CGAL users discussion list

[Alessandro Attanasi <>]

Hi Philipp,

I understood the behaviour with the STL, I was wondering about C way usage of CGAL as the following , that is working fine even if I used malloc() and free() while for the function lower_hull_points_2() a similar approach doesn't work at all, I was expecting the same behaviour but I missing something.

thanks!

// definition of the input points

  K::Point_2 p1(2,5);

  K::Point_2 p2(0,4);

  K::Point_2 p3(0,0);

  K::Point_2 p4(4,0);

  K::Point_2 p5(4,4);

  K::Point_2 p6(1,3);

  K::Point_2 p7(1,1);

  K::Point_2 p8(3,1);

  K::Point_2 p9(3,3);

  K::Point_2 p10(2,2);

  

  int totPoints = 10;

 // array storing input points

  K::Point_2 *points = NULL;

  points = (K::Point_2*)malloc(totPoints * sizeof(K::Point_2));

  

  points[0] = p1;

  points[1] = p2;

  points[2] = p3;

  points[3] = p4;

  points[4] = p5;

  points[5] = p6;

  points[6] = p7;

  points[7] = p8;

  points[8] = p9;

  points[9] = p10;

  

  // array storing results

  K::Point_2 *result = NULL;

  result = (K::Point_2*)malloc(totPoints * sizeof(K::Point_2));

ptr = CGAL::ch_akl_toussaint( points, points+totPoints, result );

 // see how many points are into the convex hull

  int ch_points = (ptr - result);

  // print out the number of points of the convex hull and the points themselves

  std::cout << "1) Points on the convex hull = " << ch_points << std::endl;

  for(int i =0;i<ch_points;i++)

  {

    std::cout << result[i] << std::endl;

  }

 free(points);

  points = NULL;

  

  free(result);

  result = NULL;

On 15 January 2014 23:58, Philipp Moeller <> wrote:

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

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