std:: compare_three_way

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Function objects
Function invocation
(C++17) (C++23)
Identity function object
(C++20)
Old binders and adaptors
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( until C++17* )
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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.

Nested types

Nested 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 >
constexpr auto operator ( ) ( T && t, U && u ) const ;

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

  • 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:

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 while comparing pointers only semantic requirements are relaxed

See also

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