std::ignore

 

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ignore

 

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

Output:

Value was inserted successfully

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