std::negation

 

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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)>.

The behavior of a program that adds specializations for negation or negation_v is undefined.

Template parameters

B

-

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

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>

Possible implementation

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

Notes

Feature-test macro:

__cpp_lib_logical_traits

Example

Run this code

#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

See also

conjunction (C++17)	variadic logical AND metafunction (class template)
disjunction (C++17)	variadic logical OR metafunction (class template)
integral_constantbool_constant (C++11)(C++17)	compile-time constant of specified type with specified value (class template)