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Difference between revisions of "cpp/types/negation"

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< cpp‎ | types
(LWG 2567)
m (→‎Example: Applied `static_assert`)
 
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{{cpp/title|negation}}
 
{{cpp/title|negation}}
−
{{cpp/types/navbar}}
+
{{cpp/meta/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | type_traits}}
+
{{dcl header|type_traits}}
−
{{dcl | since=c++17 | num=1 | 1=
+
{{dcl|since=c++17|1=
−
template<class B>
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template< class B >
 
struct negation;
 
struct negation;
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Forms the logical negation of the type trait {{tt|B}}.
+
Forms the [[enwiki:Negation|logical negation]] of the type trait {{c|B}}.
  
−
The type {{c|std::negation<B>}} is a {{concept|UnaryTypeTrait}} with a BaseCharacteristic of {{c|std::bool_constant<!bool(B::value)>}}.
+
The type {{c|std::negation<B>}} is a {{named req|UnaryTypeTrait}} with a base characteristic of {{c|std::bool_constant<!bool(B::value)>}}.
 +
 
 +
{{cpp/types/nospec|pv}}
  
 
===Template parameters===
 
===Template parameters===
 
{{par begin}}
 
{{par begin}}
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{{par | B | any type such that {{c|B::value}} is convertible to {{c|bool}} }}
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{{par|B|any type such that the expression {{c|bool(B::value)}} is a valid constant expression }}
 
{{par end}}
 
{{par end}}
  
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=== Helper variable template ===
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===Helper variable template===
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{{dcl begin}}
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{{ddcl|since=c++17|1=
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{{dcl | since=c++17 | 1=
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template< class B >
−
template<class B>
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constexpr bool negation_v = negation<B>::value;
 
constexpr bool negation_v = negation<B>::value;
 
}}
 
}}
−
{{dcl end}}
 
  
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{{cpp/types/integral_constant/inherit | {{tt|B}} has a member {{tt|::value}} that is {{c|false}} }}
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{{cpp/types/integral_constant/inherit|{{tt|B}} has a member {{c|::value}} that is {{c|false}} when explicitly converted to {{c|bool}}}}
  
 
===Possible implementation===
 
===Possible implementation===
 
{{eq fun
 
{{eq fun
−
| 1=
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|1=
 
template<class B>
 
template<class B>
−
struct negation : std::bool_constant<!B::value> { };
+
struct negation : std::bool_constant<!bool(B::value)> { };
 
}}
 
}}
 +
 +
===Notes===
 +
{{feature test macro|__cpp_lib_logical_traits|std=C++17|value=201510L|[[cpp/meta#Operations on traits|Logical operator type traits]]}}
  
 
===Example===
 
===Example===
−
{{example}}
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{{example
 +
|code=
 +
#include <type_traits>
 +
 
 +
static_assert(
 +
    std::is_same<
 +
        std::bool_constant<false>,
 +
        typename std::negation<std::bool_constant<true>>::type>::value,
 +
    "");
 +
static_assert(
 +
    std::is_same<
 +
        std::bool_constant<true>,
 +
        typename std::negation<std::bool_constant<false>>::type>::value,
 +
    "");
 +
 
 +
static_assert(std::negation_v<std::bool_constant<true>> == false);
 +
static_assert(std::negation_v<std::bool_constant<false>> == true);
 +
 
 +
int main() {}
 +
}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/types/dsc conjunction}}
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{{dsc inc|cpp/types/dsc conjunction}}
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{{dsc inc | cpp/types/dsc disjunction}}
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{{dsc inc|cpp/types/dsc disjunction}}
 +
{{dsc inc|cpp/types/dsc integral_constant}}
 
{{dsc end}}
 
{{dsc end}}
  
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[[de:cpp/types/negation]]
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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[[es:cpp/types/negation]]
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[[fr:cpp/types/negation]]
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[[it:cpp/types/negation]]
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[[ja:cpp/types/negation]]
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[[pt:cpp/types/negation]]
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[[ru:cpp/types/negation]]
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[[zh:cpp/types/negation]]
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Latest revision as of 12:18, 24 September 2024

 
C++
 
Metaprogramming library
Type traits
Type categories
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(C++11)
(C++11)  
(C++11)
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Type properties
(C++11)
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(C++11)(until C++20*)
(C++11)(deprecated in C++20)
(C++11)
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(C++20)
(C++23)
(C++17)
Type trait constants
(C++11)(C++17)(C++11)(C++11)
Metafunctions
(C++17)
(C++17)
negation
(C++17)
Supported operations
Relationships and property queries
Type modifications
(C++11)(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)
(C++11)
(C++20)
(C++11)
(C++17)

(C++11)
(C++11)
(C++20)
(C++11)
(C++11)(until C++20*)(C++17)
(C++20)
Compile-time rational arithmetic
Compile-time integer sequences
(C++14)
 
Defined in header <type_traits>
template< class B >
struct negation;
 (since C++17)

Forms the logical negation of the type trait B.

The type std::negation<B> is a UnaryTypeTrait with a base characteristic of std::bool_constant<!bool(B::value)>.

If the program adds specializations for std::negation or std::negation_v, the behavior is undefined.

Contents

[edit] Template parameters

B - any type such that the expression bool(B::value) is a valid constant expression

[edit] Helper variable template

template< class B >
constexpr bool negation_v = negation<B>::value;
 (since C++17)
[edit]

Inherited from std::integral_constant

Member constants

value
[static]
 true if B has a member ::value that is false when explicitly converted to bool, false otherwise
(public static member constant)

Member functions

operator bool
 converts the object to bool, returns value
(public member function)
operator()
(C++14)
 returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

[edit] Possible implementation

template<class B>
struct negation : std::bool_constant<!bool(B::value)> { };

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_logical_traits 201510L (C++17) Logical operator type traits

[edit] Example

Run this code
#include <type_traits>
 
static_assert(
    std::is_same<
        std::bool_constant<false>,
        typename std::negation<std::bool_constant<true>>::type>::value,
    "");
static_assert(
    std::is_same<
        std::bool_constant<true>,
        typename std::negation<std::bool_constant<false>>::type>::value,
    "");
 
static_assert(std::negation_v<std::bool_constant<true>> == false);
static_assert(std::negation_v<std::bool_constant<false>> == true);
 
int main() {}

[edit] See also

(C++17)
 variadic logical AND metafunction
(class template) [edit]
(C++17)
 variadic logical OR metafunction
(class template) [edit]
(C++11)(C++17)
 compile-time constant of specified type with specified value
(class template) [edit]
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