C++ named requirements: EqualityComparable

 

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C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

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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	meets BooleanTestable (until C++20)	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++20)

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
LWG 2114 (P2167R3)	C++98	convertibility to bool was too weak to reflect the expectation of implementations	requirements strengthened

See also

equality_comparableequality_comparable_with (C++20)

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