cpp/io/basic filebuf/seekpos

Repositions the file pointer, if possible, to the position indicated by. If the associated file is not open, fails immediately.

Reposition performs as follows: @1@ If the file is open for writing, writes the put area and any unshift sequences required by the currently imbued locale, using overflow.

@2@ Repositions the file pointer, as if by calling std.

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

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

Return value
on success or on failure.