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Showing content from https://en.cppreference.com/w/cpp/language/../memory/../named_req/LessThanComparable.html below:

C++ named requirements: LessThanComparable - cppreference.com

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

[edit] Requirements

The type T satisfies LessThanComparable if given expressions a, b and c of type T or const T(since C++11), the following expression is valid and has its specified effects:

Expression Type Effects a < b meets BooleanTestable
(until C++20) Establishes strict weak ordering relation with the following properties:

For all a, !(a < a) yields true.
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).

models boolean-testable
(since C++20) [edit] Notes

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).

[edit] 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 2114
(P2167R3) C++98 convertibility to bool was too weak to reflect the expectation of implementations requirements strengthened [edit] See also

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