Boost::Python bindings for classes with inheritance

Question

I'm trying to figure whats wrong with some autogenerated (with Pyste) boost::python code, but have no luck so far.

There is C++ library, Magick++, which provides two classes, Magick::Drawable and Magick::DrawableRectangle:

https://www.imagemagick.org/subversion/ImageMagick/trunk/Magick++/lib/Magick++/Drawable.h

class MagickDLLDecl DrawableBase:
   public std::unary_function<MagickCore::DrawingWand,void>
{...}
class MagickDLLDecl Drawable
{
  public:
    // Constructor
    Drawable ( void );
    // Construct from DrawableBase
    Drawable ( const DrawableBase& original_ );
...
}
class MagickDLLDecl DrawableRectangle : public DrawableBase
{ ... }

These are used as arguments for Image.draw(): https://www.imagemagick.org/subversion/ImageMagick/trunk/Magick++/lib/Magick++/Image.h

// Draw on image using a single drawable
void            draw ( const Drawable &drawable_ );
// Draw on image using a drawable list
void            draw ( const std::list<Magick::Drawable> &drawable_ );

I'm trying to make python bindings for it, there are autogenned wrappers for all the classes,

http://bitbucket.org/dan.kluev/pythonmagick/src/65d45c998ef3/src/_Drawable.cpp

http://bitbucket.org/dan.kluev/pythonmagick/src/65d45c998ef3/src/_DrawableRectangle.cpp

http://bitbucket.org/dan.kluev/pythonmagick/src/65d45c998ef3/src/_Image.cpp

Problem is, due to indirect class casts from DrawableBase to Drawable, these wrappers do not work:

>>> import PythonMagick
>>> image = PythonMagick.Image()
>>> square = PythonMagick._PythonMagick.DrawableRectangle(0,0,200,200)
>>> image.draw(square)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Boost.Python.ArgumentError: Python argument types in
    Image.draw(Image, DrawableRectangle)
did not match C++ signature:
    draw(Magick::Image {lvalue}, std::list<Magick::Drawable, std::allocator<Magick::Drawable> >)
    draw(Magick::Image {lvalue}, Magick::Drawable)
# But abstract Drawable() does work:
>>> image.draw(PythonMagick._PythonMagick.Drawable())
>>>

Is there any better way to handle this than write my own draw() wrapper in C++, which would cast PyObject into Drawable?

This should be tagged with 'pyste' as well as 'boost-python'. — Matthew Scouten
– Matthew Scouten, Commented Dec 2, 2010 at 17:14
@Matthew, pyste was used before to autogen code, but is dropped now, and code is supported w/out it. — Daniel Kluev
– Daniel Kluev, Commented Dec 3, 2010 at 1:35

Matthew Scouten · Accepted Answer · 2010-12-02 17:12:11Z

1

If you want BP to implicitly convert objects foy you , you must tell BP that thay are implicitly convertable. put something like this in your bp::code:

boost::python::implicitly_convertible<SourceType,DestType>();

I do not know how to induce Pyste to do this.

answered Dec 2, 2010 at 17:12

Matthew Scouten

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2 Comments

Daniel Kluev Over a year ago

Thanks a lot, that indeed kind of worked. It runs fine if I do implicitly_convertible<DrawableRectangle,Drawable>, but segfaults on implicitly_convertible<DrawableBase,Drawable>. Oh well, copypasting all Drawables will do it too.

Matthew Scouten Over a year ago

Hmm. As far as I know it ought to work. Maybe the segfault is in the wrapped lib? Or maybe a bad interaction.

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