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Showing content from https://en.cppreference.com/w/cpp/language/../utility/variadic/../compare/compare_three_way.html below:

std::compare_three_way - cppreference.com

struct compare_three_way;

Function object for performing comparisons. Deduces the parameter types and the return type of the function call operator.

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

Given the expression std::forward<T>(t) <=> std::forward<U>(u) as expr:

• If expr results in a call to built-in operator<=> comparing pointers, given the composite pointer type of t and u as P:

• Compares the two converted pointers (of type P) in the implementation-defined strict total order over pointers:

• If t precedes u, returns std::strong_ordering::less.
• If u precedes t, returns std::strong_ordering::greater.
• Otherwise, returns std::strong_ordering::equal.

• If the conversion sequence from T to P or the conversion sequence from U to P is not equality-preserving, the behavior is undefined.

• Otherwise:

• Returns the result of expr.
• If std::three_way_comparable_with<T, U> is not modeled, the behavior is undefined.

This overload participates in overload resolution only if std::three_way_comparable_with<T, U> is satisfied.

#include <compare>
#include <iostream>
 
struct Rational
{
    int num;
    int den; // > 0
 
    // Although the comparison X <=> Y will work, a direct call
    // to std::compare_three_way{}(X, Y) requires the operator==
    // be defined, to satisfy the std::three_way_comparable_with.
    constexpr bool operator==(Rational const&) const = default;
};
 
constexpr std::weak_ordering operator<=>(Rational lhs, Rational rhs)
{
    return lhs.num * rhs.den <=> rhs.num * lhs.den;
}
 
void print(std::weak_ordering value)
{
    value < 0 ? std::cout << "less\n" :
    value > 0 ? std::cout << "greater\n" :
                std::cout << "equal\n";
}
 
int main()
{
    Rational a{6, 5};
    Rational b{8, 7};
    print(a <=> b);
    print(std::compare_three_way{}(a, b));
}

Output:

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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