std::initializer_list

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std::initializer_list
 

(not to be confused with member initializer list)

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

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.

Whether two underlying arrays that are created in list-initialization and have overlapping lifetime are distinct are unspecified, i.e. they may overlap. 

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 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

 creates an empty initializer list
(public member function)
Capacity
returns the number of elements in the initializer list
(public member function)
Iterators
returns a pointer to the first element
(public member function)
returns a pointer to one past the last element
(public member function)

Non-member functions

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

Notes

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

Example

Run this code
#include <initializer_list>
#include <iostream>
#include <vector>
 
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
}

Output: 

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
CWG 1290 C++11 the lifetime of the underlying array referenced by
the initializer_list was not correctly specified 		specified same as other temporary objects
CWG 1418 C++11 the type of the underlying array lacked const const added
LWG 2129 C++11 specializing initializer_list was allowed
but not guaranteed to work 		made ill-formed
P2752R3 C++11 two underlying arrays with overlapping lifetime
must be distinct 		they may overlap

See also

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