std::initializer_list

std::initializer_list is a class type which is commonly used with constructor to pass initializer list as argument. For example:

#include <iostream>
 
class IntArray
{
    int *m_arr{};
    int m_length{};
 
public:
    IntArray() = default;
    IntArray(int length)
        : m_length{length}
    {
        m_arr = new int[static_cast<std::size_t>(length)];
    }
 
    IntArray(std::initializer_list<int> list)
        : IntArray(static_cast<int>(list.size()))
    {
        std::copy(list.begin(), list.end(), m_arr);
    }
 
    int *begin() const
    {
        return m_arr;
    }
 
    int *end() const
    {
        return m_arr + m_length;
    }
 
    friend std::ostream &operator<<(std::ostream &out, const IntArray &arr)
    {
        out << '[';
        for (const int &value : arr)
        {
            out << value << ',';
        }
        out << ']';
 
        return out;
    }
};
 
int main(int argc, char const *argv[])
{
    std::initializer_list<int> a{1, 2, 3, 4};
 
    std::cout << *a.begin() << '\n';
 
    IntArray arr{1, 2, 3, 4, 5};
    std::cout << arr << '\n';
    return 0;
}

This enables us to use our custom IntArray with initializer list. When compiler encounters initializer list, it converts the list to std::initializer_list object and we already defined a constructor with parameter matching the datatype, the constructor gets used to create the array.

References

1. https://www.learncpp.com/cpp-tutorial/stdinitializer_list/