CGAL users discussion list

[Philipp Moeller <>]

Alessandro Attanasi 
<>
 writes:

> Ok, but why that procedure was working using all convex_hull
> functions?

It was working because you were using std::vector with a back_inserter
which takes care of those things. At least this is what your code
did. There can be situations where using a malloc'd pointer as an
OutputIterator can work (with a POD-type assignment effectively becomes
a memcpy, IIRC this is still undefined behavior though).

> I started from the cgal example
> http://doc.cgal.org/latest/Convex_hull_2/Convex_hull_2_2array_convex_hull_2_8cpp-example.html
>
> I freed the malloc at the end :)

Which is exactly the wrong thing to do because it won't call the
destructors of the elements. 

>
>
>
> On 15 January 2014 23:31, Philipp Moeller <
> >
>  wrote:
>
>> Alessandro Attanasi 
>> <>
>>  writes:
>>
>> > Sorry, really stupid error :P. It works as expected!
>> >
>> > Just last question about this problem: if I want to use C pointers
>> instead
>> > of stl iterators, why this code is wrong? It gives to me zero points on
>> the
>> > lower hull (I tried to use the same logic of the example for convex hull
>> > computation)
>> >
>> > K::Point_2 *ptr2;
>> >
>> >   K::Point_2 *result2 = NULL;
>> >
>> >   result2 = (K::Point_2*)malloc(totPoints * sizeof(K::Point_2));
>> >
>> >   ptr2 = CGAL::lower_hull_points_2( points, points+totPoints, result2 );
>>
>> Why would you ever call malloc instead of new in such a situation? It is
>> exactly where your problem comes from. You cannot simply use raw
>> uninitialized memory as an OutputIterator. You should wrap the pointer
>> into a raw_storage_iterator provided by the standard header memory. [1]
>>
>> You also need to take special care when releasing the memory (call the
>> destructor explicitly).
>>
>> >
>> >
>> >   ch_points = (ptr2 - result2);
>> >
>> >   std::cout << "6) Points on the lower hull = " << ch_points <<
>> std::endl;
>> >
>> >   for(int i =0;i<ch_points;i++)
>> >
>> >   {
>> >
>> >     std::cout << result2[i] << std::endl;
>> >
>> >   }
>>
>> Footnotes:
>> [1]  http://en.cppreference.com/w/cpp/memory/raw_storage_iterator