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# std::experimental::conjunction

From cppreference.com
< cpp‎ | experimental

| Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 2/2016, see `std::conjunction` (since C++17)

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

Forms the logical conjunction of the type traits `B...`.

The BaseCharacteristic of a specialization conjunction<B1, ..., BN> is the first `Bi` for which Bi::value == false, or if every Bi::value != false, the BaseCharacteristic is `BN`.

If sizeof...(B) == 0, the BaseCharacteristic is std::true_type.

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

### [edit] Template parameters

B... | - | every type must be usable as a base class and define member B::value that is convertible to bool

### [edit] Helper variable template

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

### [edit] 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<B1::value != false, conjunction<Bn...>, B1>  {};

### [edit] Notes

A specialization of `conjunction` does not necessarily have a BaseCharacteristic of either std::true_type or std::false_type: it simply inherits the base characteristic of the first B whose ::value, converted to bool, is false, or the base characteristic of 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.

### [edit] 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
    }

### [edit] See also

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

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