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std::strong_ordering - cppreference.com

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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
a valid value indicating less-than (ordered before) relationship
(public static member constant)
a valid value indicating equivalence (neither ordered before nor ordered after), the same as equal
(public static member constant)
a valid value indicating equivalence (neither ordered before nor ordered after), the same as equivalent
(public static member constant)
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.

std::strong_ordering::operator partial_ordering

constexpr operator partial_ordering() const noexcept;

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.

std::strong_ordering::operator weak_ordering

constexpr operator weak_ordering() const noexcept;

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==

friend constexpr bool operator==( strong_ordering v, /*unspecified*/ u ) noexcept;

 (1)

friend constexpr bool 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<

friend constexpr bool operator<( strong_ordering v, /*unspecified*/ u ) noexcept;

 (1)

friend constexpr bool 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<=

friend constexpr bool operator<=( strong_ordering v, /*unspecified*/ u ) noexcept;

 (1)

friend constexpr bool 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>

friend constexpr bool operator>( strong_ordering v, /*unspecified*/ u ) noexcept;

 (1)

friend constexpr bool 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>=

friend constexpr bool operator>=( strong_ordering v, /*unspecified*/ u ) noexcept;

 (1)

friend constexpr bool 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<=>

friend constexpr strong_ordering operator<=>( strong_ordering v, /*unspecified*/ u ) noexcept;

 (1)

friend constexpr 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

#include <compare>
#include <iostream>

struct Point
{
    int x{}, y{};

    friend constexpr 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;
    }

    friend std::ostream& operator<<(std::ostream& os, Point s)
    {
        return os << '(' << s.x << ',' << s.y << ')';
    }
};

void print_three_way_comparison(const auto& p, const auto& q)
{
    const auto cmp{p <=> q};
    std::cout << p
              << (cmp < 0 ? " <  " : cmp > 0 ? " >  " : " == " ) // compares with 0
              << q << '\n';
}

void print_two_way_comparison(const auto& p, const auto& 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

the result type of 3-way comparison that supports all 6 operators and is not substitutable
(class) [edit]
the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values
(class) [edit]