std::compare_3way

template< class T, class U >
constexpr auto compare_3way( const T& a, const U& b );

Compares two values using three-way comparison and produces a result of the strongest applicable comparison category type.

In detail:

• If the expression a <=> b is well-formed, returns its result
• Otherwise, if the expressions a == b and a < b are both well-formed and convertible to bool,
• if a == b equals true, returns std::strong_ordering::equal
• otherwise, if a < b equals true, returns std::strong_ordering::less
• otherwise, returns std::strong_ordering::greater
• Otherwise, if the expression a == b is well-formed and convertible to bool (but a < b is not),
• if a == b equals true, returns std::strong_equality::equal
• otherwise, returns std::strong_equality::nonequal
• Otherwise (if neither a <=> b nor a == b are well-formed), the function is defined as deleted.

Parameters

Return value

As defined above.

Notes

This function is useful in generic programming, since it uses < and == as fallbacks when <=> is not available.

Example

See also