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

	   1.  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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