template< class... B >
struct conjunction;
Forms the logical conjunction of the type traits B..., effectively performing a logical AND on the sequence of traits.
The specialization std::conjunction<B1, ..., BN> has a public and unambiguous base that is
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.
If the program adds specializations for std::conjunction or std::conjunction_v, the behavior is undefined.
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;
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, explicitly converted to bool, is false, or from 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.
The short-circuit instantiation differentiates conjunction from fold expressions: a fold expression, like (... && Bs::value), instantiates every B in Bs, while std::conjunction_v<Bs...> stops instantiation once the value can be determined. This is particularly useful if the later type is expensive to instantiate or can cause a hard error when instantiated with the wrong type.
#include <iostream>
#include <type_traits>
// func is enabled if all Ts... have the same type as T
template<typename T, typename... Ts>
std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...)
{
std::cout << "All types in pack are the same.\n";
}
// otherwise
template<typename T, typename... Ts>
std::enable_if_t<!std::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...)
{
std::cout << "Not all types in pack are the same.\n";
}
template<typename T, typename... Ts>
constexpr bool all_types_are_same = std::conjunction_v<std::is_same<T, Ts>...>;
static_assert(all_types_are_same<int, int, int>);
static_assert(not all_types_are_same<int, int&, int>);
int main()
{
func(1, 2, 3);
func(1, 2, "hello!");
}
Output:
All types in pack are the same. Not all types in pack are the same.[edit] See also
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