C++ named requirements: EqualityComparable

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 (since C++11) 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).

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

specifies that operator == is an equivalence relation

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