std::experimental::disjunction
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
inB1, ..., BN
for which bool(Bi::value) == true, orBN
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 shortcircuiting: 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
Contents 
[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
This section is incomplete Reason: no example 
[edit] See also
(C++17) 
variadic logical OR metafunction (class template) 