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std::negation

From cppreference.com
< cpp‎ | types
 
 
Metaprogramming library
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(C++11)
(C++14)  
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(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++14)
(C++11)
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
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Type trait constants
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negation
(C++17)
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(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
 
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 >
inline constexpr bool negation_v = negation<B>::value;
(since C++17)

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

#include <iostream>
#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,
    "");
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << std::negation<std::bool_constant<true>>::value << '\n';
    std::cout << std::negation<std::bool_constant<false>>::value << '\n';
}

Output:

false
true

[edit] See also

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