std::compare_three_way

From cppreference.com
< cpp‎ | utility
 
C++
 
Utilities library
General utilities
Date and time
Function objects
Formatting library (C++20)
(C++11)
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)
Elementary string conversions
(C++17)
(C++17)
(C++17)
 
Function objects
Function wrappers
(C++11)
(C++23)
(C++11)
(C++11)
Partial function application
(C++20)(C++23)
(C++11)
(C++11)
(C++11)
(C++11)
Function invocation
(C++17)(C++23)
Identity function object
(C++20)
Reference wrappers
(C++11)
(C++11)(C++11)
(C++20)(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)
Negators
(C++17)
Searchers
(C++17)
(C++17)
(C++17)
Constrained comparators
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
compare_three_way
(C++20)
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.

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 (see below).

Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

  • The inputs to an expression consist of its operands.
  • The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

Example

Run this code
#include <iostream>
#include <compare>
 
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)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/compare/compare_three_way&oldid=136934"