Usage of initlializer_list<>. (in function)

#include <iostream>
#include <initializer_list>
#include <vector>

using namespace std;

int max(initializer_list<int> xyz)
{
    vector<int> a = xyz;

    int const_size = xyz.size();

    int _iterator = 0;

    while(_iterator < const_size - 1)
    {
        if(a.at(_iterator) > a.at(_iterator + 1))
        {
            return a.at(_iterator);
        }
        else
            return a.at(_iterator + 1);
    }
}

int main()
{
    cout << max({10, 20});

    return 0;
}