with static storage duration is described in [basic.start] and [stmt.dcl].
— end note]
5
#
A declaration D of a variable with linkage shall not have an initializer if D inhabits a block scope.
6
#
To zero-initialize an object or reference of type T means:
  • (6.1)
    if T is a scalar type ([basic.types.general]), the object is initialized to the value obtained by converting the integer literal 0 (zero) to T;79
  • (6.2)
    if T is a (possibly cv-qualified) non-union class type, its padding bits ([basic.types.general]) 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;
  • (6.3)
    if T is a (possibly cv-qualified) union type, its padding bits ([basic.types.general]) are initialized to zero bits and the object's first non-static named data member is zero-initialized;
  • (6.4)
    if T is an array type, each element is zero-initialized;
  • (6.5)
    if T is a reference type, no initialization is performed.
7
#
To default-initialize an object of type T means:
  • (7.1)
    If T is a (possibly cv-qualified) class type ([class]), constructors are considered.
    The applicable constructors are enumerated ([over.match.ctor]), and the best one for the initializer () is chosen through overload resolution ([over.match]).
    The constructor thus selected is called, with an empty argument list, to initialize the object.
  • (7.2)
    If T is an array type, the semantic constraints of default-initializing a hypothetical element shall be met and each element is default-initialized.
  • (7.3)
    Otherwise, no initialization is performed.
8
#
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
  • (8.1)
    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,
  • (8.2)
    if T is a union with at least one non-static data member, exactly one variant member has a default member initializer,
  • (8.3)
    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
  • (8.4)
    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.
9
#
To value-initialize an object of type T means:
  • (9.1)
    If T is a (possibly cv-qualified) class type ([class]), then let C be the constructor selected to default-initialize the object, if any.
    If C is not user-provided, the object is first zero-initialized.
    In all cases, the object is then default-initialized.
  • (9.2)
    If T is an array type, the semantic constraints of value-initializing a hypothetical element shall be met and each element is value-initialized.
  • (9.3)
    Otherwise, the object is zero-initialized.
10
#
A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed.
11
#
[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]
12
#
If no initializer is specified for an object, the object is default-initialized.
13
#
If the entity being initialized does not have class or array type, the expression-list in a parenthesized initializer shall be a single expression.
14
#
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 ([except.throw]), handling an exception ([except.handle]), and aggregate member initialization other than by a designated-initializer-clause ([dcl.init.aggr]), is called copy-initialization.
[Note 5: 
Copy-initialization can invoke a move ([class.copy.ctor]).
— end note]
15
#
The initialization that occurs is called direct-initialization.
16
#
The semantics of initializers are as follows.
The destination type is the cv-unqualified 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.
  • (16.1)
    If the initializer is a (non-parenthesized) braced-init-list or is = braced-init-list, the object or reference is list-initialized ([dcl.init.list]).
  • (16.2)
    If the destination type is a reference type, see [dcl.init.ref].
  • (16.3)
    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 of wchar_t, and the initializer is a string-literal, see [dcl.init.string].
  • (16.4)
    If the initializer is (), the object is value-initialized.
    [Note 6: 
    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 arguments and returning an X.
    The form () can appear in certain other initialization contexts ([expr.new], [expr.type.conv], [class.base.init]).
    — end note]
  • (16.5)
    Otherwise, if the destination type is an array, the object is initialized as follows.
    The initializer shall be of the form ( expression-list ).
    Let , …, be the elements of the expression-list.
    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 1  ≤ i  ≤ k, 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.
  • (16.6)
    Otherwise, if the destination type is a class type:
    • (16.6.1)
      If the initializer expression is a prvalue and the cv-unqualified version of the source type is the same as the destination type, the initializer expression is used to initialize the destination object.
      [Example 2: 
      T x = T(T(T())); value-initializes x.
      — end example]
    • (16.6.2)
      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 as or is derived from the class of the destination type, constructors are considered.
      The applicable constructors are enumerated ([over.match.ctor]), and the best one is chosen through overload resolution ([over.match]).
      Then:
      • (16.6.2.1)
        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).
      • (16.6.2.2)
        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]).
        Let , …, be the elements of the expression-list.
        If k is greater than n, the program is ill-formed.
        The element is copy-initialized with for 1  ≤ i  ≤ k.
