Std::compare_3way (3) Linux Manual Page

std::compare_3way – std::compare_3way

Synopsis

Defined in header<algorithm>

    template <class T, class U>
    (since C++ 20)

        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

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.

Example

// Run this code

#include <iostream>

#include <compare>

#include <algorithm>
// does not support <=>

struct Rational_1 {
    int num;
    int den; // > 0
};
inline constexpr bool operator<(Rational_1 lhs, Rational_1 rhs)

{
    return lhs.num * rhs.den < rhs.num * lhs.den;
}
inline constexpr bool operator==(Rational_1 lhs, Rational_1 rhs)

{
    return lhs.num * rhs.den == rhs.num * lhs.den;
}
// supports <=>

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";
    else if (value < 0)
        std::cout << "less";
    else
        std::cout << "greater";
    std::cout << "\n";
}
int main()

{
    Rational_1 a{1, 2};
    Rational_1 b{3, 4};

    //  print(a <=> b);                //doesn't work
    print(std::compare_3way(a, b)); // works, defaults to < and ==
    Rational_2 c{6, 5};
    Rational_2 d{8, 7};
    print(c <=> d);                 // works
    print(std::compare_3way(c, d)); // works
}

Output:

  less
  greater
  greater

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

std::compare_3way (3) Linux Manual Page