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

This section is incomplete
Reason: no example

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)