std::experimental::disjunction (3) - Linux Manuals

std::experimental::disjunction: std::experimental::disjunction


std::experimental::disjunction - std::experimental::disjunction


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

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

Helper variable template

template<class... B> (library fundamentals TS v2)
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<bool(B1::value), B1, disjunction<Bn...>> { };


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.


 This section is incomplete
 Reason: no example

See also

disjunction variadic logical OR metafunction
            (class template)