NAME¶

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 >
constexpr bool conjunction_v = (library fundamentals TS v2)
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

#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');
}

Output:¶

All types are the same.
Types differ.

See also¶

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