std::initializer_list<T>::initializer_list

std::initializer_list<T>::initializer_list - cppreference.com

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`initializer_list() noexcept;`		(since C++11) (constexpr since C++14)

Constructs an empty initializer list.

Parameters

(none)

Complexity

Constant

Notes

Despite a lack of constructors, it is possible to create non-empty initializer lists. Instances of std::initializer_list are implicitly constructed when:

a braced-init-list is used in list-initialization, including function-call list initialization and assignment expressions (not to be confused with constructor initializer lists)
a braced-init-list is bound to auto, including in a ranged for loop

Example

#include <initializer_list>
#include <iostream>

int main()
{
    std::initializer_list<int> empty_list;
    std::cout << "empty_list.size(): " << empty_list.size() << '\n';

    // create initializer lists using list-initialization
    std::initializer_list<int> digits{1, 2, 3, 4, 5};
    std::cout << "digits.size(): " << digits.size() << '\n';

    // special rule for auto means 'fractions' has the
    // type std::initializer_list<double>
    auto fractions = {3.14159, 2.71828};
    std::cout << "fractions.size(): " << fractions.size() << '\n';

    // create constexpr initializer list (since C++14)
    static constexpr auto ab = {'a', 'b'};
    static_assert(ab.size() == 2 and *ab.begin() == 'a');
}

Output:

empty_list.size(): 0
digits.size(): 5
fractions.size(): 2