std::experimental::disjunction

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Technical specifications

Filesystem library (filesystem TS)

Library fundamentals (library fundamentals TS)

Library fundamentals 2 (library fundamentals 2 TS)

Extensions for parallelism (parallelism TS)

Concepts (concepts TS)

Extensions for concurrency (concurrency TS)

 

Library fundamentals v2

experimental::propagate_const

experimental::not_fn

experimental::observer_ptr

experimental::make_array

experimental::ostream_joiner

experimental::gcd

experimental::lcm

experimental::source_location

experimental::randint

detection idiom

uniform container erasure

logical operator type traits

 

Defined in header <experimental/type_traits>
template<class... B> struct disjunction;		(library fundamentals TS v2)

Forms the logical disjunction of the type traits B....

The BaseCharacteristic of a specialization 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

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;		(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<B1::value != false, B1, disjunction<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, disjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 2.

See also

disjunction (C++17)

variadic logical OR metafunction (class template)