        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]) can appear, 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 3: struct A { int a; int&& r; }; int f(); int n = 10; A a1{1, f()}; // OK, lifetime is extended A a2(1, f()); // well-formed, but dangling reference A a3{1.0, 1}; // error: narrowing conversion A a4(1.0, 1); // well-formed, but dangling reference A a5(1.0, std::move(n)); // OK — end example]
        — end note]
      • (16.6.2.3)
        Otherwise, the initialization is ill-formed.
    • (16.6.3)
      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.
  • (16.7)
    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.
  • (16.8)
    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.
  • (16.9)
    Otherwise, the initial value of the object being initialized is the (possibly converted) value of the initializer expression.
    A standard conversion sequence ([conv]) is used to convert the initializer expression to a prvalue 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 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]
17
#
An immediate invocation ([expr.const]) that is not evaluated where it appears ([dcl.fct.default], [class.mem.general]) is evaluated and checked for whether it is a constant expression at the point where the enclosing initializer is used in a function call, a constructor definition, or an aggregate initialization.
18
#
An initializer-clause followed by an ellipsis is a pack expansion ([temp.variadic]).
19
#
Initialization includes the evaluation of all subexpressions of each initializer-clause of the initializer (possibly nested within braced-init-lists) and the creation of any temporary objects for function arguments or return values ([class.temporary]).
20
#
If the initializer is a parenthesized expression-list, the expressions are evaluated in the order specified for function calls ([expr.call]).
21
#
The same identifier shall not appear in multiple designators of a designated-initializer-list.
22
#
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.
23
#
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]) can be the declaration within the class definition and not the definition (if any) outside it.
— end note]
79)79)
As specified in [conv.ptr], converting an integer literal whose value is 0 to a pointer type results in a null pointer value.

9.4.2 Aggregates [dcl.init.aggr]

1
#
An aggregate is an array or a class ([class]) with
[Note 1: 
Aggregate initialization does not allow accessing protected and private base class' members or constructors.
— end note]
2
#
The elements of an aggregate are:
  • (2.1)
    for an array, the array elements in increasing subscript order, or
  • (2.2)
    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.
3
#
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:
  • (3.1)
    If the initializer list is a brace-enclosed 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.
  • (3.2)
    If the initializer list is a brace-enclosed initializer-list, the explicitly initialized elements of the aggregate are those for which an element of the initializer list appertains to the aggregate element or to a subobject thereof (see below).
  • (3.3)
    Otherwise, the initializer list must be {}, and there are no explicitly initialized elements.
4
#
For each explicitly initialized element:
  • (4.1)
    If the element is an anonymous union member and the initializer list is a brace-enclosed designated-initializer-list, the element is initialized by the braced-init-list { D }, where D is the designated-initializer-clause naming a member of the anonymous union member.
    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]
  • (4.2)
    Otherwise, if the initializer list is a brace-enclosed designated-initializer-list, the element is initialized with the brace-or-equal-initializer of the corresponding designated-initializer-clause.
    If that initializer is of the form = assignment-expression and a narrowing conversion ([dcl.init.list]) is required to convert the expression, the program is ill-formed.
    [Note 2: 
    The form of the initializer determines whether copy-initialization or direct-initialization is performed.
    — end note]
  • (4.3)
    Otherwise, the initializer list is a brace-enclosed initializer-list.
    If an initializer-clause appertains to the aggregate element, then the aggregate element is copy-initialized from the initializer-clause.
    Otherwise, the aggregate element is copy-initialized from a brace-enclosed initializer-list consisting of all of the initializer-clauses that appertain to subobjects of the aggregate element, in the order of appearance.
    [Note 3: 
    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 } }; 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]
5
#
For a non-union aggregate, each element that is not an explicitly initialized element is initialized as follows:
  • (5.1)
    If the element has a default member initializer ([class.mem]), the element is initialized from that initializer.
  • (5.2)
    Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).
  • (5.3)
    Otherwise, the program is ill-formed.
If the aggregate is a union and the initializer list is empty, then
  • (5.4)
    if any variant member has a default member initializer, that member is initialized from its default member initializer;
  • (5.5)
    otherwise, the first member of the union (if any) is copy-initialized from an empty initializer list.
