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