table of contents
std::experimental::conjunction(3) | C++ Standard Libary | std::experimental::conjunction(3) |
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)
2024.06.10 | http://cppreference.com |