9 Declarations [dcl]

9.5 Initializers [dcl.init]

9.5.5 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 };