C++ concepts: EqualityComparable
From cppreference.com
The type must work with == operator and the result should have standard semantics.
[edit] Requirements
The type T
satisfies EqualityComparable
if
Given

a
,b
, andc
, expressions of typeT
orconst 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:

[edit] Notes
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).