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Subject: Re: [geometry] Using Boost.Geometry with a legacy class hierarchy
From: Mats Taraldsvik (mats.taraldsvik_at_[hidden])
Date: 2012-02-22 06:53:22

Hi Barend,

On 02/21/2012 08:31 PM, Barend Gehrels wrote:
> Hi Mats,
> On 21-2-2012 12:43, Mats Taraldsvik wrote:
>> Hi Barend,
>> On 02/20/2012 07:21 PM, Barend Gehrels wrote:
>>> Hi Mats,
>>> On 20-2-2012 9:03, Mats Taraldsvik wrote:
>>>> The object hierarcy is a bit more complex than my current example
>>>> (also contains 2d, 3d points, curves etc.), and objects are stored
>>>> in pointer to base/super-containers. Therefore, I can't remove the
>>>> pointer requirement, I'm afraid. I guess creating separate pointer
>>>> and non-pointer specializations (e.g. point_type and
>>>> point_type_ptr) since they are so different, means either a lot of
>>>> duplicated code, or a very difficult problem...
>>>> I was also hoping to overcome the pointer-to-base container issue
>>>> also, but thought it was best to take one step at a time. :)
>>> OK, so the first solution I mailed is still based on pointers. Does
>>> that work for you?
>> After re-reading your email, I realise that I misinterpreted it --
>> the version you suggested, using references instead of pointers,
>> should work, since it does not modify my hierarchy.
>> --
>> However, since I the structure is a bit more complex, and I need to
>> worry about casting from base pointers etc. later, I want to use the
>> pointer version, as long as it works and behaves correctly.
>> I managed to fix the issue in QRing, without modifying Boost.Geometry. :)
>> Basically, I needed to change the ConstRingIterator :
>> from:
>> typedef RingIteratorImpl<std::vector<QLineString*>::const_iterator,
>> const QPoint*> ConstRingIterator;
>> to:
>> typedef RingIteratorImpl<std::vector<QLineString*>::const_iterator,
>> QPoint* const> ConstRingIterator;
>> and then it woked perfectly.
>> However, I'm not sure if the semantics are right -- shouldn't the
>> QPoint be const as well as the pointer (i.e. const QPoint* const)?
>> When ConstRingIterator is changed to (const QPoint* const), it does
>> not compile (perhaps it is because of lacking specializations (I have
>> defined QPoint* and const QPoint*).
>> I updated my gist with the latest version [1] (a diff userscript is
>> available for convenience, if you're not using git [2])
> OK, great that it compiles.
> I agree, the const-pointer does not mean that the pointee cannot be
> modified so indeed it should be expected as const Qpoint* const (or,
> in notation I prefer, QPoint const* const).
> The following does compile now:
> void test(QRing const& ring)
> {
> boost::range_iterator<QRing const>::type cri = range_begin(ring);
> (*cri)->x = 5;
> std::cout << boost::geometry::wkt(*cri) << std::endl;
> }
> Which should not compile - it gives the wrong expectations.
> The problem with the pointer-version is that QPoint* and QPoint const*
> has to be specialized separately, and are therefore considered
> differently in our library. That is not easy to solve. We will discuss
> that internally.

Ok, so even though I define the four "versions" of QPoint (QPoint
const*const, QPoint* const, QPoint const* and QPoint*), Boost.Geometry
can't properly handle/distinguish between the QPoint* and QPoint const*?

I guess the only or best available workaround is to iterate through the
QPoint*, and return references to QPoint?

I hope you can solve this. :) If you want to use the code as an example
of adapting legacy classes/objects in the documentation, I'll be happy
to write a few accompanying paragraphs. :)

> Regards, Barend


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