6
#
[Example 3: 
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').
struct A { string a; int b = 42; int c = -1; };
A{.c=21} has the following steps:
  • (6.1)
    Initialize a with {}
  • (6.2)
    Initialize b with = 42
  • (6.3)
    Initialize c with = 21
— end example]
7
#
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.
8
#
An aggregate that is a class can also be initialized with a single expression not enclosed in braces, as described in [dcl.init].
9
#
The destructor for each element of class type other than an anonymous union member is potentially invoked ([class.dtor]) from the context where the aggregate initialization occurs.
[Note 4: 
This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]).
— end note]
10
#
The number of elements ([dcl.array]) in an array of unknown bound initialized with a brace-enclosed initializer-list is the number of explicitly initialized elements of the array.
[Example 4: 
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]
[Example 5: 
In struct X { int i, j, k; }; X a[] = { 1, 2, 3, 4, 5, 6 }; X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } }; a and b have the same value.
— end example]
An array of unknown bound shall not be initialized with an empty braced-init-list {}.80
[Note 5: 
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 6: struct S { int y[] = { 0 }; // error: non-static data member of incomplete type }; — end example]
— end note]
11
#
[Note 6: 
Static data members, non-static data members of anonymous union members, and unnamed bit-fields are not considered elements of the aggregate.
[Example 7: 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]
12
#
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} }; // OK const A& a2 { A{} }; // error }; A a{a,a}; // OK struct B { int n = B{}.n; // error }; — end example]
13
#
When initializing a multidimensional array, the initializer-clauses initialize the elements with the last (rightmost) index of the array varying the fastest ([dcl.array]).
[Example 9: 
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]
14
#
Each initializer-clause in a brace-enclosed initializer-list is said to appertain to an element of the aggregate being initialized or to an element of one of its subaggregates.
Considering the sequence of initializer-clauses, and the sequence of aggregate elements initially formed as the sequence of elements of the aggregate being initialized and potentially modified as described below, each initializer-clause appertains to the corresponding aggregate element if
  • (14.1)
    the aggregate element is not an aggregate, or
  • (14.2)
    the initializer-clause begins with a left brace, or
  • (14.3)
    the initializer-clause is an expression and an implicit conversion sequence can be formed that converts the expression to the type of the aggregate element, or
  • (14.4)
    the aggregate element is an aggregate that itself has no aggregate elements.
Otherwise, the aggregate element is an aggregate and that subaggregate is replaced in the list of aggregate elements by the sequence of its own aggregate elements, and the appertainment analysis resumes with the first such element and the same initializer-clause.
[Note 7: 
These rules apply recursively to the aggregate's subaggregates.
[Example 10: 
In struct S1 { int a, b; }; struct S2 { S1 s, t; }; S2 x[2] = { 1, 2, 3, 4, 5, 6, 7, 8 }; S2 y[2] = { { { 1, 2 }, { 3, 4 } }, { { 5, 6 }, { 7, 8 } } }; x and y have the same value.
— end example]
— end note]
This process continues until all initializer-clauses have been exhausted.
If any initializer-clause remains that does not appertain to an element of the aggregate or one of its subaggregates, the program is ill-formed.
[Example 11: char cv[4] = { 'a', 's', 'd', 'f', 0 }; // error: too many initializers — end example]
15
#
[Example 12: 
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]
16
#
[Note 8: 
The initializer for an empty subaggregate is needed if any initializers are provided for subsequent elements.
[Example 13: 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]
— end note]
17
#
[Example 14: 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]
18
#
[Note 9: 
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]
19
#
[Note 10: 
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]
20
#
When a union is initialized with an initializer list, there shall not be more than one explicitly initialized element.
[Example 15: 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]
21
#
[Note 11: 
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]
80)80)
The syntax provides for empty braced-init-lists, but nonetheless C++ does not have zero length arrays.

9.4.3 Character arrays [dcl.init.string]

1
#
An array of ordinary character type ([basic.fundamental]), char8_t array, char16_t array, char32_t array, or wchar_t array may 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]).
Additionally, an array of char or unsigned char may be initialized by a UTF-8 string literal, or by such a string literal enclosed in braces.
Successive characters of the value of the string-literal initialize the elements of the array, with an integral conversion ([conv.integral]) if necessary for the source and destination value.
[Example 1: 
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]
2
#
There shall not be more initializers than there are array elements.
[Example 2: 
char cv[4] = "asdf"; // error is ill-formed since there is no space for the implied trailing '\0'.
