# std::experimental::conjunction

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< cpp‎ | experimental

Defined in header `<experimental/type_traits>` |  |
template<class... B> struct conjunction; |  | (library fundamentals TS v2)

Forms the logical conjunction of the type traits `B...`, effectively performing a logical AND on the sequence of traits.

The specialization std::experimental::conjunction<B1, ..., BN> has a public and unambiguous base that is

- if sizeof...(B) == 0, std::true_type; otherwise
- the first type `Bi` in `B1, ..., BN` for which bool(Bi::value) == false, or `BN` if there is no such type.

The member names of the base class, other than `conjunction` and `operator=`, are not hidden and are unambiguously available in `conjunction`.

Conjunction is short-circuiting: if there is a template type argument `Bi` with bool(Bi::value) == false, then instantiating conjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for `j > i`.

### Template parameters

B... | - | every template argument `Bi` for which Bi::value is instantiated must be usable as a base class and define member `value` that is convertible to bool

### Helper variable template

template<class... B> constexpr bool conjunction_v = conjunction<B...>::value; |  | (library fundamentals TS v2)

### Possible implementation

    template<class...> struct conjunction : std::true_type { };
    template<class B1> struct conjunction<B1> : B1 { };
    template<class B1, class... Bn>
    struct conjunction<B1, Bn...>
        : std::conditional_t<bool(B1::value), conjunction<Bn...>, B1> {};

### Notes

A specialization of `conjunction` does not necessarily inherit from either std::true_type or std::false_type: it simply inherits from the first B whose ::value, converted to bool, is false, or from the very last B when all of them convert to true. For example, conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 4.

### Example

Run this code

    // func is enabled if all Ts... have the same type
    template<typename T, typename... Ts>
    std::enable_if_t<std::experimental::conjunction_v<std::is_same<T, Ts>...> >
    func(T, Ts...) {
     // TODO something to show
    }

### See also

conjunction (C++17) | variadic logical AND metafunction (class template)

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