C++ named requirements: EqualityComparable

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C++ named requirements

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


The type T satisfies EqualityComparable if


  • a, b, and c, expressions of type T or const T

The following expressions must be valid and have their specified effects

Expression Return type Requirements
a == b implicitly convertible to bool 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


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).