22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.8 Comparisons [comparisons]

22.10.8.8 Class compare_three_way [comparisons.three.way]

namespace std { struct compare_three_way { template<class T, class U> constexpr auto operator()(T&& t, U&& u) const; using is_transparent = unspecified; }; }

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template<class T, class U>
  constexpr auto operator()(T&& t, U&& u) const;

Constraints: T and U satisfy three_way_comparable_with .

Preconditions: If the expression std::forward<T>(t) <=> std::forward<U>(u) results in a call to a built-in operator <=> comparing pointers of type P, the conversion sequences from both T and U to P are equality-preserving ([concepts.equality]); otherwise, T and U model three_way_comparable_with .

Effects:

(3.1)
If the expression std::forward<T>(t) <=> std::forward<U>(u) results in a call to a built-in operator <=> comparing pointers of type P, returns strong_ordering::less if (the converted value of) t precedes u in the implementation-defined strict total order over pointers ([defns.order.ptr]), strong_ordering::greater if u precedes t, and otherwise strong_ordering::equal.
(3.2)
Otherwise, equivalent to: return std::forward<T>(t) <=> std::forward<U>(u);