[edit]Notes

Avoid expression or a volatile bit-field value cannot be assigned tostd::ignore.

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

Some code guides recommend usingstd::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 tovoid. For variables that have names, but whose value is unused, one may cast those tovoid or declare those variables with[[maybe_unused]].

[edit]Example

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

#include <iostream>#include <set>#include <string>#include <tuple> [[nodiscard]]int dontIgnoreMe(){return42;} 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.

[edit]Defect reports

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

[edit]See also

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