Geometry Boost List: Re: [geometry] Using Boost.Geometry with a legacy class hierarchy

Subject: Re: [geometry] Using Boost.Geometry with a legacy class hierarchy
From: Barend Gehrels (barend_at_[hidden])
Date: 2012-02-21 14:31:53

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.

Regards, Barend