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std::initializer_list(3) C++ Standard Libary std::initializer_list(3)


std::initializer_list - std::initializer_list


Defined in header <initializer_list>
template< class T > (since C++11)
class initializer_list;

An object of type std::initializer_list<T> is a lightweight proxy object that
provides access to an array of objects of type const T.

A std::initializer_list object is automatically constructed when:

* a braced-init-list is used to list-initialize an object, where the corresponding
constructor accepts an std::initializer_list parameter
* a braced-init-list is used as the right operand of assignment or as a function
call argument, and the corresponding assignment operator/function accepts an
std::initializer_list parameter
* a braced-init-list is bound to auto, including in a ranged for loop

Initializer lists may be implemented as a pair of pointers or pointer and length.
Copying a std::initializer_list does not copy the underlying objects.

The underlying array is a temporary array of type const T[N], in which each element
is copy-initialized (except that narrowing conversions are invalid) from the
corresponding element of the original initializer list. The lifetime of the
underlying array is the same as any other temporary object, except that initializing
an initializer_list object from the array extends the lifetime of the array exactly
like binding a reference to a temporary (with the same exceptions, such as for
initializing a non-static class member). The underlying array may be allocated in
read-only memory.

The program is ill-formed if an explicit or partial specialization of
std::initializer_list is declared.

Member types

Member type Definition
value_type T
reference const T&
const_reference const T&
size_type std::size_t
iterator const T*
const_iterator const T*

Member functions

constructor creates an empty initializer list
(public member function)


size returns the number of elements in the initializer list
(public member function)


begin returns a pointer to the first element
(public member function)
end returns a pointer to one past the last element
(public member function)

Non-member functions

std::begin(std::initializer_list) overloads std::begin
(C++11) (function template)
std::end(std::initializer_list) specializes std::end
(C++11) (function template)
Free function templates overloaded for std::initializer_list
rbegin returns a reverse iterator to the beginning of a
crbegin container or array
(C++14) (function template)
rend returns a reverse end iterator for a container or
crend array
(C++14) (function template)
empty checks whether the container is empty
(C++17) (function template)
data obtains the pointer to the underlying array
(C++17) (function template)


// Run this code

#include <iostream>
#include <vector>
#include <initializer_list>

template <class T>
struct S {
std::vector<T> v;
S(std::initializer_list<T> l) : v(l) {
std::cout << "constructed with a " << l.size() << "-element list\n";
void append(std::initializer_list<T> l) {
v.insert(v.end(), l.begin(), l.end());
std::pair<const T*, std::size_t> c_arr() const {
return {&v[0], v.size()}; // copy list-initialization in return statement
// this is NOT a use of std::initializer_list

template <typename T>
void templated_fn(T) {}

int main()
S<int> s = {1, 2, 3, 4, 5}; // copy list-initialization
s.append({6, 7, 8}); // list-initialization in function call

std::cout << "The vector size is now " << s.c_arr().second << " ints:\n";

for (auto n : s.v)
std::cout << n << ' ';
std::cout << '\n';

std::cout << "Range-for over brace-init-list: \n";

for (int x : {-1, -2, -3}) // the rule for auto makes this ranged-for work
std::cout << x << ' ';
std::cout << '\n';

auto al = {10, 11, 12}; // special rule for auto

std::cout << "The list bound to auto has size() = " << al.size() << '\n';

// templated_fn({1, 2, 3}); // compiler error! "{1, 2, 3}" is not an expression,
// it has no type, and so T cannot be deduced
templated_fn<std::initializer_list<int>>({1, 2, 3}); // OK
templated_fn<std::vector<int>>({1, 2, 3}); // also OK


constructed with a 5-element list
The vector size is now 8 ints:
1 2 3 4 5 6 7 8
Range-for over brace-init-list:
-1 -2 -3
The list bound to auto has size() = 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
the lifetime of the underlying array
CWG 1290 C++11 referenced by specified same as other
the initializer_list was not correctly temporary objects
CWG 1418 C++11 the type of the underlying array lacked const added
specializing initializer_list was
LWG 2129 C++11 allowed made ill-formed
but not guaranteed to work

See also

span a non-owning view over a contiguous sequence of objects
(C++20) (class template)
basic_string_view read-only string view
(C++17) (class template)