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std::ios_base::xalloc(3)      C++ Standard Libary     std::ios_base::xalloc(3)

NAME
       std::ios_base::xalloc - std::ios_base::xalloc

Synopsis
	  static int xalloc();

	  Returns  a unique (program-wide) index value that can	be used	to ac-
       cess one	long and
	  one void* elements in	the private storage of std::ios_base by	 call-
       ing iword() and
	  pword(). The call to xalloc does not allocate	memory.

	  This	function is thread-safe; concurrent access by multiple threads
       does not	result
	  in a data race.
	  (since C++14)

	  Effectively	increments   a	 private   static   data   member   of
       std::ios_base, as if by
	  executing  return index++;, if index is the name of that static mem-
       ber
	  (which may be	std::atomic to support concurrent access  by  multiple
       threads,	or
	  otherwise synchronized)
	  (since C++14)

Parameters
	  (none)

Return value
	  Unique integer for use as pword/iword	index.

Example
	  Uses base class pword	storage	for runtime type identification	of de-
       rived stream
	  objects.

       // Run this code

	#include <iostream>

	template<class charT, class traits = std::char_traits<charT> >
	class mystream : public	std::basic_ostream<charT, traits>
	{
	 public:
	    static const int xindex;
	    mystream(std::basic_ostream<charT, traits>&	ostr) :
		std::basic_ostream<charT, traits>(ostr.rdbuf())
	    {
		 this->pword(xindex) = this;
	    }

	    void myfn()
	    {
		*this << "[special handling for	mystream]";
	    }
	};

	//  each  specialization  of mystream obtains a	unique index from xal-
       loc()
	template<class charT, class traits>
	const int mystream<charT, traits>::xindex = std::ios_base::xalloc();

	// This	I/O manipulator	will be	able to	recognize  ostreams  that  are
       mystreams
	// by looking up the pointer stored in pword
	template<class charT, class traits>
	std::basic_ostream<charT,traits>&		mymanip(std::basic_os-
       tream<charT,traits>& os)
	{
	 if (os.pword(mystream<charT,traits>::xindex) == &os)
	    static_cast<mystream<charT,traits>&>(os).myfn();
	 return	os;
	}

	int main()
	{
	    std::cout << "cout,	narrow-character test "	<< mymanip << '\n';

	    mystream<char> myout(std::cout);
	    myout << "myout, narrow-character test " <<	mymanip	<< '\n';

	    std::wcout << "wcout, wide-character test "	<< mymanip << '\n';

	    mystream<wchar_t> mywout(std::wcout);
	    mywout << "mywout, wide-character test " <<	mymanip	<< '\n';
	}

Output:
	cout, narrow-character test
	myout, narrow-character	test [special handling for mystream]
	wcout, wide-character test
	mywout,	wide-character test [special handling for mystream]

See also
		resizes	the private storage if necessary  and  access  to  the
       void* element at
	  pword	the given index
		(public	member function)
		resizes	 the  private  storage	if necessary and access	to the
       long element at the
	  iword	given index
		(public	member function)

http://cppreference.com		  2022.07.31	      std::ios_base::xalloc(3)

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