std::compare_3way

C++

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Defined in header <algorithm>
template< class T, class U > constexpr auto compare_3way( const T& a, const U& b );		(since C++20)

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

a, b

-

the values to compare

Return value

As defined above.

Notes

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

See also

lexicographical_compare_3way (C++20)	compares two ranges using three-way comparison (function template)
strong_equality (C++20)	the result type of 3-way comparison that supports only equality/inequality and is substitutable (class)
weak_equality (C++20)	the result type of 3-way comparison that supports only equality/inequality and is not substitutable (class)
strong_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators and is substitutable (class)
weak_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators and is not substitutable (class)
partial_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class)