# std::negation

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< cpp‎ | types

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 `std::negation` or `std::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 | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_logical_traits` | 201510L | (C++17) | Logical operator type 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_constant bool_constant (C++11)(C++17) | compile-time constant of specified type with specified value (class template)

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