std::strong_ordering

 

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Defined in header <compare>
class strong_ordering;		(since C++20)

The class type std::strong_ordering is the result type of a three-way comparison that:

• Admits all six relational operators (==, !=, <, <=, >, >=).

• Implies substitutability: if a is equivalent to b, f(a) is also equivalent to f(b), where f denotes a function that reads only comparison-salient state that is accessible via the argument's public const members. In other words, equivalent values are indistinguishable.
• Does not allow incomparable values: exactly one of a < b, a == b, or a > b must be true.

Constants

The type std::strong_ordering has four valid values, implemented as const static data members of its type:

Name	Definition
inlineconstexpr std::strong_ordering less [static]	a valid value indicating less-than (ordered before) relationship (public static member constant)
inlineconstexpr std::strong_ordering equivalent [static]	a valid value indicating equivalence (neither ordered before nor ordered after), the same as equal (public static member constant)
inlineconstexpr std::strong_ordering equal [static]	a valid value indicating equivalence (neither ordered before nor ordered after), the same as equivalent (public static member constant)
inlineconstexpr std::strong_ordering greater [static]	a valid value indicating greater-than (ordered after) relationship (public static member constant)

Conversions

std::strong_ordering is the strongest of the three comparison categories: it is not implicitly-convertible from any other category and is implicitly-convertible to the other two.

operator partial_ordering

implicit conversion to std::partial_ordering (public member function)

std::strong_ordering::operator partial_ordering

constexpr operator partial_ordering()constnoexcept;

Return value

std::partial_ordering::less if v is less, std::partial_ordering::greater if v is greater, std::partial_ordering::equivalent if v is equal or equivalent.

operator weak_ordering

implicit conversion to std::weak_ordering (public member function)

std::strong_ordering::operator weak_ordering

constexpr operator weak_ordering()constnoexcept;

Return value

std::weak_ordering::less if v is less, std::weak_ordering::greater if v is greater, std::weak_ordering::equivalent if v is equal or equivalent.

Comparisons

Comparison operators are defined between values of this type and literal ​0​. This supports the expressions a <=> b ==0 or a <=> b <0 that can be used to convert the result of a three-way comparison operator to a boolean relationship; see std::is_eq, std::is_lt, etc.

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::strong_ordering is an associated class of the arguments.

The behavior of a program that attempts to compare a strong_ordering with anything other than the integer literal ​0​ is undefined.

operator==operator<operator>operator<=operator>=operator<=>

compares with zero or a strong_ordering (function)

operator==

friendconstexprbool operator==( strong_ordering v, /*unspecified*/ u )noexcept;	(1)
friendconstexprbool operator==( strong_ordering v, strong_ordering w )noexcept=default;	(2)

Parameters

v, w	-	std::strong_ordering values to check
u	-	an unused parameter of any type that accepts literal zero argument

Return value

1)true if v is equivalent or equal, false if v is less or greater

2)true if both parameters hold the same value, false otherwise. Note that equal is the same as equivalent.

operator<

friendconstexprbool operator<( strong_ordering v, /*unspecified*/ u )noexcept;	(1)
friendconstexprbool operator<(/*unspecified*/ u, strong_ordering v )noexcept;	(2)

Parameters

v	-	a std::strong_ordering value to check
u	-	an unused parameter of any type that accepts literal zero argument

Return value

1)true if v is less, and false if v is greater, equivalent, or equal

2)true if v is greater, and false if v is less, equivalent, or equal

operator<=

friendconstexprbool operator<=( strong_ordering v, /*unspecified*/ u )noexcept;	(1)
friendconstexprbool operator<=(/*unspecified*/ u, strong_ordering v )noexcept;	(2)

Parameters

v	-	a std::strong_ordering value to check
u	-	an unused parameter of any type that accepts literal zero argument

Return value

1)true if v is less, equivalent, or equal, and false if v is greater

2)true if v is greater, equivalent, or equal, and false if v is less

operator>

friendconstexprbool operator>( strong_ordering v, /*unspecified*/ u )noexcept;	(1)
friendconstexprbool operator>(/*unspecified*/ u, strong_ordering v )noexcept;	(2)

Parameters

v	-	a std::strong_ordering value to check
u	-	an unused parameter of any type that accepts literal zero argument

Return value

1)true if v is greater, and false if v is less, equivalent, or equal

2)true if v is less, and false if v is greater, equivalent, or equal

operator>=

friendconstexprbool operator>=( strong_ordering v, /*unspecified*/ u )noexcept;	(1)
friendconstexprbool operator>=(/*unspecified*/ u, strong_ordering v )noexcept;	(2)

Parameters

v	-	a std::strong_ordering value to check
u	-	an unused parameter of any type that accepts literal zero argument

Return value

1)true if v is greater, equivalent, or equal, and false if v is less

2)true if v is less, equivalent, or equal, and false if v is greater

operator<=>

friendconstexpr strong_ordering operator<=>( strong_ordering v, /*unspecified*/ u )noexcept;	(1)
friendconstexpr strong_ordering operator<=>(/*unspecified*/ u, strong_ordering v )noexcept;	(2)

Parameters

v	-	a std::strong_ordering value to check
u	-	an unused parameter of any type that accepts literal zero argument

Return value

1)v.

2)greater if v is less, less if v is greater, otherwise v.

Example

Run this code

#include <compare>#include <iostream>   struct Point {int x{}, y{};   friendconstexpr std::strong_ordering operator<=>(Point lhs, Point rhs){if(lhs.x< rhs.x or (lhs.x== rhs.x and lhs.y< rhs.y))return std::strong_ordering::less;if(lhs.x> rhs.x or (lhs.x== rhs.x and lhs.y> rhs.y))return std::strong_ordering::greater;return std::strong_ordering::equivalent;}   friendstd::ostream& operator<<(std::ostream& os, Point s){return os <<'('<< s.x<<','<< s.y<<')';}};   void print_three_way_comparison(constauto& p, constauto& q){constauto cmp{p <=> q};std::cout<< p <<(cmp <0?" < ": cmp >0?" > ":" == ")// compares with 0<< q <<'\n';}   void print_two_way_comparison(constauto& p, constauto& q){std::cout<< p <<(p < q ?" < ": p > q ?" > ":" == ")// compares p and q<< q <<'\n';}   int main(){const Point p1{0, 1}, p2{0, 1}, p3{0, 2};   print_three_way_comparison(p1, p2); print_two_way_comparison(p1, p2);   print_three_way_comparison(p2, p3); print_two_way_comparison(p2, p3);   print_three_way_comparison(p3, p2); print_two_way_comparison(p3, p2);}

Output:

(0,1) == (0,1) (0,1) == (0,1) (0,1) < (0,2) (0,1) < (0,2) (0,2) > (0,1) (0,2) > (0,1)

See also

weak_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators and is not substitutable (class)
partial_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class)