std::compare_three_way
Defined in header <compare>


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.
Contents 
[edit] Member types
Member type  Definition 
is_transparent

/* unspecified */ 
[edit] Member functions
operator() 
obtains the result of threeway 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>  

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 invoke a builtin 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 implementationdefined 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 the conversion sequences from both T
and U
to P
are equalitypreserving (see below).
[edit] 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).
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.
[edit] Example
#include <iostream> #include <compare> struct Rational_2 { int num; int den; // > 0 }; constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs) { return lhs.num * rhs.den <=> rhs.num * lhs.den; } void print(std::weak_ordering value) { if (value == 0) std::cout << "equal\n"; else if (value < 0) std::cout << "less\n"; else std::cout << "greater\n"; } int main() { Rational_2 c{6,5}; Rational_2 d{8,7}; print(c <=> d); print(std::compare_three_way{}(c,d)); }
Output:
greater greater
[edit] 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) 