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    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 specialization of std::initializer_list, the object is initialized as described below.
    • Otherwise, if T is a class type, 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 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.

    See also