— end example]
3
#
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]

1
#
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 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]
2
#
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.
3
#
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 2: int& r1; // error: initializer missing extern int& r2; // OK — end example]
4
#
Given types “cv1 T1” and “cv2 T2”, “cv1 T1” is reference-related to “cv2 T2” if T1 is similar ([conv.qual]) to T2, or T1 is a base class of T2.
“cv1 T1” is reference-compatible with “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.
5
#
A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:
  • (5.1)
    If the reference is an lvalue reference and the initializer expression
    • (5.1.1)
      is an lvalue (but is not a bit-field), and “cv1 T1” is reference-compatible with “cv2 T2”, or
    • (5.1.2)
      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 T3”81 (this conversion is selected by enumerating the applicable conversion functions ([over.match.ref]) and choosing the best one through overload resolution),
    then the reference binds 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 2: 
    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 3: 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]
  • (5.2)
    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; // error: not an lvalue and reference not const int i = 2; double& rd3 = i; // error: type mismatch and reference not const — end example]
  • (5.3)
    Otherwise, if the initializer expression
    • (5.3.1)
      is an rvalue (but not a bit-field) or an lvalue of function type and “cv1 T1” is reference-compatible with “cv2 T2”, or
    • (5.3.2)
      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 of type “cv3 T3” or an lvalue of function type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3” (see [over.match.ref]),
    then the initializer expression in the first case and the converted expression in the second case is called the converted initializer.
    If the converted initializer is a prvalue, let its type be denoted by T4; the temporary materialization conversion ([conv.rval]) is applied, considering the type of the prvalue to be “cv1 T4” ([conv.qual]).
    In any case, the reference binds 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(); // binds 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; // binds to the A subobject of the result of the conversion int i2 = 42; int&& rri = static_cast<int&&>(i2); // binds directly to i2 B&& rrb = x; // binds directly to the result of operator B constexpr int f() { const int &x = 42; const_cast<int &>(x) = 1; // undefined behavior return x; } constexpr int z = f(); // error: not a constant expression — end example]
  • (5.4)
    Otherwise, T1 shall not be reference-related to T2.
    • (5.4.1)
      If T1 or T2 is a class type, 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.
    • (5.4.2)
      Otherwise, the initializer expression is implicitly converted to a prvalue of type “T1”.
      The temporary materialization conversion is applied, considering the type of the prvalue to be “cv1 T1”, and the reference is bound to the result.
    [Example 6: struct Banana { }; struct Enigma { operator const Banana(); }; struct Alaska { operator Banana&(); }; void enigmatic() { typedef const Banana ConstBanana; Banana &&banana1 = ConstBanana(); // error Banana &&banana2 = Enigma(); // error Banana &&banana3 = Alaska(); // error } const double& rcd2 = 2; // rcd2 refers to temporary with type const double and 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 reference-related type struct X { operator int&(); }; int&& rri2 = X(); // error: result of conversion function is // lvalue of reference-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 referenced type), the reference is said to bind directly to the initializer expression.
6
#
[Note 3: 
[class.temporary] describes the lifetime of temporaries bound to references.
— end note]
81)81)
This requires a conversion function ([class.conv.fct]) returning a reference type.

9.4.5 List-initialization [dcl.init.list]

1
#
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.
Direct-initialization that is not list-initialization is called direct-non-list-initialization.
[Note 1: 
List-initialization can be used
[Example 1: 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]
2
#
A constructor is an initializer-list constructor if its first parameter is of type std​::​initializer_list<E> or reference to cv 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 template std​::​initializer_list is not predefined; if a standard library declaration ([initializer.list.syn], [std.modules]) of std​::​initializer_list is not reachable from ([module.reach]) a use of std​::​initializer_list — even an implicit use in which the type is not named ([dcl.spec.auto]) — the program is ill-formed.
3
#
List-initialization of an object or reference of type cv T is defined as follows:
  • (3.1)
    If the braced-init-list contains a designated-initializer-list and T is not a reference type, 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 2: 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]
  • (3.2)
    If T is an aggregate class and the initializer list has a single element of type cv1 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).
  • (3.3)
    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.
  • (3.4)
    Otherwise, if T is an aggregate, aggregate initialization is performed ([dcl.init.aggr]).
