std::compare_three_way

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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.

Implementation-defined strict total order over pointers

The function call operator yields the implementation-defined strict total order over pointers if the <=> operator between arguments invokes a built-in comparison operator for a pointer, even if the built-in <=> operator does not.

The implementation-defined strict total order is consistent with the partial order imposed by built-in comparison operators (<=>, <, >, <=, and >=), and consistent among following standard function objects:

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 std::three_way_comparable_with<T, U> // with different semantic requirements

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 not invoke a built-in operator comparing pointers, the behavior is undefined if std::three_way_comparable_with<T, U> is not modeled.

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 conversion sequences from both T and U to P are equality-preserving.

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 <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 while comparing pointers only semantic requirements relaxed

See also

(C++20)
 function object implementing x == y
(class)
(C++20)
 function object implementing x != y
(class)
(C++20)
 function object implementing x < y
(class)
(C++20)
 function object implementing x > y
(class)
(C++20)
 function object implementing x <= y
(class)
(C++20)
 function object implementing x >= y
(class)
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