NAME¶

std::compare_three_way - std::compare_three_way

Synopsis¶

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

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

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 >
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.

Example¶

// Run this code

#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:¶

greater
greater

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 3530 C++20 syntactic checks were relaxed only semantic requirements are
while comparing pointers relaxed

See also¶

ranges::equal_to constrained function object implementing x == y
(C++20) (class)
ranges::not_equal_to constrained function object implementing x != y
(C++20) (class)
ranges::less constrained function object implementing x < y
(C++20) (class)
ranges::greater constrained function object implementing x > y
(C++20) (class)
ranges::less_equal constrained function object implementing x <= y
(C++20) (class)
ranges::greater_equal constrained function object implementing x >= y
(C++20) (class)