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

NAME
       std::strstreambuf::seekoff - std::strstreambuf::seekoff

Synopsis
	  protected:

	  virtual pos_type seekoff(off_type off,
	  ios_base::seekdir way,

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

	  Repositions	std::basic_streambuf::gptr  and/or  std::basic_stream-
       buf::pptr, if
	  possible, to the position that corresponds to	exactly	off characters
       from beginning,
	  end, or current position of the get and/or put area of the buffer.

	    * If which includes	ios_base::in and this buffer is	open for read-
       ing, then
	      repositions the read pointer  std::basic_streambuf::gptr	inside
       the get area as
	      described	below
	    *  If  which  includes  ios_base::out  and this buffer is open for
       writing,	then
	      repositions the write pointer std::basic_streambuf::pptr	inside
       the put area as
	      described	below
	    *  If  which  includes both	ios_base::in and ios_base::out and the
       buffer is open for
	      both reading and writing,	and when is  either  ios_base::beg  or
       ios_base::end,
	      then  repositions	 both read and write pointers as described be-
       low.
	    * Otherwise, this function fails.

	  If the pointer (either gptr or pptr or both) is repositioned,	it  is
       done as follows:

	  1)  If  the pointer to be repositioned is a null pointer and the new
       offset newoff
	  would	be non-zero, this function fails.
	  2) The new pointer offset newoff of type off_type is determined
	  a) if	way == ios_base::beg, then newoff is zero
	  b) if	way == ios_base::cur, then newoff is the current  position  of
       the pointer
	  (gptr()-eback() or pptr()-pbase())
	  c)  if way ==	ios_base::end, then newoff is the length of the	entire
       initialized part
	  of the buffer	(if overallocation is used, the	high watermark pointer
       minus the
	  beginning pointer)
	  3) If	newoff + off is	negative or out	of bounds of  the  initialized
       part of the
	  buffer, the function fails
	  4)  Otherwise,  the  pointer is assigned as if by gptr() = eback() +
       newoff +	off or
	  pptr() = pbase() + newoff + off

Parameters
	  off	- relative position to set the next pointer(s) to
		  defines base position	to apply the relative  offset  to.  It
       can be one of the
		  following constants:

	  way	- Constant Explanation
		  beg	   the beginning of a stream
		  end	   the ending of a stream
		  cur	   the current position	of stream position indicator
		  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
	  pos_type(newoff) on success, pos_type(off_type(-1)) on  failure  and
       if pos_type
	  cannot represent the resulting stream	position.

Example
       // Run this code

	#include <iostream>
	#include <strstream>

	int main()
	{
	    char a[] = "123";
	    std::strstream ss(a, sizeof	a); // in/out
	    std::cout << "put pos = " << ss.tellp()
		      << " get pos = " << ss.tellg() <<	'\n';

	    // absolute	positioning both pointers
	    ss.rdbuf()->pubseekoff(1,  std::ios_base::beg);  //	move both for-
       ward
	    std::cout << "put pos = " << ss.tellp()
		      << " get pos = " << ss.tellg() <<	'\n';

	    // try to move both	pointers 1 forward from	current	position
	    if(-1 == ss.rdbuf()->pubseekoff(1, std::ios_base::cur))
		std::cout <<  "moving  both  pointers  from  current  position
       failed\n";
	    std::cout << "put pos = " << ss.tellp()
		      << " get pos = " << ss.tellg() <<	'\n';

	    // move the	write pointer 1	forward, but not the read pointer
	    // can also	be called as ss.seekp(1, std::ios_base::cur);
	    ss.rdbuf()->pubseekoff(1, std::ios_base::cur, std::ios_base::out);
	    std::cout << "put pos = " << ss.tellp()
		      << " get pos = " << ss.tellg() <<	'\n';

	    ss << 'a'; // write	at put position
	    std::cout << "Wrote	'a' at put position, the buffer	is now:	'";
	    std::cout.write(a, sizeof a);
	    std::cout << "'\n";
	    char ch;
	    ss >> ch;
	    std::cout << "reading at get position gives	'" << ch << "'\n";
	}

Output:
	put pos	= 0 get	pos = 0
	put pos	= 1 get	pos = 1
	moving both pointers from current position failed
	put pos	= 1 get	pos = 1
	put pos	= 2 get	pos = 1
	Wrote 'a' at put position, the buffer is now: '12a'
	reading	at get position	gives '2'

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

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

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