List-initialization (since C++11)

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

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

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

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

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

List initialization is performed in the following situations:

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

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

[edit] Explanation

The effects of list-initialization of an object of type (possibly cv-qualified) T are:

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

• 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 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:

• Otherwise, if the brace-enclosed initializer list has no initializer clause, T is value-initialized.

[edit] 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}

[edit] 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

[edit] 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.

[edit] Example

#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]]
}

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

[edit] Defect reports

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

[edit] 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