std::partial_ordering
Defined in header <compare> |
||
---|---|---|
class partial_ordering; |
(since C++20) |
The class type std::partial_ordering
is the result type of a three-way comparison that.
- admits all six relational operators (==, !=, <, <=, >, >=)
- does not imply substitutability: if a is equivalent to b, f(a) may not be 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 may be distinguishable.
- admits incomparable values: a < b, a == b, and a > b may all be false
Constants
The type std::partial_ordering
has four valid values, implemented as const static data members of its type:
Member constant | Definition |
---|---|
less(inline constexpr)
[static]
|
a valid value of the type std::partial_ordering indicating less-than (ordered before) relationship (public static member constant) |
equivalent(inline constexpr)
[static]
|
a valid value of the type std::partial_ordering indicating equivalence (neither ordered before nor ordered after) (public static member constant) |
greater(inline constexpr)
[static]
|
a valid value of the type std::partial_ordering indicating greater-than (ordered after) relationship (public static member constant) |
unordered(inline constexpr)
[static]
|
a valid value of the type std::partial_ordering indicating relationship with an incomparable value (public static member constant) |
Conversions
std::partial_ordering
cannot be implicitly converted to other comparison category types, while both std::strong_ordering
and std::weak_ordering
are implicitly-convertible to partial_ordering
.
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::partial_ordering
is an associated class of the arguments.
The behavior of a program that attempts to compare a partial_ordering
with anything other than the integer literal 0
is undefined.
operator==operator<operator>operator<=operator>=operator<=>
|
compares with zero or a partial_ordering (function) |
operator==
friend constexpr bool operator==( partial_ordering v, /*unspecified*/ u ) noexcept; |
(1) | |
friend constexpr bool operator==( partial_ordering v, partial_ordering w ) noexcept = default; |
(2) |
Parameters
v, w | - | std::partial_ordering values to check |
u | - | an unused parameter of any type that accepts literal zero argument |
Return value
true
if
v
is
equivalent
,
false
if
v
is
less
,
greater
, or
unordered
true
if both parameters hold the same value,
false
otherwise
operator<
friend constexpr bool operator<( partial_ordering v, /*unspecified*/ u ) noexcept; |
(1) | |
friend constexpr bool operator<( /*unspecified*/ u, partial_ordering v ) noexcept; |
(2) |
Parameters
v | - | a std::partial_ordering value to check |
u | - | an unused parameter of any type that accepts literal zero argument |
Return value
true
if
v
is
less
, and
false
if
v
is
greater
,
equivalent
, or
unordered
true
if
v
is
greater
, and
false
if
v
is
less
,
equivalent
, or
unordered
operator<=
friend constexpr bool operator<=( partial_ordering v, /*unspecified*/ u ) noexcept; |
(1) | |
friend constexpr bool operator<=( /*unspecified*/ u, partial_ordering v ) noexcept; |
(2) |
Parameters
v | - | a std::partial_ordering value to check |
u | - | an unused parameter of any type that accepts literal zero argument |
Return value
true
if
v
is
less
or
equivalent
, and
false
if
v
is
greater
or
unordered
true
if
v
is
greater
or
equivalent
, and
false
if
v
is
less
or
unordered
operator>
friend constexpr bool operator>( partial_ordering v, /*unspecified*/ u ) noexcept; |
(1) | |
friend constexpr bool operator>( /*unspecified*/ u, partial_ordering v ) noexcept; |
(2) |
Parameters
v | - | a std::partial_ordering value to check |
u | - | an unused parameter of any type that accepts literal zero argument |
Return value
true
if
v
is
greater
, and
false
if
v
is
less
,
equivalent
, or
unordered
true
if
v
is
less
, and
false
if
v
is
greater
,
equivalent
, or
unordered
operator>=
friend constexpr bool operator>=( partial_ordering v, /*unspecified*/ u ) noexcept; |
(1) | |
friend constexpr bool operator>=( /*unspecified*/ u, partial_ordering v ) noexcept; |
(2) |
Parameters
v | - | a std::partial_ordering value to check |
u | - | an unused parameter of any type that accepts literal zero argument |
Return value
true
if
v
is
greater
or
equivalent
, and
false
if
v
is
less
or
unordered
true
if
v
is
less
or
equivalent
, and
false
if
v
is
greater
or
unordered
operator<=>
friend constexpr partial_ordering operator<=>( partial_ordering v, /*unspecified*/ u ) noexcept; |
(1) | |
friend constexpr partial_ordering operator<=>( /*unspecified*/ u, partial_ordering v ) noexcept; |
(2) |
Parameters
v | - | a std::partial_ordering value to check |
u | - | an unused parameter of any type that accepts literal zero argument |
Return value
v
.
greater
if
v
is
less
,
less
if
v
is
greater
, otherwise
v
.
Notes
The built-in operator<=>
between floating-point values uses this ordering: the positive zero and the negative zero compare equivalent
, but can be distinguished, and NaN values compare unordered
with any other value.
Example
See also
(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) |
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