std::initializer_list

 

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std::initializer_list

Member functions

initializer_list::initializer_list

Capacity

initializer_list::size

Iterators

initializer_list::begin

initializer_list::end

Non-member functions

begin(std::initializer_list)

end(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 (that may be allocated in read-only memory).

A std::initializer_list object is automatically constructed when:

• a brace-enclosed initializer list is used to list-initialize an object, where the corresponding constructor accepts an std::initializer_list parameter,
• a brace-enclosed initializer 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 brace-enclosed initializer list is bound to auto, including in a ranged for loop.

std::initializer_list may be implemented as a pair of pointers or pointer and length. Copying a std::initializer_list does not copy the backing array of the corresponding initializer list.

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

Member types

Name	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)
Capacity
size	returns the number of elements in the initializer list (public member function)
Iterators
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) (C++11)	overloads std::begin (function template)
std::end(std::initializer_list) (C++11)	specializes std::end (function template)
Free function templates overloaded for std::initializer_list
rbegincrbegin (C++14)	returns a reverse iterator to the beginning of a container or array (function template)
rendcrend (C++14)	returns a reverse end iterator for a container or array (function template)
empty (C++17)	checks whether the container is empty (function template)
data (C++17)	obtains the pointer to the underlying array (function template)

Notes

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

Example

Run this code

#include <cassert>
#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 now has " << 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';
    auto la = al; // a shallow-copy of top-level proxy object
    assert(la.begin() == al.begin()); // guaranteed: backing array is the same
 
    std::initializer_list<int> il{-3, -2, -1};
    assert(il.begin()[2] == -1); // note the replacement for absent operator[]
    il = al; // shallow-copy
    assert(il.begin() == al.begin()); // guaranteed
 
//  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 now has 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
LWG 2129	C++11	std::initializer_list could have explicit specializations or partial specializations	the program is ill-formed in this case

See also

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