[geos-devel] WKB parser templates status

[strk at refractions.net]

Few months later ...
I've found my default WKB io parameter class (biostringstream)
to be not-usable in older compilers. GCC-3.3 doesn't allow
basic_stringstream<unsigned char>, so we should provide a template
specializzation for it.

The point is that I'm not sure we should provide adapters, but
rather use whatever STL provides with an acceptably wider
availability.

Do you have suggestions about what to use for binary input/output ?
A commonly used class would be used to set interface constraints
(currently .read and .write).

--strk;


On Fri, Apr 29, 2005 at 07:34:58PM +0200, strk at refractions.net wrote:
> The template classes for WKB parser seems to be in a usable
> state. This is a report and a request for testing and/or
> discussing the idea beyond them.
> 
> WKB parser contains the first implementation of template
> classes into the GEOS code base. And my first implementation
> also. For the way I get it templates provide good performance
> w/out loosing the lack of compiler control we have in the
> C world with macros.
> 
> <rant>
> I like the idea, as we're developing
> free software, not one of those black boxes that require
> object-level extensibility as a mean of closing the sources
> (isn't this a use for virtual functions?).
> </rant>
> 
> Beside that template function can make code bigger, as every
> possible parameter will result in a class implementation
> duplication. WKB parsing was a good candidate as Martin idea
> of 'extensibility' has potentially two uses: a streamed WKB
> and a buffered WKB, of which only the second is actually
> implemented.
> 
> In GEOS this became:
> 	template <class T> ByteOrderDataInStreamT;
> 	template <class T> WKBWriterT;
> 	template <class T> WKBReaderT;
> 
> The parameter is something that can be .read(where, howmany)
> from for WKBReaderT and ByteOrderDataInStreamT and something
> that can be .write(what, howmany) to for WKBWriterT.
> "where" and "what" are 'pointer-to-byte' types.
> 
> I added a typedef for 'byte' in geos/geom.h:
> 
> 	typedef unsigned char byte;
> 
> A typedef for a convenient WKB parser parameter in geos/io.h:
> 
> 	typedef basic_stringstream<byte> biostringstream;
> 
> And typedefs for "default" template class instances in
> their respective headers (geos/WKBWriterT.h and geos/WKBReaderT.h):
> 
> 	typedef WKBWriterT<biostringstream> WKBWriter;
> 	typedef WKBReaderT<biostringstream> WKBReader;
> 
> All defininions will be available by including the geos.h header itself.
> 
> doc/example.cpp contains example code, where WKBWriter and WKBReader
> are used to double-check parser correctness by writing WKB, reading
> it back and comparing IN and OUT using the compareTo() function.
> 
> There is a single question left on this regard (comparing)
> 
> 	1) The ::compareTo() does not consider LINEARRING and LINESTRING
> 	   as the same thing. Should it ? There's nothing as LINEARRING
> 	   in the WKB spec, so the OUT geometry (the one read from WKB)
> 	   will *never* match the IN if that is a LINEARRING.
> 	   Using ::normalize() does not help, but does probably change
> 	   vertex orders, which somehow *invalidate* the test as WKB
> 	   reading and writing should not change vertex order.
> 	   Current code in doc/example.cpp uses normalize() and
> 	   compareTo().
> 
> Code and documentation (doxygen) is in CVS.
> 
> --strk;
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