20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.8 Comparisons [comparisons]

20.14.8.8 Class compare_­three_­way [comparisons.three.way]

In this subclause, BUILTIN-PTR-THREE-WAY(T, U) for types T and U is a boolean constant expression.
BUILTIN-PTR-THREE-WAY(T, U) is true if and only if <=> in the expression declval<T>() <=> declval<U>() resolves to a built-in operator comparing pointers.
struct compare_three_way { template<class T, class U> requires three_­way_­comparable_­with<T, U> || BUILTIN-PTR-THREE-WAY(T, U) constexpr auto operator()(T&& t, U&& u) const; using is_transparent = unspecified; };
template<class T, class U> requires three_­way_­comparable_­with<T, U> || BUILTIN-PTR-THREE-WAY(T, U) constexpr auto operator()(T&& t, U&& u) const;
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]).
Effects:
  • 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.
  • Otherwise, equivalent to: return std​::​forward<T>(t) <=> std​::​forward<U>(u);