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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.


[edit] 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

[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<bool(B1::value), conjunction<Bn...>, B1> {};

[edit] 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.

[edit] Example

#include <experimental/type_traits>
#include <iostream>
// func is enabled if all Ts... have the same type
template<typename T, typename... Ts>
constexpr std::enable_if_t<std::experimental::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...)
    std::cout << "All types are the same.\n";
template<typename T, typename... Ts>
constexpr std::enable_if_t<!std::experimental::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...)
    std::cout << "Types differ.\n";
int main()
    func(1, 2'7, 3'1);    
    func(1, 2.7, '3');    


All types are the same.
Types differ.

[edit] See also

variadic logical AND metafunction
(class template) [edit]