operator==, !=, <, <=, >, >=, <=>(std::optional)
From cppreference.com
Defined in header <optional>
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Compare two optional objects |
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template
<
class T, class U >
constexpr bool operator== ( const optional<T> & lhs, const optional<U> & rhs ) ; |
(1) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator! = ( const optional<T> & lhs, const optional<U> & rhs ) ; |
(2) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator< ( const optional<T> & lhs, const optional<U> & rhs ) ; |
(3) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator<= ( const optional<T> & lhs, const optional<U> & rhs ) ; |
(4) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator> ( const optional<T> & lhs, const optional<U> & rhs ) ; |
(5) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator>= ( const optional<T> & lhs, const optional<U> & rhs ) ; |
(6) | (since C++17) |
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template
<
class T, std::three_way_comparable_with
<T> U >
constexpr
std::compare_three_way_result_t
<T, U>
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(7) | (since C++20) |
Compare an optional object with a nullopt |
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template
<
class T >
constexpr bool operator== ( const optional<T> & opt, std::nullopt_t ) noexcept ; |
(8) | (since C++17) |
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template
<
class T >
constexpr bool operator== ( std::nullopt_t, const optional<T> & opt ) noexcept ; |
(9) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator! = ( const optional<T> & opt, std::nullopt_t ) noexcept ; |
(10) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator! = ( std::nullopt_t, const optional<T> & opt ) noexcept ; |
(11) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator< ( const optional<T> & opt, std::nullopt_t ) noexcept ; |
(12) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator< ( std::nullopt_t, const optional<T> & opt ) noexcept ; |
(13) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator<= ( const optional<T> & opt, std::nullopt_t ) noexcept ; |
(14) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator<= ( std::nullopt_t, const optional<T> & opt ) noexcept ; |
(15) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator> ( const optional<T> & opt, std::nullopt_t ) noexcept ; |
(16) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator> ( std::nullopt_t, const optional<T> & opt ) noexcept ; |
(17) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator>= ( const optional<T> & opt, std::nullopt_t ) noexcept ; |
(18) | (since C++17) (until C++20) |
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template
<
class T >
constexpr bool operator>= ( std::nullopt_t, const optional<T> & opt ) noexcept ; |
(19) | (since C++17) (until C++20) |
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template
<
class T >
constexpr std::strong_ordering |
(20) | (since C++20) |
Compare an optional object with a value |
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template
<
class T, class U >
constexpr bool operator== ( const optional<T> & opt, const U& value ) ; |
(21) | (since C++17) |
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template
<
class U, class T >
constexpr bool operator== ( const U& value, const optional<T> & opt ) ; |
(22) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator! = ( const optional<T> & opt, const U& value ) ; |
(23) | (since C++17) |
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template
<
class U, class T >
constexpr bool operator! = ( const U& value, const optional<T> & opt ) ; |
(24) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator< ( const optional<T> & opt, const U& value ) ; |
(25) | (since C++17) |
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template
<
class U, class T >
constexpr bool operator< ( const U& value, const optional<T> & opt ) ; |
(26) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator<= ( const optional<T> & opt, const U& value ) ; |
(27) | (since C++17) |
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template
<
class U, class T >
constexpr bool operator<= ( const U& value, const optional<T> & opt ) ; |
(28) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator> ( const optional<T> & opt, const U& value ) ; |
(29) | (since C++17) |
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template
<
class U, class T >
constexpr bool operator> ( const U& value, const optional<T> & opt ) ; |
(30) | (since C++17) |
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template
<
class T, class U >
constexpr bool operator>= ( const optional<T> & opt, const U& value ) ; |
(31) | (since C++17) |
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template
<
class U, class T >
constexpr bool operator>= ( const U& value, const optional<T> & opt ) ; |
(32) | (since C++17) |
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template
<
class T, std::three_way_comparable_with
<T> U >
constexpr
std::compare_three_way_result_t
<T, U>
|
(33) | (since C++20) |
Performs comparison operations on optional objects.
1-7) Compares two
optional objects, lhs and rhs. The contained values are compared (using the corresponding operator of T) only if both lhs and rhs contain values. Otherwise,
- lhs is considered equal to rhs if, and only if, both lhs and rhs
- lhs is considered less than rhs if, and only if, rhs contains a value and lhs
1-6) Let @ denote the corresponding comparison operator, for each of these functions:
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If the corresponding expression *lhs @ *rhs is ill-formed or its result is not convertible to bool |
(until C++26) |
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This overload participates in overload resolution only if the corresponding expression *lhs @ *rhs is well-formed and its result is convertible to bool |
(since C++26) |
8-20) Compares opt with a
nullopt. Equivalent to (1-6) when comparing to an optional that does not contain a value.
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The |
(since C++20) |
21-33) Compares opt with a value. The values are compared (using the corresponding operator of
T) only if opt contains a value. Otherwise, opt is considered less than
value
21-32) Let @ denote the corresponding comparison operator, for each of these functions:
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If the corresponding expression *opt @ value or value @ *opt (depending on the positions of the operands) is ill-formed or its result is not convertible to bool |
(until C++26) |
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This overload participates in overload resolution only if all following conditions are satisfied:
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(since C++26) |
Parameters
| lhs, rhs, opt | - | an optional object to compare
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| value | - | value to compare to the contained value |
Return value
1)
lhs.has_value
(
)
!
= rhs.has_value
(
)
?
false
:
(lhs.has_value ( ) == false ? true : *lhs == *rhs)
(lhs.has_value ( ) == false ? true : *lhs == *rhs)
2)
lhs.has_value
(
)
!
= rhs.has_value
(
)
?
true
:
(lhs.has_value ( ) == false ? false : *lhs ! = *rhs)
(lhs.has_value ( ) == false ? false : *lhs ! = *rhs)
3)
!rhs ?
false
:
(
!lhs ?
true
:
*lhs <
*rhs)
4)
!lhs ?
false
:
(
!rhs ?
true
:
*lhs >
*rhs)
5)
!lhs ?
true
:
(
!rhs ?
false
:
*lhs <=
*rhs)
6)
!rhs ?
true
:
(
!lhs ?
false
:
*lhs >=
*rhs)
7)
lhs && rhs ?
*lhs <=>
*rhs : lhs.has_value
(
)
<=> rhs.has_value
(
)
8,9) !opt
10,11) opt.has_value()
12) false
13) opt.has_value()
14) !opt
15) true
16) opt.has_value()
17) false
18) true
19) !opt
20)
opt.has_value() <=> false
21)
opt.has_value
(
)
?
*opt == value :
false
22)
opt.has_value
(
)
? value ==
*opt :
false
23)
opt.has_value
(
)
?
*opt !
= value :
true
24)
opt.has_value
(
)
? value !
=
*opt :
true
25)
opt.has_value
(
)
?
*opt < value :
true
26)
opt.has_value
(
)
? value <
*opt :
false
27)
opt.has_value
(
)
?
*opt <= value :
true
28)
opt.has_value
(
)
? value <=
*opt :
false
29)
opt.has_value
(
)
?
*opt > value :
false
30)
opt.has_value
(
)
? value >
*opt :
true
31)
opt.has_value
(
)
?
*opt >= value :
false
32)
opt.has_value
(
)
? value >=
*opt :
true
33)
opt.has_value
(
)
?
*opt <=> value : std::
strong_ordering
::
less
Exceptions
1-7) May throw implementation-defined exceptions.
21-33) Throws when and what the comparison throws.
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_constrained_equality |
202403L |
(C++26) | constrained comparison operators for std::optional |
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2945 | C++17 | order of template parameters inconsistent for compare-with-T cases | made consistent |