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std::ignore

From cppreference.com
< cpp‎ | utility‎ | tuple
 
 
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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.

[edit] Example

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

#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

[edit] See also

creates a tuple of lvalue references or unpacks a tuple into individual objects
(function template) [edit]