std::conjunction

 

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Defined in header <type_traits>
template<class... B> struct conjunction;	(1)	(since C++17)

Forms the logical conjunction of the type traits B....

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

B...

-

every type must be usable as a base class and define member B::value that is convertible to bool

Helper variable template

template<class... B> constexpr bool conjunction_v = conjunction<B...>::value;		(since C++17)

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

Run this code

// func is enabled if all Ts... have the same type
template<typename T, typename... Ts>
std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...> >
func(T, Ts...) {
 // TODO somethng to show
}

See also

negation (C++17)	logical NOT metafunction (class template)
disjunction (C++17)	variadic logical OR metafunction (class template)