List-initialization (since C++11)

 

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Initialization

Initializer

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Direct initialization

Copy initialization

List initialization (C++11)

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Initializes an object from braced-init-list.

Syntax

Direct-list-initialization

T object { arg1, arg2, ... };	(1)
T { arg1, arg2, ... }	(2)
new T { arg1, arg2, ... }	(3)
Class { T member { arg1, arg2, ... }; };	(4)
Class::Class() : member { arg1, arg2, ... } {...	(5)

Copy-list-initialization

T object = { arg1, arg2, ... };	(6)
function ({ arg1, arg2, ... })	(7)
return { arg1, arg2, ... };	(8)
object [{ arg1, arg2, ... }]	(9)
object = { arg1, arg2, ... }	(10)
U ({ arg1, arg2, ... })	(11)
Class { T member = { arg1, arg2, ... }; };	(12)

List initialization is performed in the following situations:

• direct-list-initialization (both explicit and non-explicit constructors are considered)

1) initialization of a named variable with a braced-init-list (that is, a possibly empty brace-enclosed list of expressions or nested braced-init-lists)

2) initialization of an unnamed temporary with a braced-init-list

3) initialization of an object with dynamic storage duration with a new-expression, where the initializer is a braced-init-list

4) in a non-static data member initializer that does not use the equals sign

5) in a member initializer list of a constructor if braced-init-list is used

• copy-list-initialization (both explicit and non-explicit constructors are considered, but only non-explicit constructors may be called)

6) initialization of a named variable with a braced-init-list after an equals sign

7) in a function call expression, with braced-init-list used as an argument and list-initialization initializes the function parameter

8) in a return statement with braced-init-list used as the return expression and list-initialization initializes the returned object

9) in a subscript expression with a user-defined operator[], where list-initialization initializes the parameter of the overloaded operator

10) in an assignment expression, where list-initialization initializes the parameter of the overloaded operator

11) functional cast expression or other constructor invocations, where braced-init-list is used in place of a constructor argument. Copy-list-initialization initializes the constructor's parameter (note; the type U in this example is not the type that's being list-initialized; U's constructor's parameter is)

12) in a non-static data member initializer that uses the equals sign

Explanation

The effects of list-initialization of an object of type T are:

• If T is an aggregate class and the braced-init-list has a single element of the same or derived type (possibly cv-qualified), 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 braced-init-list has a single element that is an appropriately-typed string literal, the array is initialized from the string literal as usual.

• Otherwise, if T is an aggregate type, aggregate initialization is performed.

• Otherwise, if the braced-init-list is empty and T is a class type with a default constructor, value-initialization is performed.

• Otherwise, if T is a specialization of std::initializer_list, the T object is direct-initialized or copy-initialized, depending on context, from a prvalue of the same type initialized from (until C++17) the braced-init-list.

• Otherwise, the constructors of T are considered, in two phases:

• All constructors that take std::initializer_list as the only argument, or as the first argument if the remaining arguments have default values, are examined, and matched by overload resolution against a single argument of type std::initializer_list

• If the previous stage does not produce a match, all constructors of T participate in overload resolution against the set of arguments that consists of the elements of the braced-init-list, with the restriction that only non-narrowing conversions are allowed. If this stage produces an explicit constructor as the best match for a copy-list-initialization, compilation fails (note, in simple copy-initialization, explicit constructors are not considered at all).

Otherwise, if： T is an enumeration type that with fixed underlying type U (which is int if the enumeration is scoped and its underlying type is not manually specified), and the initialization is direct-list-initialization, and the braced-init-list has only one initializer v that is implicitly convertible to U, and the conversion from v to U is non-narrowing, then the enumeration is initialized with the result of converting v to U.

(since C++17)

• Otherwise (if T is not a class type), if the braced-init-list has only one element and either T is not a reference type or is a reference type whose referenced type is same as or is a base class of the type of the element, T is direct-initialized (in direct-list-initialization) or copy-initialized (in copy-list-initialization), except that narrowing conversions are not allowed.

• Otherwise, if T is a reference type that is not compatible with the type of the element:

a prvalue temporary of the type referenced by T is copy-list-initialized, and the reference is bound to that temporary (this fails if the reference is a non-const lvalue reference).

(until C++17)

a prvalue is generated. The prvalue initializes its result object by copy-list-initialization. The prvalue is then used to direct-initialize the reference (this fails if the reference is a non-const lvalue 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 (since C++20).

(since C++17)

• Otherwise, if the braced-init-list has no elements, T is value-initialized.

Narrowing conversions

List-initialization limits the allowed implicit conversions by prohibiting the following:

• conversion from a floating-point type to an integer type

• conversion from a long double to double or to float and conversion from double to float, except where the source is a constant expression and overflow does not occur

• conversion from an integer type to a floating-point type, except where the source is a constant expression whose value can be stored exactly in the target type

• conversion from integer or unscoped enumeration type to integer type that cannot represent all values of the original, except where source is a constant expression whose value can be stored exactly in the target type

• conversion from a pointer type or pointer-to-member type to bool

Notes

Every initializer clause is sequenced before any initializer clause that follows it in the braced-init-list. This is in contrast with the arguments of a function call expression, which are unsequenced (until C++17)indeterminately sequenced (since C++17).

