Boost.Python, callbacks and inheritance

Timo Savola timo.savola at iki.fi
Sat Nov 17 20:05:31 CET 2001


On Fri, 2001-11-09 at 13:45, Michael Hudson wrote:

> Timo Savola <timo.savola at iki.fi> writes:
> 
> > Now I'm doing an extension module, or rather a system that I want to
> > control via Python. I have a C++ class hierarchy that I want to be
> > accessible from Python. 
> 
> Have you looked at Boost::Python?  I haven't, but I've heard it's good
> for this sort of thing.

I've now converted my code to use Boost.Python and quite happy with it.
But I do have one major problem.

I have a C++ class (Color) with only pure virtual methods. All other
classes with methods that have Color as the type of their argument
obviously always get an instance of a class derived from Color. One of
these other classes (ColorStream) also has only pure virtual methods,
and is meant to be implemented with Python.

	class Color {
		virtual float red() = 0;
		virtual float green() = 0;
		virtual float blue() = 0;
	}

	class_builder<Color> color(module, "Color");
	color.def(&Color::red, "red");
	color.def(&Color::green, "green");
	color.def(&Color::blue, "blue");

	class ColorStream {
		virtual void put(Color &) = 0;
	}

	struct ColorStreamCallback : ColorStream {
		ColorStreamCallback(PyObject *s)
		{
			self = s;
		}

		void put(Color &color)
		{
			callback<void>::call_method(self, "put", color);
		}

	private:
		PyObject *self;
	}

	class_builder<ColorStream, ColorStreamCallback>
		color_stream(module, "ColorStream");
	color_stream.def(constructor<>());

The problem surfaces in the put function of ColorStreamCallback. The
call_method function of callback<void> doesn't accept the reference to
the non-concrete Color. I tried playing around with auto_ptr<Color>, but
just ended up with incomprehensible errors with template unrolling. I'm
more or less clueless.

Timo





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