std::compare_strong_order_fallback
<compare>
inline constexpr /* unspecified */
compare_strong_order_fallback = /* unspecified */;
requires /* see below */
constexpr std::strong_ordering
Performs three-way comparison on subexpressions t and u and produces a result of type std::strong_ordering, even if the operator <=> is unavailable.
If std::decay_t <T> and std::decay_t <U> are the same type, std::compare_strong_order_fallback(t, u) is expression-equivalent to:
- std::strong_order (t, u), if it is a well-formed expression;
- otherwise, t == u ? std::strong_ordering::equal :
t < u ? std::strong_ordering::less :
std::strong_ordering::greater
- if the expressions t == u and t < u are both well-formed and each of decltype(t == u) and decltype(t < u) models boolean-testable, except that t and u are evaluated only once.
In all other cases, std::compare_strong_order_fallback(t, u) is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
Customization point objects
The name std::compare_strong_order_fallback denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.
[edit] Example
Reason: no example
[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++20 | the fallback mechanism only required return types to be convertible to bool |
constraints strengthened |
[edit] See also
std::strong_ordering(customization point object)[edit]