std::totally_ordered, std::totally_ordered_with - cppreference.com (original) (raw)

1. The concept std::totally_ordered specifies that the comparison operators ==,!=,<,>,<=,>= on a type yield results consistent with a strict total order on the type.

2. The concept std::totally_ordered_with specifies that the comparison operators ==,!=,<,>,<=,>= on (possibly mixed) T and U operands yield results consistent with a strict total order. Comparing mixed operands yields results equivalent to comparing the operands converted to their common type.

3. The exposition-only concept __PartiallyOrderedWith_ specifies that a value of type T and a value of type U can be compared in a partial order with each other (in either order) using <, >, <=, and >=, and the results of the comparisons are consistent.

Contents

• 1 Semantic requirements
• 2 Equality preservation
• 3 Implicit expression variations
• 4 References
• 5 See also

[edit] Semantic requirements

These concepts are modeled only if they are satisfied and all concepts they subsume are modeled.

1. std::totally_ordered<T> is modeled only if, given lvalues a, b and c of type const std::remove_reference_t<T>:

• Exactly one of bool(a < b), bool(a > b) and bool(a == b) is true;
• If bool(a < b) and bool(b < c) are both true, then bool(a < c) is true;
• bool(a > b) == bool(b < a)
• bool(a >= b) == !bool(a < b)
• bool(a <= b) == !bool(b < a)

1. std::totally_ordered_with<T, U> is modeled only if, given

• t and t2, lvalues denoting distinct equal objects of types const std::remove_reference_t<T> and std::remove_reference_t<T> respectively, and
• u and u2, lvalues denoting distinct equal objects of types const std::remove_reference_t<U> and std::remove_reference_t<U> respectively,

let C be std::common_reference_t<const std::remove_reference_t<T>&, const std::remove_reference_t<U>&>, and, given an expression E and a type C, let CONVERT_TO<C>(E) be:

the following are true:

• bool(t < u) == bool(CONVERT_TO<C>(t2) < CONVERT_TO<C>(u2))
• bool(t > u) == bool(CONVERT_TO<C>(t2) > CONVERT_TO<C>(u2))
• bool(t <= u) == bool(CONVERT_TO<C>(t2) <= CONVERT_TO<C>(u2))
• bool(t >= u) == bool(CONVERT_TO<C>(t2) >= CONVERT_TO<C>(u2))
• bool(u < t) == bool(CONVERT_TO<C>(u2) < CONVERT_TO<C>(t2))
• bool(u > t) == bool(CONVERT_TO<C>(u2) > CONVERT_TO<C>(t2))
• bool(u <= t) == bool(CONVERT_TO<C>(u2) <= CONVERT_TO<C>(t2))
• bool(u >= t) == bool(CONVERT_TO<C>(u2) >= CONVERT_TO<C>(t2))

1. __PartiallyOrderedWith<T, U> is modeled only if given

• any lvalue t of type const std::remove_reference_t<T>, and
• any lvalue u of type const std::remove_reference_t<U>,

the following are true:

• t < u, t <= u, t > u, t >= u, u < t, u <= t, u > t, and u >= t have the same domain;
• bool(t < u) == bool(u > t);
• bool(u < t) == bool(t > u);
• bool(t <= u) == bool(u >= t); and
• bool(u <= t) == bool(t >= u).

[edit] Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

[edit] Implicit expression variations

A requires expression that uses an expression that is non-modifying for some constant lvalue operand also requires implicit expression variations.

[edit] References

• C++23 standard (ISO/IEC 14882:2024):

• 18.5.5 Concept totally_ordered [concept.totallyordered]

• C++20 standard (ISO/IEC 14882:2020):

• 18.5.4 Concept totally_ordered [concept.totallyordered]

[edit] See also