The type must work with == operator and the result should have standard semantics.
The type T
satisfies EqualityComparable
if.
Given.
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 equivalence relation with the following properties:

To satisfy this requirement, types that do not have builtin comparison operators have to provide a userdefined 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)
.
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http://en.cppreference.com/w/cpp/concept/EqualityComparable