operator==, <=>(std::reference_wrapper)

 

C++

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Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

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)

 

Function objects

Function wrappers function (C++11) move_only_function (C++23) copyable_function (C++26) function_ref (C++26) mem_fn (C++11) bad_function_call (C++11)	Partial function application bind_frontbind_back (C++20)(C++23) bind (C++11) is_bind_expression (C++11) is_placeholder (C++11) _1, _2, _3, ... (C++11)
Function invocation invokeinvoke_r (C++17)(C++23) Identity function object identity (C++20)	Reference wrappers reference_wrapper (C++11) refcref (C++11)(C++11) unwrap_referenceunwrap_ref_decay (C++20)(C++20)
Operator wrappers plus minus negate multiplies divides modulus bit_and bit_or bit_not (C++14)   bit_xor equal_to not_equal_to greater less greater_equal less_equal logical_and logical_or logical_not	Transparent operator wrappers plus<> (C++14) minus<> (C++14) negate<> (C++14) multiplies<> (C++14)   divides<> (C++14) modulus<> (C++14) bit_and<> (C++14) bit_or<> (C++14) bit_not<> (C++14) bit_xor<> (C++14) equal_to<> (C++14) not_equal_to<> (C++14) greater<> (C++14) less<> (C++14) greater_equal<> (C++14) less_equal<> (C++14) logical_and<> (C++14) logical_or<> (C++14) logical_not<> (C++14)
Negators not_fn (C++17) Searchers default_searcher (C++17) boyer_moore_searcher (C++17) boyer_moore_horspool_searcher (C++17)	Constrained comparators ranges::equal_to (C++20) ranges::not_equal_to (C++20) ranges::greater (C++20) ranges::less (C++20) ranges::greater_equal (C++20) ranges::less_equal (C++20) compare_three_way (C++20)
Old binders and adaptors
unary_function (until C++17*) binary_function (until C++17*) ptr_fun (until C++17*) pointer_to_unary_function (until C++17*) pointer_to_binary_function (until C++17*)   mem_fun (until C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (until C++20*) not2 (until C++20*)	binder1stbinder2nd (until C++17*)(until C++17*) bind1stbind2nd (until C++17*)(until C++17*) mem_fun_ref (until C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (until C++20*) binary_negate (until C++20*)

 

std::reference_wrapper

Member functions

reference_wrapper::reference_wrapper

reference_wrapper::operator=

reference_wrapper::getreference_wrapper::operator T&

reference_wrapper::operator()

Non-member functions

operator==operator<=> (C++26)(C++26)

Deduction guides (C++17)

Helper classes

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

 

friend constexpr bool     operator==( reference_wrapper lhs, reference_wrapper rhs );	(1)	(since C++26)
friend constexpr bool     operator==( reference_wrapper lhs, reference_wrapper<const T> rhs );	(2)	(since C++26)
friend constexpr bool     operator==( reference_wrapper lhs, const T& ref );	(3)	(since C++26)
friend constexpr auto     operator<=>( reference_wrapper lhs, reference_wrapper rhs );	(4)	(since C++26)
friend constexpr auto     operator<=>( reference_wrapper lhs, reference_wrapper<const T> rhs );	(5)	(since C++26)
friend constexpr auto     operator<=>( reference_wrapper lhs, const T& ref );	(6)	(since C++26)

Performs comparison operations on reference_wrapper objects.

1,2) Compares two reference_wrapper objects. The objects compare equal if and only if lhs.get() and rhs.get() are equal.

• Both overloads participate in overload resolution only if lhs.get() == rhs.get() is well-formed and its result is convertible to bool.
• The overload (2) participates in overload resolution only if std::is_const_v<T> is false.

3) Compares reference_wrapper object with a reference. The parameters compare equal if and only if lhs.get() is equal to ref.

• The overload (3) participates in overload resolution only if lhs.get() == ref is well-formed and its result is convertible to bool.

4) Compares two reference_wrapper objects as if by synth-three-way(lhs.get(), rhs.get()). The return type is synth-three-way-result<T>. This overload participates in overload resolution only if the return type is well-formed.

5) Compares two reference_wrapper objects as if by synth-three-way(lhs.get(), rhs.get()). The return type is synth-three-way-result<T>. This overload participates in overload resolution only if std::is_const_v<T> is false and the return type is well-formed.

6) Compares reference_wrapper object with a reference as if by synth-three-way(lhs.get(), ref). The return type is synth-three-way-result<T>. This overload participates in overload resolution only if the return type is well-formed.

The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively.

Parameters

lhs, rhs	-	reference_wrapper object to compare
ref	-	reference to compare to the reference_wrapper object

Return value

1,2) lhs.get() == rhs.get().

3) lhs.get() == ref.

4,5) synth-three-way(lhs.get(), rhs.get()).

6) synth-three-way(lhs.get(), ref).

Exceptions

Throws when and what the comparison throws.

Notes

The return types of (4-6) are deduced from return statements to avoid hard error when instantiating a std::reference_wrapper<T> with synth-three-way-result<T> being ill-formed.

Feature-test macro	Value	Std	Feature
__cpp_lib_reference_wrapper	202403L	(C++26)	Comparisons for std::reference_wrapper