SegmentString CHANGES (was: [geos-devel] SegmentString modeling)

[strk at refractions.net]

Just to have this in archive, the SegmentString class changed
it's semantic by *not* promising constness of the CoordinateSequence
passed at construction time.

Also, the getCoordinatesRO() method has been dropped and the
getCoordinates() has been changed to NOT return a clone, but
the CoordinateSequence passed at construction time.

--strk;

On Sat, Feb 18, 2006 at 04:32:06PM +0100, Mateusz Å?oskot wrote:
> strk at refractions.net wrote:
> >On Fri, Feb 17, 2006 at 06:55:02PM +0100, Mateusz Ã
?oskot wrote:
> >>strk at refractions.net wrote:
> >>
> >>>Need some help from C++ gurus. One of the new JTS classes expects to 
> >>>modify the Coordinates associated with a SegmentString.
> >>
> >>I wonder why SegmentString::pts is declared as
> >>const CoordinateSequence* pts;
> >>
> >>As I see, in JTS sources SegmentString::pts is not declared as const.
> >>I'm not a Java guru but IMHO const in Java has similar meaning
> >>as in C++.
> >
> >
> >I've been the 'const' pusher in GEOS. I tried adding
> >const-correctness here and there to help maintainance
> >and reduce unneeded copies.
> 
> I mostly agree but in this case const does not make better 
> const-correctness. Second, it causes problems as now while porting new 
> class which requires mutable access to SegmentString::pts which is 
> defined as const.
> 
> OK, I see one case when current solution makes const-correctness required:
> 
> void foo(const CoordinateSequence &a)
> {
>     // foo promises to not to modify 'a' object so
>     // it requires SegmentString to hold 'a' as const pointer - as
>     // it currently is defined
> 
>     // IMPORTANT!
>     // if SegmentString holds 'a' by non-const pointer then foo
>     // will fail because it cannot cast from const to non-const
> 
>     SegmentString b(&a);
> }
> 
> This sentence market as IMPORTANT! is very important :-) in case you 
> decide to move SegmentString::pts from const to non-const declaration - 
> what I also suggested in my previous replies.
> So, if there is some similar function defined as foo above then problems 
> may occur.
> I hope my explanation is clear. If my english it very poor then please 
> let me know I'll try to make it clearer ;-)
> 
> Yesterday, I consulted the problem with my friend and he suggested to 
> move this case to more "modern C++" design solution based on policies.
> Policy-based design of classes was introduced by Andrei Alexandrescu
> 
> http://en.wikipedia.org/wiki/Policy-based_design
> 
> and comes extremely handy in such cases, especially when you may expect 
> some design changes (just as this one about moving from const -> 
> non-const problem) in future.
> 
> Here is simple example how to handle const and non-const versions of the 
> same type using policies:
> 
> struct use_const
> {
>     typedef const A * pointer_type;
> };
> 
> struct dont_use_const
> {
>     typedef A * pointer_type;
> };
> 
> template <class ConstCorrectness>
> class B
> {
>     typedef typename ConstCorrectness::pointer_type pointer_type;
>     pointer_type a;
> public:
>     B(pointer_type p) : a(p) {}
>     pointer_type get() { return a; }
> };
> 
> 
> Now, you can define const version:
> 
> B<use_const> b(&a);
> 
> and non-const version:
> 
> B<dont_use_const> b(&a);
> 
> Both versions are interchangable and follows the Liskov Substitution 
> Principle:
> 
> http://www.objectmentor.com/resources/articles/lsp.pdf
> 
> 
> >I could take the const out of SegmentString but I guess
> >JTS code itself is not enough to tell what the intended
> >behaviour is.
> 
> That's usually the big problem when porting from Java or C# to C++ :-)
> 
> Cheers
> -- 
> Mateusz Łoskot
> http://mateusz.loskot.net
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> geos-devel at geos.refractions.net
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