std::compare_three_way

From cppreference.com
< cpp‎ | utility
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)
(C++20)
Swap and type operations
(C++20)
(C++14)
(C++11)
(C++23)
(C++11)
(C++23)
(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++11)
(C++17)
(C++17)
(C++23)



Function objects
Function invocation
(C++17)(C++23)
Identity function object
(C++20)
Transparent operator wrappers
(C++14)
(C++14)
(C++14)
(C++14)  
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)

Old binders and adaptors
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)  
(until C++17*)
(until C++17*) (until C++17*) (until C++17*) (until C++17*)
(until C++20*)
(until C++20*)
(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*) (until C++17*) (until C++17*) (until C++17*)
(until C++20*)
(until C++20*)
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
  • Otherwise:

This overload participates in overload resolution only if std::three_way_comparable_with<T, U>

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)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/compare/compare_three_way&oldid=168288