Defined in header <experimental/type_traits>
template<class... B>
struct disjunction;
(library fundamentals TS v2)

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

The BaseCharacteristic of a specialization disjunction<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::false_type.

Disjunction is short-circuiting: if there is a template type argument Bi with Bi::value != false, then instantiating disjunction<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 disjunction_v = disjunction<B...>::value;
(library fundamentals TS v2)

Possible implementation

template<class...> struct disjunction : std::false_type { };
template<class B1> struct disjunction<B1> : B1 { };
template<class B1, class... Bn>
struct disjunction<B1, Bn...> : std::conditional_t<B1::value != false, B1, disjunction<Bn...>>  { };


A specialization of disjunction 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 true, or the base characteristic of the very last B when all of them convert to false. For example, disjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 2.


See also

variadic logical OR metafunction
(class template)