List-initialization (since C++11)

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Initialization
 

Initializes an object from a brace-enclosed initializer list.

Syntax

Direct-list-initialization

T object { arg1, arg2, ... };

T object{.des1 = arg1 , .des2 { arg2 } ... };

(since C++20)
 (1)
T { arg1, arg2, ... }

T {.des1 = arg1 , .des2 { arg2 } ... }

(since C++20)
 (2)
new T { arg1, arg2, ... }

new T {.des1 = arg1 , .des2 { arg2 } ... }

(since C++20)
 (3)
Class { T member { arg1, arg2, ... }; };

Class { T member {.des1 = arg1 , .des2 { arg2 } ... }; };

(since C++20)
 (4)
Class::Class() : member { arg1, arg2, ... } {...

Class::Class() : member {.des1 = arg1 , .des2 { arg2 } ...} {...

(since C++20)
 (5)

Copy-list-initialization

T object = { arg1, arg2, ... };

T object = {.des1 = arg1 , .des2 { arg2 } ... };

(since C++20)
 (6)
function ({ arg1, arg2, ... })

function ({.des1 = arg1 , .des2 { arg2 } ... })

(since C++20)
 (7)
return { arg1, arg2, ... };

return {.des1 = arg1 , .des2 { arg2 } ... };

(since C++20)
 (8)
object [{ arg1, arg2, ... }]

object [{.des1 = arg1 , .des2 { arg2 } ... }]

(since C++20)
 (9)
object = { arg1, arg2, ... }

object = {.des1 = arg1 , .des2 { arg2 } ... }

(since C++20)
 (10)
U ({ arg1, arg2, ... })

U ({.des1 = arg1 , .des2 { arg2 } ... })

(since C++20)
 (11)
Class { T member = { arg1, arg2, ... }; };

Class { T member = {.des1 = arg1 , .des2 { arg2 } ... }; };

(since C++20)
 (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 brace-enclosed initializer list
2) initialization of an unnamed temporary with a brace-enclosed initializer list
3) initialization of an object with dynamic storage duration with a new-expression, where the initializer is a brace-enclosed initializer 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 a brace-enclosed initializer 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 brace-enclosed initializer list after an equals sign
7) in a function call expression, with a brace-enclosed initializer list used as an argument and list-initialization initializes the function parameter
8) in a return statement with a brace-enclosed initializer 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 a brace-enclosed initializer 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 is 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 (possibly cv-qualified) T are:

  • If the brace-enclosed initializer list contains a designated initializer list and T is not a reference type, T must be an aggregate class. The ordered identifiers in the designators of the designated initializer list must form a subsequence of the ordered identifiers in the direct non-static data members of T. Aggregate initialization is performed.
(since C++20)
  • If T is an aggregate class and the brace-enclosed initializer list, which does not contain a designated initializer list,(since C++20) has a single initializer clause of the same or derived type (possibly cv-qualified), the object is initialized from that initializer clause (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization).
  • Otherwise, if T is a character array and the brace-enclosed initializer list has a single initializer clause that is an appropriately-typed string literal, the array is initialized from the string literal as usual.
  • Otherwise, if the brace-enclosed initializer list is empty and T is a class type with a default constructor, value-initialization is performed.
  • Otherwise, if T is a class type, the constructors of T are considered, in two phases:
  • 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 initializer clauses of the brace-enclosed initializer 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, the brace-enclosed initializer list has only one initializer v, and all following conditions are satisfied, then the enumeration is initialized with the result of converting v to U:
    • The initialization is direct-list-initialization.
    • v is of scalar type.
    • v is implicitly convertible to U.
    • The conversion from v to U is non-narrowing.
(since C++17)
  • Otherwise (if T is not a class type), if the brace-enclosed initializer list has only one initializer clause 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 initializer clause, 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 initializer clause:
  • 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 brace-enclosed initializer list has no initializer clause, T is value-initialized.

List-initializing std::initializer_list

An object of type std::initializer_list<E> is constructed from an initializer list as if the compiler generated and materialized(since C++17) a prvalue of type “array of N const E”, where N is the number of initializer clauses 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 initializer clause of the initializer list, and the std::initializer_list<E> object is constructed to refer to that array. A constructor or conversion function selected for the copy is required to be accessible in the context of the initializer list. If a narrowing conversion is required to initialize any of the elements, the program is ill-formed.

The backing array has the same lifetime as any other temporary object, except that initializing an std::initializer_list object from the backing array extends the lifetime of the array exactly like binding a reference to a temporary. 

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 above will be implemented in a way roughly equivalent to below,
// assuming that the compiler can construct an initializer_list object with a pair of
// pointers, and with the understanding that `__b` does not outlive the call to `f`.
 
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));
}

Whether all backing arrays are distinct (that is, are stored in non-overlapping objects) is unspecified: 

bool fun(std::initializer_list<int> il1, std::initializer_list<int> il2)
{
    return il2.begin() == il1.begin() + 1;
}
 
bool overlapping = fun({1, 2, 3}, {2, 3, 4}); // the result is unspecified:
                                              // the back arrays can share
                                              // storage within {1, 2, 3, 4}

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 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 conversion result is a constant expression and one of the following conditions is satisfied:
    • The converted value is finite, and the conversion does not overflow.
    • The values before and after the conversion are not finite.
  • 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
    • 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
    • the 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 brace-enclosed initializer 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 brace-enclosed initializer 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 brace-enclosed initializer 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 brace-enclosed initializer list as std::initializer_list in copy-list-initialization.

Also because a brace-enclosed initializer 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 brace-enclosed initializer list of a single initializer clause of the same type, but non-aggregates consider std::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.

Feature-test macro Value Std Feature
__cpp_initializer_lists 200806L (C++11) List-initialization and std::initializer_list

Example

Run this code
#include <iostream>
#include <map>
#include <string>
#include <vector>
 
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 an lvalue reference to a temporary array
    int&& r1 = {1}; // binds an 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 << ' ';
    std::cout << '\n';
 
    [](...){}(d, ar, r1, uc1); // has effect of [[maybe_unused]]
}

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 brace-enclosed initializer list of a
single initializer clause always bound the reference to a temporary 		bind to that initializer
clause if valid
CWG 1290 C++11 the lifetime of the backing array was not correctly specified specified same as other
temporary objects
CWG 1324 C++11 initialization considered first for initialization from {} aggregate initialization
considered first
CWG 1418 C++11 the type of the backing array lacked const const added
CWG 1467 C++11 same-type initialization of aggregates and character
arrays was prohibited; initializer-list constructors had
priority over copy constructors for single-clause lists 		same-type initialization
allowed; single-clause
lists initialize directly
CWG 1494 C++11 when list-initializing a reference with an initializer clause 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 2252 C++17 enumerations could be list-initialized from non-scalar values prohibited
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
CWG 2627 C++11 a narrow bit-field of a larger integer type can be promoted to
a smaller integer type, but it was still a narrowing conversion 		it is not a
narrowing conversion
CWG 2713 C++20 references to aggregate classes could not
be initialized by designated initializer lists 		allowed
CWG 2830 C++11 list-initialization did not ignore the top-level cv-qualification ignores
CWG 2864 C++11 floating-point conversions that overflow were not narrowing they are narrowing
P1957R2 C++11 conversion from a pointer/pointer-to-member
to bool was not narrowing 		considered narrowing
P2752R3 C++11 backing arrays with overlapping lifetime could not overlap they may overlap

See also

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