Does Cython support dynamic_cast?

[Jasmine Sandhu]

Say I have the following C++ class:

namespace shapes {

// Simply a base class with virtual destructor

class DLL Polygon {

public:

Polygon();

virtual ~Polygon();

virtual string Type();

string _type;

virtual double getArea()=0;

};

class DLL Rectangle : public virtual Polygon {      // virtual inheritance

    public:

        double area;

Rectangle(double x0, double y0, double x1, double y1);

~Rectangle();

double getLength(); // Add method that does not exist in Base class

string Type();    // override base function

double getArea();   // implement virtual method

    };

Since Rectangle inherits virtual Polygon, in C++, I would need to do a dynamic cast

to access the methods for the Rectangle class. See example below:

        Polygon * polygon = new Rectangle(1,2,3,4);

Rectangle * rect;

rect = dynamic_cast<Rectangle *>(polygon);        // dynamic cast

cout << polygon->Type() << endl;

cout << "area: "<< polygon->getArea() << endl;

cout << rect->getLength() << endl;

I know cython supports the following for casting:

    cdef Polygon * polygon  

    cdef Rectangle * rect

    polygon = new Rectangle(x0,y0,x1,y1)

    rect = <Rectangle *>polygon                   // static cast

However, since Rectangle does virtual inheritance of Polygon, I get a compiler error:

Rectangle/cy_rectangle.cpp:1557: error: cannot convert from base ‘shapes::Polygon’ to derived type ‘shapes::Rectangle’ via virtual base ‘shapes::Polygon’

Can I do a dynamic_cast in Cython?

Thanks,

Jasmine

[Bradley Froehle]

You can think of the dynamic_cast as a function with a template parameter which Cython doesn't yet support.  But you can implement it as:

cdef extern from "*"

    Rectangle* dynamic_cast_rectangle_ptr "dynamic_cast<Rectangle*>" (Polygon*) except NULL

cdef Polygon *polygon

cdef Rectangle *rect

polygon = new Rectangle(x0, y0, x1, y1)

rect = dynamic_cast_rectangle_ptr(polygon)

[Jasmine Sandhu]

On Thursday, January 10, 2013 4:48:44 PM UTC-8, Bradley Froehle wrote:

You can think of the dynamic_cast as a function with a template parameter which Cython doesn't yet support.  But you can implement it as:

cdef extern from "*"

    Rectangle* dynamic_cast_rectangle_ptr "dynamic_cast<Rectangle*>" (Polygon*) except NULL

Thanks Brad - this is indeed very useful. 

I got it to work by playing around with it, but could you please explain the syntax:

What does cdef extern from * mean? Doesn't it have to be externed from some header file?

It looks like the following defines the function and give its implementation all in one line:

Rectangle* dynamic_cast_rectangle_ptr "dynamic_cast<Rectangle*>" (Polygon*) except NULL

Thanks,

Jasmine

[Bradley Froehle]

Sorry, I meant cdef extern from *, which is short for "no include is necessary" (http://docs.cython.org/src/userguide/external_C_code.html).  

[Jasmine Sandhu]

On Friday, January 11, 2013 6:20:20 PM UTC-8, Bradley Froehle wrote:

Sorry, I meant cdef extern from *, which is short for "no include is necessary" (http://docs.cython.org/src/userguide/external_C_code.html).  

Great, thank you for the link and explanation. 

This solved my problem.

Jasmine