< cpp‎ | utility
Defined in header <compare>
inline namespace /* unspecified */ {

    inline constexpr /* unspecified */
        compare_partial_order_fallback = /* unspecified */;

(since C++20)
Call signature
template< class T, class U >

    requires /* see below */
constexpr std::partial_ordering

    compare_partial_order_fallback(T&& t, U&& u) noexcept(/* see below */);

Performs three-way comparison on t an u and produces a result of type std::partial_ordering, even if the operator <=> is unavailable.

Let t and u be expressions and T and U denote decltype((t)) and decltype((u)) respectively, std::compare_partial_order_fallback(t, u) is expression-equivalent to:

  • If std::is_same_v<std::decay_t<T>, std::decay_t<U>> == true:
    • the expression is expression-equivalent to std::partial_order(t, u), if it is a well-formed expression;
    • Otherwise, if t == u and t < u are both well-formed and convertible to bool, the expression is expression-equivalent to
t == u ? std::partial_ordering::equivalent :
t < u  ? std::partial_ordering::less :
u < t  ? std::partial_ordering::greater :
except that t and u are evaluated only once.
  • In all other cases, std::compare_partial_order_fallback(t, u) is ill-formed.


[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] Customization point objects

The name std::compare_partial_order_fallback denotes a customization point object, which is a function object of a literal semiregular class type (denoted, for exposition purposes, as compare_partial_order_fallback_ftor). All instances of compare_partial_order_fallback_ftor are equal. Thus, std::compare_partial_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_partial_order_fallback above, compare_partial_order_fallback_ftor will satisfy std::invocable<const compare_partial_order_fallback_ftor&, Args...>. Otherwise, no function call operator of compare_partial_order_fallback_ftor participates in overload resolution.

[edit] Example

[edit] See also

performs 3-way comparison and produces a result of type std::partial_ordering
(customization point object) [edit]