    [Example 3: 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]
  • (3.5)
    Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.
  • (3.6)
    Otherwise, if T is a specialization of std​::​initializer_list, the object is constructed as described below.
  • (3.7)
    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>); // #1 S(std::initializer_list<int>); // #2 S(std::initializer_list<S>); // #3 S(); // #4 // ... }; S s1 = { 1.0, 2.0, 3.0 }; // invoke #1 S s2 = { 1, 2, 3 }; // invoke #2 S s3{s2}; // invoke #3 (not the copy constructor) S s4 = { }; // invoke #4 — end example]
    [Example 5: struct Map { Map(std::initializer_list<std::pair<std::string,int>>); }; Map ship = {{"Sophie",14}, {"Surprise",28}}; — end example]
    [Example 6: 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]
  • (3.8)
    Otherwise, if T is an enumeration with a fixed underlying type ([dcl.enum]) U, the initializer-list has a single element v of scalar type, 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 v to U, the program is ill-formed.
    [Example 7: 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]
  • (3.9)
    Otherwise, if the initializer list is not a designated-initializer-list and 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 8: int x1 {2}; // OK int x2 {2.0}; // error: narrowing — end example]
  • (3.10)
    Otherwise, if T is a reference type, a prvalue is generated.
    The prvalue initializes its result object by copy-list-initialization from the initializer list.
    The prvalue is then used to direct-initialize the reference.
    The type of the prvalue is the type referenced by T, unless T is “reference to array of unknown bound of U”, in which case the type of the prvalue 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>); // #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 struct A { } a; struct B { explicit B(const A&); }; const B& b2{a}; // error: cannot copy-list-initialize B temporary from A struct C { int x; }; C&& c = { .x = 1 }; // OK — end example]
  • (3.11)
    Otherwise, if the initializer list has no elements, the object is value-initialized.
    [Example 10: int** pp {}; // initialized to null pointer — end example]
  • (3.12)
    Otherwise, the program is ill-formed.
    [Example 11: 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]
4
#
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-clause that 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 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]
5
#
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; this is called the initializer list's backing array.
Each element of the backing 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 5: 
A constructor or conversion function selected for the copy needs 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.
[Note 6: 
Backing arrays are potentially non-unique objects ([intro.object]).
— end note]
6
#
The backing array has the same lifetime as any other temporary object ([class.temporary]), 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 12: void f(std::initializer_list<double> il); void g(float x) { f({1, x, 3}); } void h() { f({1, 2, 3}); } struct A { mutable int i; }; void q(std::initializer_list<A>); void r() { q({A{1}, A{2}, A{3}}); }
The initialization will be implemented in a way roughly equivalent to this: void g(float x) { const double __a[3] = {double{1}, double{x}, double{3}}; // backing array f(std::initializer_list<double>(__a, __a+3)); } void h() { static constexpr double __b[3] = {double{1}, double{2}, double{3}}; // backing array f(std::initializer_list<double>(__b, __b+3)); } void r() { const A __c[3] = {A{1}, A{2}, A{3}}; // backing array q(std::initializer_list<A>(__c, __c+3)); } assuming that the implementation can construct an initializer_list object with a pair of pointers, and with the understanding that __b does not outlive the call to f.
— end example]
[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 } {} // 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]
7
#
A narrowing conversion is an implicit conversion
  • (7.1)
    from a floating-point type to an integer type, or
  • (7.2)
    from a floating-point type T to another floating-point type whose floating-point conversion rank is neither greater than nor equal to that of T, except where the result of the conversion is a constant expression and either its value is finite and the conversion did not overflow, or the values before and after the conversion are not finite, or
  • (7.3)
    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
  • (7.4)
    from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where
    • (7.4.1)
      the source is a bit-field whose width w is less than that of its type (or, for an enumeration type, its underlying type) and the target type can represent all the values of a hypothetical extended integer type with width w and with the same signedness as the original type or
    • (7.4.2)
      the source is a constant expression whose value after integral promotions will fit into the target type, or
  • (7.5)
    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; // x is not a constant expression const int y = 999; const int z = 99; char c1 = x; // OK, though it potentially narrows (in this case, it does narrow) char c2{x}; // error: potentially narrows 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: potentially narrows float f2 { 7 }; // OK, 7 can be exactly represented as a float bool b = {"meow"}; // error: narrows 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]