std::disjunction

template<class... B>
struct disjunction;

Forms the logical disjunction of the type traits B..., effectively performing a logical or on the sequence of traits.

The BaseCharacteristic of a specialization std::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

Helper variable template

template<class... B>
constexpr bool disjunction_v = disjunction<B...>::value;

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...>>  { };

Notes

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, std::disjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 2.

Example

See also