Difference between revisions of "cpp/utility/compare/compare strong order fallback"
(Wording update.) |
(Uses {{c multi}}.) |
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If {{c/core|std::decay_t<T>}} and {{c/core|std::decay_t<U>}} are the same type, {{c|std::compare_strong_order_fallback(t, u)}} is [[cpp/language/expressions#Expression-equivalence|expression-equivalent]] to: | If {{c/core|std::decay_t<T>}} and {{c/core|std::decay_t<U>}} are the same type, {{c|std::compare_strong_order_fallback(t, u)}} is [[cpp/language/expressions#Expression-equivalence|expression-equivalent]] to: | ||
* {{c|std::strong_order(t, u)}}, if it is a well-formed expression; | * {{c|std::strong_order(t, u)}}, if it is a well-formed expression; | ||
| − | * otherwise, | + | * otherwise, {{c multi |
| − | {{ | + | |1=t == u ? std::strong_ordering::equal : |
| − | t == u ? std::strong_ordering::equal : | + | |2=t < u ? std::strong_ordering::less : |
| − | t < u ? std::strong_ordering::less : | + | |3= std::strong_ordering::greater |
| − | + | ||
}} | }} | ||
: if the expressions {{c|1=t == u}} and {{c|t < u}} are both well-formed and each of {{c/core|1=decltype(t == u)}} and {{c/core|decltype(t < u)}} models {{lti|cpp/concepts/boolean-testable}}, except that {{c|t}} and {{c|u}} are evaluated only once. | : if the expressions {{c|1=t == u}} and {{c|t < u}} are both well-formed and each of {{c/core|1=decltype(t == u)}} and {{c/core|decltype(t < u)}} models {{lti|cpp/concepts/boolean-testable}}, except that {{c|t}} and {{c|u}} are evaluated only once. | ||
Revision as of 19:38, 4 May 2023
| Defined in header <compare>
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| inline namespace /* unspecified */ { inline constexpr /* unspecified */ |
(since C++20) | |
| Call signature |
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| template< class T, class U > requires /* see below */ |
(since C++20) | |
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.
Contents |
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. For exposition purposes, the cv-unqualified version of its type is denoted as __compare_strong_order_fallback_fn.
All instances of __compare_strong_order_fallback_fn are equal. The effects of invoking different instances of type __compare_strong_order_fallback_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, std::compare_strong_order_fallback can be copied freely and its copies can be used interchangeably.
Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to std::compare_strong_order_fallback above, __compare_strong_order_fallback_fn models
- std::invocable<__compare_strong_order_fallback_fn, Args...>,
- std::invocable<const __compare_strong_order_fallback_fn, Args...>,
- std::invocable<__compare_strong_order_fallback_fn&, Args...>, and
- std::invocable<const __compare_strong_order_fallback_fn&, Args...>.
Otherwise, no function call operator of __compare_strong_order_fallback_fn participates in overload resolution.
Example
| This section is incomplete Reason: no example |
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2167R3 | C++20 | the fallback mechanism only required return types to be convertible to bool | constraints strengthened |
See also
| (C++20) |
performs 3-way comparison and produces a result of type std::strong_ordering(customization point object) |
