C++ - std::tie and std::ignore

c++11 defines utility tie to return tuple that can be used in lvalue.

c++ reference has have one line definition

Creates a tuple of lvalue references to its arguments or instances of std::ignore.

in plain English, this works like unpack tuple in python or destructing in javascript. So, in the following example, func return tuple of two elements and using tie and unpacking it in x and y variables

#include <iostream>
#include <utility>
#include <string>
#include <tuple> // for tuple
using namespace std;

std::tuple<int, int> func()
{
    return make_tuple(1, 2);
}
int main()
{
    int x, y;
    std::tie(x, y) = func();

    cout << x << endl
         << y << endl;
}

so, what does std::ignore do here?

std::ignore is defined in cpp reference

An object of unspecified type such that any value can be assigned to it with no effect. Intended for use with std::tie when unpacking a std::tuple, as a placeholder for the arguments that are not used.

which means that std::ignore can be used to ignore the element in the returned tuple. in this example, i care only about the second element in tuple so, i have to use std::ignore in the first element.

#include <iostream>
#include <utility>
#include <string>
#include <tuple> // for tuple
using namespace std;

std::tuple<int, int> func()
{
    return make_tuple(1, 2);
}
int main()
{
    int x, y;
    std::tie(std::ignore, y) = func();

    cout << x << endl
         << y << endl;

}