Swig4D class cast proposal

gistfile1.d

/*
 * C++ Class structure:
 * 
 * Shape
 * |- Circle
 * |- Rectangle
 *    |-Square
 * 
 */

extern(C)
{
    void* swig_cpp_cast_shape_circle(void*);
    void* swig_cpp_cast_shape_rectangle(void*);
    void* swig_cpp_cast_shape_square(void*);
    void* swig_cpp_cast_rectangle_square(void*);
}

public class Shape
{
    T opCast(T) if (is(T : Shape) && __traits(compiles, T.castFrom(this)))
    {
        return T.castFrom(this);
    }
}

public class Circle
{
    static Circle castFrom(Shape shape)
    {
        void* ptr = swig_cpp_cast_shape_circle(shape.getCPtr());
        if(ptr == null)
            return null;
        else
            return new Circle(ptr);
    }

    T opCast(T) if (is(T : Circle) && __traits(compiles, T.castFrom(this)))
    {
        return T.castFrom(this);
    }
}

public class Rectangle
{
    static Rectangle castFrom(Shape shape)
    {
        void* ptr = swig_cpp_cast_shape_rectangle(shape.getCPtr());
        if(ptr == null)
            return null;
        else
            return new Rectangle(ptr);
    }

    T opCast(T) if (is(T : Rectangle) && __traits(compiles, T.castFrom(this)))
    {
        return T.castFrom(this);
    }
}

public class Square
{
    static Square castFrom(Shape shape)
    {
        void* ptr = swig_cpp_cast_shape_square(shape.getCPtr());
        if(ptr == null)
            return null;
        else
            return new Square(ptr);
    }

    static Square castFrom(Rectangle rect)
    {
        void* ptr = swig_cpp_cast_rectangle_square(rect.getCPtr());
        if(ptr == null)
            return null;
        else
            return new Square(ptr);
    }
    
    T opCast(T) if (is(T : Square) && __traits(compiles, T.castFrom(this)))
    {
        return T.castFrom(this);
    }
}

unittest
{
    void* ptr = someFunctionReturningAShapePtr();
    Shape shape = new Shape(ptr);
    auto circle = cast(Circle)shape;
    if(!Circle)
    {
        auto rectangle = cast(Rectangle)shape;
        if(rectangle)
            auto square = cast(Square)rectangle;
    }
}