The type must work with < operator and the result should have standard semantics.
c, expressions of type
The following expressions must be valid and have their specified effects
| a < b
|| implicitly convertible to bool
|| Establishes strict weak ordering relation with the following properties
- For all
a, !(a < a)
- If a < b then !(b < a)
- If a < b and b < c then a < c
- Defining equiv(a, b) as !(a < b) && !(b < a), if equiv(a, b) and equiv(b, c), then equiv(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
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).