std::compare_three_way

 

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Defined in header <compare>
struct compare_three_way;		(since C++20)

Function object for performing comparisons. Deduces the parameter types of the function call operator from the arguments (but not the return type).

Member types

Member type	Definition
is_transparent	/* unspecified */

Member functions

operator()

obtains the result of three-way comparison on both arguments (public member function)

std::compare_three_way::operator()

template<class T, class U>     requires three_way_comparable_with<T, U> ||              /* std::declval<T>() <=> std::declval<U>() resolves to                 a built-in operator comparing pointers */ constexpr auto operator()(T&& t, U&& u) const;

Compares t and u, equivalent to return std::forward<T>(t) <=> std::forward<U>(u);, except when that expression resolves to a call to a builtin operator<=> comparing pointers.

When a call would invoke a built-in operator comparing pointers of type P, the result is instead determined as follows:

• Returns std::strong_ordering::less if the (possibly converted) value of the first argument precedes the (possibly converted) value of the second argument in the implementation-defined strict total ordering over all pointer values of type P. This strict total ordering is consistent with the partial order imposed by the builtin operators <, >, <=, and >=.
• Otherwise, returns std::strong_ordering::greater if (possibly converted) value of the second argument precedes the (possibly converted) value of the first argument in the same strict total ordering.
• Otherwise, returns std::strong_ordering::equal.

The behavior is undefined unless the the conversion sequences from both T and U to P are equality-preserving (see below).

Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

Example

Run this code

#include <iostream>
#include <compare>
 
struct Rational_2 {
    int num;
    int den; // > 0
};
 
inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs)
{
    return lhs.num * rhs.den <=> rhs.num * lhs.den;
}
 
void print(std::weak_ordering value)
{
    if (value == 0)
        std::cout << "equal\n";
    else if (value < 0)
        std::cout << "less\n";
    else
        std::cout << "greater\n";
}
 
int main()
{
    Rational_2 c{6,5};
    Rational_2 d{8,7};
    print(c <=> d);
    print(std::compare_three_way{}(c,d));
}

Output:

less
greater
greater