std::compare_3way
From cppreference.com
Defined in header <algorithm>
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template< class T, class U > constexpr auto compare_3way( const T& a, const U& b ); |
(since C++20) | |
Compares two values using three-way comparison and produces a result of the strongest applicable comparison category type.
In detail:
- If the expression a <=> b is well-formed, returns its result
- Otherwise, if the expressions a == b and a < b are both well-formed and convertible to bool,
- if a == b equals true, returns std::strong_ordering::equal
- otherwise, if a < b equals true, returns std::strong_ordering::less
- otherwise, returns std::strong_ordering::greater
- Otherwise, if the expression a == b is well-formed and convertible to bool (but a < b is not),
- if a == b equals true, returns std::strong_equality::equal
- otherwise, returns std::strong_equality::nonequal
- Otherwise (if neither a <=> b nor a == b are well-formed), the function is defined as deleted.
Parameters
a, b | - | the values to compare |
Return value
As defined above.
Notes
This function is useful in generic programming, since it uses < and == as fallbacks when <=> is not available.
Example
Run this code
#include <iostream> #include <compare> #include <algorithm> //does not support <=> struct Rational_1 { int num; int den; // > 0 }; inline constexpr bool operator<(Rational_1 lhs, Rational_1 rhs) { return lhs.num * rhs.den < rhs.num * lhs.den; } inline constexpr bool operator==(Rational_1 lhs, Rational_1 rhs) { return lhs.num * rhs.den == rhs.num * lhs.den; } //supports <=> struct Rational_2 { int num; int den; // > 0 }; inline 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"; else if (value < 0) std::cout << "less"; else std::cout << "greater"; std::cout << "\n"; } int main() { Rational_1 a{1,2}; Rational_1 b{3,4}; // print(a <=> b); //doesn't work print(std::compare_3way(a,b)); //works, defaults to < and == Rational_2 c{6,5}; Rational_2 d{8,7}; print(c <=> d); //works print(std::compare_3way(c,d)); //works }
Output:
less greater greater
See also
(C++20) |
compares two ranges using three-way comparison (function template) |
(C++20) |
the result type of 3-way comparison that supports only equality/inequality and is substitutable (class) |
(C++20) |
the result type of 3-way comparison that supports only equality/inequality and is not substitutable (class) |
(C++20) |
the result type of 3-way comparison that supports all 6 operators and is substitutable (class) |
(C++20) |
the result type of 3-way comparison that supports all 6 operators and is not substitutable (class) |
(C++20) |
the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class) |