std::ignore

 

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std::tuple

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tuple::tuple

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make_tuple

tie

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operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

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get(std::tuple)

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tuple-like (C++23)

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tuple_size<std::tuple>

tuple_element<std::tuple>

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basic_common_reference<std::tuple> (C++23)

common_type<std::tuple> (C++23)

formatter<std::tuple> (C++23)

ignore

Deduction guides(C++17)

 

Defined in header <tuple>
Defined in header <utility>
	(1)
const /*ignore-type*/ ignore;			(since C++11) (until C++14)
constexpr /*ignore-type*/ ignore;			(since C++14) (inline since c++17)
		(2)
struct /*ignore-type*/ {     template< class T >     const /*ignore-type*/& operator=( const T& ) const noexcept     {         return *this;     } };				(since C++11) (until C++14) (exposition only*)
struct /*ignore-type*/ {     template< class T >     constexpr const /*ignore-type*/& operator=( const T& ) const noexcept     {         return *this;     } };				(since C++14) (exposition only*)

1) An object such that any value can be assigned to it with no effect.

2) The type of std::ignore.

Notes

A void expression or a volatile bit-field value cannot be assigned to std::ignore.

std::ignore is intended for use with std::tie when unpacking a std::tuple, as a placeholder for the arguments that are not used, but can be used for any unwanted assignment.

Some code guides recommend using std::ignore to avoid warnings from unused return values of [[nodiscard]] functions, even though an assignment isn't required.

For ignoring values not requiring assignment, one may cast to void. For variables that have names, but whose value is unused, one may cast those to void or declare those variables with [[maybe_unused]].

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

Output:

Value was inserted successfully.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2773	C++14	std::tuple was made constexpr but std::ignore was not yet	made constexpr
P2968R2	C++11	the behavior of std::ignore outside of std::tie was not formally specified	made fully specified

See also

tie (C++11)

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