std::disjunction
| Defined in header
<type_traits>
|
||
| template<class... B>
struct disjunction; |
(1) | (since C++17) |
Forms the logical disjunction of the type traits B..., effectively performing a logical or on the sequence of traits.
The BaseCharacteristic of a specialization std::disjunction <B1, ..., BN> is the first Bi for which Bi::value != false, or if every Bi::value == false, the BaseCharacteristic is BN.
If sizeof...(B) == 0, the BaseCharacteristic is std::false_type.
Disjunction is short-circuiting: if there is a template type argument Bi with 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 type must be usable as a base class and define member B::value that is convertible to bool |
[edit] Helper variable template
| template<class... B>
constexpr bool disjunction_v = disjunction<B...>::value; |
(since C++17) | |
[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<B1::value != false, B1, disjunction<Bn...>> { }; |
[edit] Notes
A specialization of disjunction does not necessarily have a BaseCharacteristic of either std::true_type or std::false_type: it simply inherits the base characteristic of the first B whose ::value, converted to bool, is true, or the base characteristic of the very last B when all of them convert to false. For example, std::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)
|
logical NOT metafunction (class template) |
| (C++17)
|
variadic logical AND metafunction (class template) |