std::disjunction

From cppreference.com
< cpp‎ | types
 
 
 
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Type categories
(C++11)
(C++14)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++14)
(C++11)
Type trait constants
Metafunctions
(C++17)
disjunction
(C++17)
(C++17)
Supported operations
Relationships and property queries
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type modifications
(C++11)(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Type transformations
(C++11)
(C++11)
(C++11)
(C++11)
(C++17)
(C++11)
(C++11)
(C++11)
(C++11)
 
Defined in header <type_traits>
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

[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

[edit] See also

(C++17)
logical NOT metafunction
(class template)
(C++17)
variadic logical AND metafunction
(class template)