A braced-init-list is not an expression and therefore has no type, e.g. decltype({1,2}) is ill-formed. Having no type implies that template type deduction cannot deduce a type that matches a braced-init-list, so given the declaration template<class T> void f(T); the expression f({1,2,3}) is ill-formed. However, the template parameter can otherwise be deduced, as is the case for std::vector<int> v(std::istream_iterator<int>(std::cin), {}), where the iterator type is deduced by the first argument but also used in the second parameter position. A special exception is made for type deduction using the keyword auto , which deduces any braced-init-list as std::initializer_list in copy-list-initialization.

Also because a braced-init-list has no type, special rules for overload resolution apply when it is used as an argument to an overloaded function call.

Aggregates copy/move initialize directly from single-element braced-init-list of the same type, but non-aggregates consider initializer_list constructors first:

struct X {}; // aggregate
 
struct Q     // non-aggregate
{
    Q() = default;
    Q(Q const&) = default;
    Q(std::initializer_list<Q>) {}
};
 
int main()
{
    X x;
    X x2 = X{x}; // copy-constructor (not aggregate initialization)
 
    Q q;
    Q q2 = Q{q}; // initializer-list constructor (not copy constructor)
}

Some compilers (e.g., gcc 10) only consider conversion from a pointer or a pointer-to-member to bool narrowing in C++20 mode.

Example

Run this code

#include <iostream>
#include <vector>
#include <map>
#include <string>
 
struct Foo
{
    std::vector<int> mem = {1, 2, 3}; // list-initialization of a non-static member
    std::vector<int> mem2;
 
    Foo() : mem2{-1, -2, -3} {} // list-initialization of a member in constructor
};
 
std::pair<std::string, std::string> f(std::pair<std::string, std::string> p)
{
    return {p.second, p.first}; // list-initialization in return statement
}
 
int main()
{
    int n0{};  // value-initialization (to zero)
    int n1{1}; // direct-list-initialization
 
    std::string s1{'a', 'b', 'c', 'd'}; // initializer-list constructor call
    std::string s2{s1, 2, 2};           // regular constructor call
    std::string s3{0x61, 'a'}; // initializer-list ctor is preferred to (int, char)
 
    int n2 = {1}; // copy-list-initialization
    double d = double{1.2}; // list-initialization of a prvalue, then copy-init
    auto s4 = std::string{"HelloWorld"}; // same as above, no temporary created since C++17
 
    std::map<int, std::string> m = // nested list-initialization
    {
        {1, "a"},
        {2, {'a', 'b', 'c'}},
        {3, s1}
    };
 
    std::cout << f({"hello", "world"}).first // list-initialization in function call
              << '\n';
 
    const int (&ar)[2] = {1, 2}; // binds a lvalue reference to a temporary array
    int&& r1 = {1}; // binds a rvalue reference to a temporary int
//  int& r2 = {2}; // error: cannot bind rvalue to a non-const lvalue ref
 
//  int bad{1.0}; // error: narrowing conversion
    unsigned char uc1{10}; // okay
//  unsigned char uc2{-1}; // error: narrowing conversion
 
    Foo f;
 
    std::cout << n0 << ' ' << n1 << ' ' << n2 << '\n'
              << s1 << ' ' << s2 << ' ' << s3 << '\n';
    for(auto p: m)
        std::cout << p.first << ' ' << p.second << '\n';
    for(auto n: f.mem)
        std::cout << n << ' ';
    for(auto n: f.mem2)
        std::cout << n << ' ';
}

Output:

world
0 1 1
abcd cd aa
1 a
2 abc
3 abcd
1 2 3 -1 -2 -3

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 1288	C++11	list-initializing a reference with a single-element braced-init-list always bound the reference to a temporary	bind to that element if valid
CWG 1324	C++11	initialization considered first for initialization from {}	aggregate initialization considered first
CWG 1467	C++11	same-type initialization of aggregates and character arrays was prohibited; initializer-list constructors had priority over copy constructors for single-element lists	same-type initialization allowed; single-element lists initialize directly
CWG 1494	C++11	when list-initializing a reference with an element of an incompatible type, it was unspecified whether the temporary created is direct-list-initialized or copy-list-initialized	it depends on the kind of initialization for the reference
CWG 2137	C++11	initializer-list constructors lost to copy constructors when list-initializing X from {X}	non-aggregates consider initializer-lists first
CWG 2267	C++11	the resolution of CWG issue 1494 made clear that temporaries could be direct-list-initialized	they are copy-list-initialized when list-initializing references
CWG 2374	C++17	direct-list-initialization of an enum allowed too many source types	restricted
P1957R2	C++11	conversion from a pointer/pointer-to -member to bool was not narrowing	made narrowing

See also

• constructor
• converting constructor
• copy assignment
• copy constructor
• copy elision
• default constructor
• explicit
• initialization
  • aggregate initialization
  • constant initialization
  • copy initialization
  • direct initialization
  • reference initialization
  • value initialization
  • zero initialization
• move assignment
• move constructor
• new