Re: [Swig-user] A way to instantiate the correct wrapper from a base class pointer?

[William S F. <ws...@fu...>]

David Piepgrass wrote:
> Suppose I get my shapes from a ShapeProvider, which is some sort of
> collection:
> 
> 	class Shape {
> 		virtual const char* Name() { return "Shape"; }
> 	};
> 	class Square : public Shape {
> 		virtual const char* Name() { return "Square"; }
> 		int SideLength() { return 10; }
> 	};
> 	class Circle : public Shape {
> 		virtual const char* Name() { return "Square"; }
> 		int Radius() { return 10; }
> 	};
> 	class ShapeProvider {
> 	private:
> 		Square s;
> 		Circle c;
> 	public:
> 		Shape* Get(int index) {
> 			if (index == 0) return &s; else return &c;
> 		}
> 	};
> 
> Unfortunately, when Get() is called, the SWIG creates a Shape wrapper
> rather than a wrapper for the actual type of the object (C# example):
> 
>   public Shape Get(int index) {
>     IntPtr cPtr = NaviPINVOKE.ShapeProvider_Get(swigCPtr, index);
>     Shape ret = (cPtr == IntPtr.Zero) ? null : new Shape(cPtr, false);
>     return ret;
>   }
> 
> What can I do to get the correct wrapper created? Preferably with only
> one P/Invoke call...
> 

See the discussion of downcasts in 
http://www.swig.org/Doc1.3/Java.html#adding_downcasts. This is mostly 
relevant to C#. You'd have to replace the JNI code in the final Vehicle 
*vehicle_factory out typemap with something more elaborate using 
delegates. Or have just a csout typemap for Vehicle *vehicle_factory 
which calls into unmanaged code to cycle through make the different 
possible C++ dynamic_cast calls.

William