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

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

Synopsis
	protected:

	virtual  pos_type  seekpos( pos_type sp,			 (depre-
     cated in C++98)
				  std::ios_base::openmode which =	(removed
     in C++26)

				      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:

	  * If the next pointer is null, the operation fails.
	  * 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.
	  *  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 seekpos()
		defines whether the input sequences,  the  output  sequence,  or
     both are
		affected.  It  can be one or a combination of the following con-
     stants:
	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 <cstring>
      #include <iostream>
      #include <strstream>

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

	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	 55	C++98	       seekpos	   returned	an     undefined
     pos_type(off_type(-1))
			  invalid stream position  on  failure	is  returned  on
     failure

See also
	seekoff   repositions the next pointer in the input sequence, output se-
     quence, or
	[virtual] both, using relative addressing
		  (virtual protected member function)
		  repositions the next pointer in the input sequence, output se-
     quence, or
	seekpos   both using absolute addressing
	[virtual]  (virtual  protected	member	function  of  std::basic_stream-
     buf<CharT,Traits>)

		  repositions the next pointer in the input sequence, output se-
     quence, 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>)

http://cppreference.com 	   2024.06.10	   std::strstreambuf::seekpos(3)
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