NAME¶

std::strong_ordering - std::strong_ordering

Synopsis¶

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:

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

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

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==
operator<
operator> compares with zero or a strong_ordering
operator<= (function)
operator>=
operator<=>

operator==

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

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 ) (1)
noexcept;
friend constexpr bool operator<( /*unspecified*/ u, strong_ordering v ) (2)
noexcept;

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

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 ) (1)
noexcept;
friend constexpr bool operator>( /*unspecified*/ u, strong_ordering v ) (2)
noexcept;

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

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

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{};

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¶

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