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.

While the behavior of std::ignore outside of std::tie is not formally specified, some code guides recommend using std::ignore to avoid warnings from unused return values of [[nodiscard]] functions.

Possible implementation

namespace detail { struct ignore_t { template <typename T> constexpr // required since C++14 void operator=(T&&) const noexcept {} }; } inline constexpr detail::ignore_t ignore; // 'const' only until C++17

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.

Run this code

#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;
    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

See also

tie (C++11)

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