std::ignore

 

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

std::tuple

Member functions

tuple::tuple

tuple::operator=

tuple::swap

Non-member functions

make_tuple

tie

forward_as_tuple

tuple_cat

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

swap(std::tuple)

get(std::tuple)

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tuple-like (C++23)

Helper classes

tuple_size<std::tuple>

tuple_element<std::tuple>

uses_allocator<std::tuple>

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 was initially intended for use with std::tie when unpacking a std::tuple, as a placeholder for the arguments that are not used.

Some code guides recommend using std::ignore to avoid warnings from unused return values of [[nodiscard]] functions.

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)