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std::basic_ios::tie(3)	       C++ Standard Libary	  std::basic_ios::tie(3)

NAME
     std::basic_ios::tie - std::basic_ios::tie

Synopsis
	std::basic_ostream<CharT,	   Traits>*	    tie()	  const;
     (1)
	std::basic_ostream<CharT,   Traits>*   tie(    std::basic_ostream<CharT,
     Traits>* str  (2)
	);

	Manages the tied stream. A tied stream is an output stream which is syn-
     chronized
	with  the  sequence  controlled by the stream buffer (rdbuf()), that is,
     flush() is
	called on the tied stream before any input/output operation on *this.

	1) Returns the current tied stream. If there is no tied stream,  a  null
     pointer is
	returned.
	2)  Sets  the current tied stream to str. Returns the tied stream before
     the
	operation. If there is no tied stream, a null pointer  is  returned.  If
     str is not
	null and tie() is reachable by traversing the linked list of tied stream
     objects
	starting from str->tie(), the behavior is undefined.

Parameters
	str - an output stream to set as the tied stream

Return value
	The tied stream, or a null pointer if there was no tied stream.

Exceptions
	May throw implementation-defined exceptions.

Notes
	By  default,  the  standard  stream  std::cout	is  tied to std::cin and
     std::cerr.
	Similarly, its wide counterpart std::wcout  is	tied  to  std::wcin  and
     std::wcerr.

Example
     // Run this code

      #include <fstream>
      #include <iomanip>
      #include <iostream>
      #include <string>

      int main()
      {
	  std::ofstream os("test.txt");
	  std::ifstream is("test.txt");
	  std::string value("0");

	  os << "Hello";
	  is >> value;

	  std::cout << "Result before tie(): " << std::quoted(value) << "\n";
	  is.clear();
	  is.tie(&os);

	  is >> value;

	  std::cout << "Result after tie(): " << std::quoted(value) << "\n";
      }

Output:
      Result before tie(): "0"
      Result after tie(): "Hello"

	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
			   two streams could be tied to each
	LWG  835 C++98	    other^[1]				    the behavior
     is undefined
			   (either directly or through another	   in this case
			   intermediate stream object)

	 1. a std::basic_ostream::flush() is an UnformattedOutputFunction, so it
     creates a
	    sentry object while being called. When flush() is called on a stream
     object, the
	    constructor of the sentry object  will  call  flush()  on  its  tied
     stream, and that
	    flush()  will  construct  another  sentry object and its constructor
     will call
	    flush() on the tied stream of that stream and so on.  Therefore,  if
     streams a and
	    b are (directly or indirectly) tied to each other, calling a.flush()
     will
	    eventually call b.flush(), which will eventually call a.flush(), and
     will result
	    in an infinite loop.

http://cppreference.com 	   2024.06.10		  std::basic_ios::tie(3)
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