std::experimental::disjunction

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

The specialization std::experimental::disjunction<B1, ..., BN>

• if sizeof...(B) == 0, std::false_type
• the first type Bi in B1, ..., BN for which bool(Bi::value) == true , or BN

The member names of the base class, other than disjunction and operator=, are not hidden and are unambiguously available in disjunction.

Disjunction is short-circuiting: if there is a template type argument Bi with bool (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

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<bool(B1::value), B1, disjunction<Bn...>>  {};

Notes

A specialization of disjunction does not necessarily inherit from of either std::true_type or std::false_type: it simply inherits from the first B whose ::value, explicitly converted to bool, is true, or from 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

Example

See also