9 Declarations [dcl.dcl]

9.4 Initializers [dcl.init]

9.4.5 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 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]
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 the header <initializer_­list> is not imported or included prior to 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.
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 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]
  • If T is an aggregate class and the initializer list has a single element of type cv U, where U is T or a class derived from T, the object is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization).
  • Otherwise, if T is a character array and the initializer list has a single element that is an appropriately-typed string-literal ([dcl.init.string]), initialization is performed as described in that subclause.
  • Otherwise, if T is an aggregate, aggregate initialization is performed ([dcl.init.aggr]).
    [Example 3: double ad[] = { 1, 2.0 }; // 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 4: 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 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]
  • Otherwise, if T is an enumeration with a fixed underlying type ([dcl.enum]) U, the initializer-list has a single element v, v can be implicitly converted to U, and the initialization is direct-list-initialization, the object is initialized with the value T(v) ([expr.type.conv]); if a narrowing conversion is required to convert 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]
  • 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 8: int x1 {2}; // OK int x2 {2.0}; // error: narrowing — end example]
  • Otherwise, if T is a reference type, a prvalue is generated.
    The prvalue initializes its result object by copy-list-initialization.
    The prvalue is then used to direct-initialize the reference.
    The type of the temporary is the type referenced by T, unless T is “reference to array of unknown bound of U”, in which case the type of the temporary is the type of x in the declaration U x[] H, where H is the initializer list.
    [Note 3:
    As usual, the binding will fail and the program is ill-formed if the reference type is an lvalue reference to a non-const type.
    — end note]
    [Example 9: struct S { S(std::initializer_list<double>); // #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 — end example]
  • Otherwise, if the initializer list has no elements, the object is value-initialized.
    [Example 10: int** pp {}; // initialized to null pointer — end example]
  • 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]
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]
An object of type std​::​initializer_­list<E> is constructed from an initializer list as if the implementation generated and materialized ([conv.rval]) a prvalue of type “array of N const E”, where N is the number of elements in the initializer list.
Each element of that array is copy-initialized with the corresponding element of the initializer list, and the std​::​initializer_­list<E> object is constructed to refer to that array.
[Note 5:
A constructor or conversion function selected for the copy is required to be accessible ([class.access]) in the context of the initializer list.
— end note]
If a narrowing conversion is required to initialize any of the elements, the program is ill-formed.
[Example 12: struct X { X(std::initializer_list<double> v); }; X x{ 1,2,3 };
The initialization will be implemented in a way roughly equivalent to this: const double __a[3] = {double{1}, double{2}, double{3}}; X x(std::initializer_list<double>(__a, __a+3)); assuming that the implementation can construct an initializer_­list object with a pair of pointers.
— end example]
The array has the same lifetime as any other temporary object ([class.temporary]), except that initializing an initializer_­list object from the array extends the lifetime of the array exactly like binding a reference to a temporary.
[Example 13: typedef std::complex<double> cmplx; std::vector<cmplx> v1 = { 1, 2, 3 }; void f() { std::vector<cmplx> v2{ 1, 2, 3 }; std::initializer_list<int> i3 = { 1, 2, 3 }; } struct A { std::initializer_list<int> i4; A() : i4{ 1, 2, 3 } {} // 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 6:
The implementation is free to allocate the array in read-only memory if an explicit array with the same initializer can 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, or
  • from a pointer type or a pointer-to-member type to bool.
[Note 7:
As indicated above, such conversions are not allowed at the top level in list-initializations.
— end note]
[Example 14: int x = 999; // 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]