CGAL users discussion list

[Efraim Fogel <>]

This method, (void _Polyline_2::push_back (const Point_2 & p)) has a 
precondition that 'segments' is not empty. It is missing, and I'll add 
it to the code.

Is it used anywhere in a way that violates this precondition?

Olivier Tournaire wrote:
> Hi,
>
> The code in the push_back method is the following :
>
>   void push_back (const Point_2 & p)
>   {
>     Point_2 pt = p;
>     Point_2 ps = segments.back().target();
>     segments.push_back (Segment_2 (ps, pt));
>   }
>
> As I use the empty constructor, the std::vector<Segment_2> segments; 
> is empty. Then, if I want to insert a point in the polyline, 
> segments.back() returns a null iterator, and, .target() on a null 
> iterator certainly crashes. So, how can we use this method ? Maybe I 
> did not understand the underlying concept of CGAL polylines ...
>
> Regards,
>
> Olivier
>
> 2008/3/4, Olivier Tournaire < 
> <mailto:>>:
>
>     Hi all,
>
>     I have a problem with a push_back on a polyline. Here are some
>     samples of my code :
>
>     typedef CGAL::Arr_polyline_traits_2< CGAL::Arr_segment_traits_2<
>     CGAL::Cartesian<int> > >::Curve_2 polyline_int;
>
>     // ...
>
>     polyline_int polyline;
>     chains.push_back( polyline ); // chains is a std::vector<
>     polyline_int >
>     MakeAChain( *copy , NLD[iNode].position , iDirection ,
>     chains[chains.size()-1] , FinalNode , FinalDirection );
>
>     // MakeAChain
>     void MakeAChain( itk::Image<unsigned char,2> &image , CGAL_Point2i
>     BeginPoint , unsigned char BeginDirection , polyline_int
>     &CurrentPolyline , CGAL_Point2i EndPoint , unsigned char
>     EndDirection )
>     {
>         // ...
>         CurrentPolyline.push_back( BeginPoint ); // crashes because
>     CurrentPolyline.segments is NULL (from the debugger)
>         // ...
>     }
>
>     I do not understand why CurrentPolyline.segments is NULL. Could
>     you please help ?
>
>     Regards.
>
>     Olivier
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