C++ named requirements: EqualityComparable

 

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C++ named requirements

Basic
DefaultConstructible CopyAssignable	CopyConstructible MoveAssignable (C++11)	MoveConstructible (C++11) Destructible
Type properties
ScalarType TriviallyCopyable (C++11)	StandardLayoutType (C++11) PODType	TrivialType (C++11) ImplicitLifetimeType
Library-Wide
EqualityComparable LessThanComparable Swappable ValueSwappable (C++11)	NullablePointer (C++11) Hash (C++11) Allocator FunctionObject	Callable Predicate BinaryPredicate Compare
Container
Container SequenceContainer ContiguousContainer (C++17)	ReversibleContainer AssociativeContainer AllocatorAwareContainer	UnorderedAssociativeContainer (C++11)
Container Elements
DefaultInsertable (C++11) CopyInsertable (C++11)	MoveInsertable (C++11) EmplaceConstructible (C++11)	Erasable (C++11)
Iterator
LegacyIterator LegacyOutputIterator LegacyInputIterator	LegacyForwardIterator LegacyBidirectionalIterator LegacyRandomAccessIterator	LegacyContiguousIterator (C++17) ConstexprIterator (C++20)
Stream I/O
FormattedInputFunction UnformattedInputFunction	FormattedOutputFunction	UnformattedOutputFunction
Formatters
BasicFormatter (C++20)	Formatter (C++20)
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SeedSequence (C++11) RandomNumberEngine (C++11)	RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11)	RandomNumberEngineAdaptor (C++11)
Concurrency
BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11)	SharedLockable (C++14) SharedTimedLockable (C++14) Mutex (C++11)	TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14)
Ranges
RangeAdaptorObject (C++20)	RangeAdaptorClosureObject (C++20)
Multidimensional View Customization
LayoutMapping (C++23)	LayoutMappingPolicy (C++23)	AccessorPolicy (C++23)
Other
CharTraits RegexTraits (C++11) BitmaskType LiteralType (C++11)	NumericType UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11)	Clock (C++11) TrivialClock (C++11)

 

The type must work with == operator and the result should have standard semantics.

Requirements

The type T satisfies EqualityComparable if given expressions a, b and c of type T or (since C++11)const T, the following expression is valid and has its specified effects:

Expression	Type	Effects
a == b	implicitly convertible to bool  (until C++23)	Establishes an equivalence relation, that is, it satisfies the following properties: For all values of a, a == a yields true. If a == b, then b == a. If a == b and b == c, then a == c.
	models boolean-testable (since C++23)

Notes

To satisfy this requirement, types that do not have built-in comparison operators have to provide a user-defined operator==.

For the types that are both EqualityComparable and LessThanComparable, the C++ standard library makes a distinction between

• Equality, which is the value of the expression a == b and
• Equivalence, which is the value of the expression !(a < b) && !(b < a).

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 283	C++98	even if T is EqualityComparable, the requirements did not apply to const T objects	they apply to const T instead of T

See also

equality_comparableequality_comparable_with (C++20)

specifies that operator == is an equivalence relation (concept)