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Showing content from https://en.cppreference.com/w/cpp/language/../string/../named_req/EqualityComparable.html below:

C++ named requirements: EqualityComparable - cppreference.com

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

[edit] 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) [edit] 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).

[edit] 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 [edit] See also

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