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std::experimental::disjunction(3) C++ Standard Libary std::experimental::disjunction(3)


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>
constexpr bool disjunction_v = (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...>> { };


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
(C++17) (class template)