std::ignore

Defined in header `<tuple>`
const /unspecified/ ignore;		(since C++11)

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.

#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 sucessfully\n";
    }
}

Output:

Value was inserted sucessfully

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

Links:

http://en.cppreference.com/w/cpp/utility/tuple/ignore

doc_CPP

2016-10-11 10:03:16

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