NAME¶

std::ignore - std::ignore

Synopsis¶

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 creates a tuple of lvalue references or unpacks a tuple into individual
(C++11) objects
(function template)