Difference between revisions of "cpp/types/conjunction"

Revision as of 00:09, 13 October 2016

Forms the logical conjunction of the type traits B..., effectively performing a logical AND on the sequence of traits.

The BaseCharacteristic of a specialization std::conjunction<B1, ..., BN> is the first Bi for which Bi::value == false, or if every Bi::value != false, the BaseCharacteristic is BN.

If sizeof...(B) == 0, the BaseCharacteristic is std::true_type.

Conjunction is short-circuiting: if there is a template type argument Bi with Bi::value == false, then instantiating conjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for j > i

Template parameters

Helper variable template

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<B1::value != false, conjunction<Bn...>, B1>  {};

Notes

A specialization of conjunction does not necessarily have a BaseCharacteristic of either std::true_type or std::false_type: it simply inherits the base characteristic of the first B whose ::value, converted to bool, is false, or the base characteristic of the very last B when all of them convert to true. For example, std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 4.

Example

#include <iostream>
#include <type_traits>
#include <experimental/type_traits>
 
// 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...) {
 std::cout << "all types in the pack are the same" << std::endl;
}
 
template<typename... Ts>
void func(Ts...) {
    std::cout << "not all types are the same" << std::endl;
}
 
int main() {
    func(1, 2, 3);
    func(1, 2, "hello!");
}

See also