std::ios_base::xalloc(3) - FreeBSD Ports 15.1.quarterly Manual Pages

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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 access
     one long and
	one void* elements in the private storage of  std::ios_base  by  calling
     iword() and
	pword(). The call to xalloc does not allocate memory.

	This  function	is  thread-safe:  concurrent  access by multiple threads
     (since C++11)
	does not result in a data race.

	Effectively increments the next available unique index.

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

Example
	Uses base class pword storage for runtime type identification of derived
     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 xalloc()
      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  mys-
     treams
      // by looking up the pointer stored in pword
      template<class CharT, class Traits>
      std::basic_ostream<CharT,    Traits>&    mymanip(std::basic_ostream<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]

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	 Applied to   Behavior as published    Correct behavior
	LWG 2143 C++11	    xalloc was not thread-safe made thread-safe

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 	   2024.06.10		std::ios_base::xalloc(3)
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