std::experimental::conjunction
| 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
BiinB1, ..., BNfor whichbool(Bi::value) == false, orBNif 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
#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
(C++17) |
variadic logical AND metafunction (class template) |
