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 > has a public and unambiguous base that is

• if sizeof... ( B ) == 0 , std:: false_type ; otherwise
• the first type Bi in B1, ..., BN for which bool ( Bi :: value ) == true , or BN if there is no such type.

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