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std::strstreambuf::seekpos(3) C++ Standard Libarystd::strstreambuf::seekpos(3)

NAME
       std::strstreambuf::seekpos - std::strstreambuf::seekpos

Synopsis
	  protected:

	  virtual pos_type seekpos(pos_type sp,

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

	  Repositions  std::basic_streambuf::gptr  and/or   std::basic_stream-
       buf::pptr, if
	  possible, to the position indicated by sp

	  If  std::ios_base::in	is set in which, attempts to reposition	gptr()
       (the next
	  pointer in the get area). If std::ios_base::out is set in which, at-
       tempts to
	  reposition pptr() (the next pointer in the put area).	If neither bit
       is set in
	  which, the operation fails.

	  Each next pointer is repositioned as follows:

	  1) If	the next pointer is null, the operation	fails
	  2) Otherwise,	the new	offset newoff (of type off_type) is determined
       by calling
	  sp.offset(). If newoff is negative, out of bounds of the buffer,  or
       invalid,	the
	  operation fails.
	  3)  Otherwise,  the  next  pointer  is  assigned  as	if by gptr() =
       eback()+newoff or
	  pptr() = pbase()+newoff.

Parameters
	  sp	- stream position, such	as one obtained	by seekoff() or	 seek-
       pos()
		  defines whether the input sequences, the output sequence, or
       both are
		  affected.  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
	  The  resultant  offset  converted  to	  pos_type   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()

Example
       // Run this code

	#include <strstream>
	#include <cstring>
	#include <iostream>

	struct mybuf : std::strstreambuf
	{
	    mybuf(const	char* str) : std::strstreambuf(str,  std::strlen(str))
       {}
	    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::strstreambuf::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";
		return rc;
	    }
	};

	int main()
	{
	    mybuf buf("12345");
	    std::iostream stream(&buf);
	    stream.seekg(2);
	}

Output:
	Before	seekpos(2),  size  of  the get area is 5 with 5	read positions
       available
	seekpos() returns 2.
	After the call,	size of	the get	area is	5 with 3 read positions	avail-
       able

See also
	  pubseekpos invokes seekpos()
		     (public	member	  function    of    std::basic_stream-
       buf<CharT,Traits>)
		     repositions  the next pointer in the input	sequence, out-
       put sequence, or
	  seekpos    both using	absolute addressing
	  [virtual]  (virtual protected	member function	of
		     std::basic_stringbuf<CharT,Traits,Allocator>)
	  seekpos    repositions the file position, using absolute addressing
	  [virtual]  (virtual protected	member	function  of  std::basic_file-
       buf<CharT,Traits>)
	  seekoff     repositions the next pointer in the input	sequence, out-
       put sequence, or
	  [virtual]  both, using relative addressing
		     (virtual protected	member function)

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

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