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

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


[edit] Template parameters

B... - every template argument Bi for which Bi::value is instantiated must be usable as a base class and define member value that is convertible to bool

[edit] Helper variable template

template<class... B>
constexpr bool disjunction_v = disjunction<B...>::value;
(library fundamentals TS v2)

[edit] 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...>>  { };

[edit] 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.

[edit] Example

[edit] See also

variadic logical OR metafunction
(class template) [edit]