std::experimental::disjunction
| Defined in header <experimental/type_traits>
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| 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
BiinB1, ..., BNfor which bool(Bi::value) == true, orBNif 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
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Helper variable template
| template< class... B > constexpr bool disjunction_v = disjunction<B...>::value; |
(library fundamentals TS v2) | |
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
| This section is incomplete Reason: no example |
See also
| (C++17) |
variadic logical OR metafunction (class template) |