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std::basic_...buf::seekpos(3) C++ Standard Libarystd::basic_...buf::seekpos(3)

NAME
       std::basic_filebuf::seekpos - std::basic_filebuf::seekpos

Synopsis
	  protected:

	  virtual pos_type seekpos( pos_type sp,

	  std::ios_base::openmode      which	 =     std::ios_base::in     |
       std::ios_base::out );

	  Repositions the file pointer,	if possible, to	the position indicated
       by sp.

	  If the associated file is not	open (is_open()==false), fails immedi-
       ately.

	  If the file is open for writing, first writes	the put	area  and  any
       unshift sequences
	  required by the currently imbued locale, using overflow()

	  Then repositions the file pointer, as	if by calling std::fsetpos()

	  If the file is open for reading, updates the get area	if necessary.

	  If sp	was not	obtained by calling seekoff() or seekpos() on the same
       file, the
	  behavior is undefined.

Parameters
	  sp	- file position	obtained by seekoff() or seekpos() called ear-
       lier on the same
		  file
		  defines  which  of  the input	and/or output sequences	to af-
       fect. It	can be one
		  or a combination of the following constants:
	  which	-
		  Constant Explanation
		  in	   affect the input sequence
		  out	   affect the output sequence

Return value
	  sp on	success	or pos_type(off_type(-1)) on failure.

Notes
	  seekpos() is called by std::basic_streambuf::pubseekpos(), which  is
       called by the
	  single-argument versions of std::basic_istream::seekg() and
	  std::basic_ostream::seekp()

	  Many	implementations	do not update the get area in seekpos(), dele-
       gating to
	  underflow() that is called by	the next sgetc()

Example
	  on some implementations, the get area	is emptied  by	seekpos()  and
       the second
	  underflow() is necessary to observe the effects

       // Run this code

	#include <fstream>
	#include <iostream>

	struct mybuf : std::filebuf
	{
	    pos_type seekpos(pos_type sp, std::ios_base::openmode which) {
		 std::cout  <<	"Before	seekpos(" << sp	<< "), size of the get
       area is "
			   << egptr()-eback() << " with	"
			   << egptr()-gptr() <<	" read positions available\n";
		 pos_type rc = std::filebuf::seekpos(sp, which);
		 std::cout << "seekpos() returns "  <<	rc  <<	".\nAfter  the
       call, "
			   << "size of the get area is "
			   << egptr()-eback() << " with	"
			   << egptr()-gptr() <<	" read positions available\n";
	// uncomment if	get area is emptied by seekpos()
	//	   std::filebuf::underflow();
	//	    std::cout  <<  "after  forced underflow(), size of the get
       area is "
	//		     <<	egptr()-eback()	<< " with "
	//		     <<	egptr()-gptr()	<<  "  read  positions	avail-
       able\n";
		return rc;
	    }
	};

	int main()
	{
	    mybuf buf;
	    buf.open("test.txt", std::ios_base::in);
	    std::istream stream(&buf);
	    stream.get(); // read one char to force underflow()
	    stream.seekg(2);
	}

Possible output:
	Before seekpos(2), size	of the get area	is 110 with 109	read positions
       available
	seekpos() returns 2.
	After  the  call,  size	of the get area	is 110 with 108	read positions
       available

See also
	  pubseekpos invokes seekpos()
		     (public	member	  function    of    std::basic_stream-
       buf<CharT,Traits>)
	  seekoff    repositions the file position, using relative addressing
	  [virtual]  (virtual protected	member function)
	  fseek	      moves the	file position indicator	to a specific location
       in a file
		     (function)

http://cppreference.com		  2022.07.31	 std::basic_...buf::seekpos(3)

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