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

NAME
       std::disjunction	- std::disjunction

Synopsis
	  Defined in header <type_traits>
	  template<class... B>		   (since C++17)
	  struct disjunction;

	  Forms	 the  logical disjunction of the type traits B..., effectively
       performing a
	  logical OR on	the sequence of	traits.

	  The specialization std::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 oper-
       ator=, are not
	  hidden and are unambiguously available in disjunction.

	  Disjunction is short-circuiting: if there is a template  type	 argu-
       ment Bi with
	  bool(Bi::value)  !=  false,  then instantiating disjunction<B1, ...,
       BN>::value does
	  not require the instantiation	of Bj::value for j > i

	  The behavior of a program that adds specializations for  disjunction
       or disjunction_v
	  is undefined.

Template parameters
	  B...	-  every template argument Bi for which	Bi::value is instanti-
       ated must be usable
		 as a base class and define member value that  is  convertible
       to bool

	 Helper	variable template

	  template<class...						    B>
       (since C++17)
	  inline constexpr bool	disjunction_v =	disjunction<B...>::value;

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...>>  {
       };

Notes
	  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, std::disjunction<std::inte-
       gral_constant<int, 2>,
	  std::integral_constant<int, 4>>::value is 2.

	  The short-circuit instantiation differentiates disjunction from fold
       expressions: a
	  fold expression like (... || Bs::value) instantiates every B in  Bs,
       while
	  std::disjunction_v<Bs...>  stops instantiation once the value	can be
       determined. This
	  is particularly useful if the	later type is expensive	to instantiate
       or can cause a
	  hard error when instantiated with the	wrong type.

	  Feature-test macro: __cpp_lib_logical_traits

Example
       // Run this code

	#include <type_traits>
	#include <cstdint>
	#include <string>

	// values_equal<a, b, T>::value	is true	if and only if a == b.
	template <auto V1, decltype(V1)	V2, typename T>
	struct values_equal : std::bool_constant<V1 == V2> {
	  using	type = T;
	};

	// default_type<T>::value is always true
	template <typename T>
	struct default_type : std::true_type {
	  using	type = T;
	};

	// Now we can use disjunction like a switch statement:
	template <int I>
	using int_of_size = typename std::disjunction<	//
	    values_equal<I, 1, std::int8_t>,		//
	    values_equal<I, 2, std::int16_t>,		//
	    values_equal<I, 4, std::int32_t>,		//
	    values_equal<I, 8, std::int64_t>,		//
	    default_type<void>				// must	be last!
	    >::type;

	static_assert(sizeof(int_of_size<1>) ==	1);
	static_assert(sizeof(int_of_size<2>) ==	2);
	static_assert(sizeof(int_of_size<4>) ==	4);
	static_assert(sizeof(int_of_size<8>) ==	8);
	static_assert(std::is_same_v<int_of_size<13>, void>);

	// checking if Foo is constructible from double	will cause a hard  er-
       ror
	struct Foo {
	    template<class T>
	    struct  sfinae_unfriendly_check { static_assert(!std::is_same_v<T,
       double>); };

	    template<class T>
	    Foo(T, sfinae_unfriendly_check<T> =	{} );
	};

	template<class... Ts>
	struct first_constructible {
	    template<class T, class...Args>
	    struct is_constructible_x :	std::is_constructible<T, Args...> {
		using type = T;
	    };
	    struct fallback {
		static constexpr bool value = true;
		using type = void; // type to return if	nothing	is found
	    };

	    template<class... Args>
	    using  with	 =  typename   std::disjunction<is_constructible_x<Ts,
       Args...>...,
						   fallback>::type;
	};

	// OK, is_constructible<Foo, double> not instantiated
	static_assert(std::is_same_v<first_constructible<std::string,	  int,
       Foo>::with<double>,
				     int>);

	static_assert(std::is_same_v<first_constructible<std::string,
       int>::with<>, std::string>);
	static_assert(std::is_same_v<first_constructible<std::string,
       int>::with<const	char*>,
				     std::string>);
	static_assert(std::is_same_v<first_constructible<std::string,
       int>::with<void*>, void>);

	int main() { }

See also
	  negation    logical NOT metafunction
	  (C++17)     (class template)
	  conjunction variadic logical AND metafunction
	  (C++17)     (class template)

http://cppreference.com		  2022.07.31		   std::disjunction(3)

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