std:: ignore

Notes

A void expression or a volatile bit-field value cannot be assigned to std::ignore .

std::ignore is intended for use with std::tie when unpacking a std::tuple , as a placeholder for the arguments that are not used, but can be used for any unwanted assignment.

Some code guides recommend using std::ignore to avoid warnings from unused return values of [[ nodiscard ]] functions, even though an assignment isn't required.

For ignoring values not requiring assignment, one may cast to void. For variables that have names, but whose value is unused, one may cast those to void or declare those variables with [[ maybe_unused ]] .

Example

1. Demonstrates the use of std::ignore together with a [[ nodiscard ]] function.
2. Unpacks a std:: pair < iterator, bool > returned by std:: set :: insert ( ) , but only saves the boolean.

#include <iostream>
#include <set>
#include <string>
#include <tuple>
 
[[nodiscard]] int dontIgnoreMe()
{
    return 42;
}
 
int main()
{
    std::ignore = dontIgnoreMe();
 
    std::set<std::string> set_of_str;
    if (bool inserted{false};
        std::tie(std::ignore, inserted) = set_of_str.insert("Test"),
        inserted)
        std::cout << "Value was inserted successfully.\n";
}

Value was inserted successfully.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also