# std::initializer_list

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(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 not guaranteed to exist after the lifetime of the original initializer list object has ended. The storage for `std::initializer_list` is unspecified (i.e. it could be automatic, temporary, or static read-only memory, depending on the situation). | (until C++14)

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.

(since C++14)

The program is ill-formed if an explicit or partial specialization of `std::initializer_list` is declared. | (since C++17)

### 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)

##### 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) | specializes std::begin (function template)
std::end(std::initializer_list) (C++11) | specializes std::end (function template)
Defined in header `<iterator>`
rbegin(std::initializer_list) (C++14) | specializes std::rbegin (function)
rend(std::initializer_list) (C++14) | specializes std::rend (function)

### Example

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
    }

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

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