std::experimental::conjunction (3) Linux Manual Page

std::experimental::conjunction – std::experimental::conjunction

Synopsis

Defined in header<experimental / type_traits>

    template <class... B>
    (library fundamentals TS v2)

        struct conjunction;

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> (library fundamentals TS v2)
constexpr bool conjunction_v = conjunction<B…>::value;

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