std::ignore (3) Linux Manual Page

std::ignore – std::ignore

Synopsis

Defined in header <tuple>
const /*unspecified*/ ignore; (since C++11)
                                          (until C++17)
inline constexpr /*unspecified*/ ignore; (since C++17)
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.

Example

unpack a pair returned by set.insert(), but only save the boolean.
// Run this code

#include <iostream>

#include <string>

#include <set>

#include <tuple>
int main()

{
    std::set<std::string> set_of_str;
    bool inserted = false;
    std::tie(std::ignore, inserted) = set_of_str.insert("Test");
    if (inserted) {
        std::cout << "Value was inserted successfully\n";
    }
}

Output:

  Value was inserted successfully

See also

    creates a tuple of lvalue references or unpacks a tuple into individual objects
tie (function template)