Difference between revisions of "cpp/types/disjunction"
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Revision as of 09:20, 28 October 2015
| Defined in header <type_traits>
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| template<class... B> struct disjunction; |
(1) | (since C++17) |
Forms the logical disjunction of the type traits B... .
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 |
Template parameters
| B... | - | every type must be usable as a base class and define member B::value that is convertible to bool |
Helper variable template
| template<class... B> constexpr bool disjunction_v = disjunction<B...>::value; |
(since C++17) | |
Possible implementation
template<class...> struct disjunction; // not defined template<> struct disjunction<> : std::false_type { }; template<class B1> struct disjunction<B1> : B1 { }; template<class B1, class B2> struct disjunction<B1, B2> : std::conditional_t<B1::value, B1, B2> { }; template<class B1, class B2, class B3, class... Bn> struct disjunction<B1, B2, B3, Bn...> : std::conditional_t<B1::value, B1, disjunction<B2, B3, Bn...>> { }; |
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.
Example
| This section is incomplete Reason: no example |
See also
| (C++17) |
logical NOT metafunction (class template) |
| (C++17) |
variadic logical AND metafunction (class template) |
