[C++-sig] inherit an instance, or something

[Roman Yakovenko]

On 11/8/06, Johan Rydberg <jrydberg at gnu.org> wrote:
>
> I have what might seem to be a strange situation.  I have a
> plugin-system which loads plugins from DSO's.  Each plugin presents a
> factory function which returns an object that implements an abstract
> interface (a class).  The interface could look something like this:
>
>   class plugin
>   {
>     virtual void set_parameter (const char *name, int value);
>     ...
>   };
>
> The plugin class is easily wrapped using Boost.Python, and a generic
> factory function could be exported which loads a DSO, finds the
> factory function, invokes it and returns the created instance;
>
>   plugin *make_plugin (const char *dso_name);
>
> The problem comes when I in python wants to inherit from a specific
> plugin.  The simplest way is probably to do something like this;
>
> (case 1)
>
>   class my_extended_plugin(plugin):
>       def __init__(self, ...):
>           plugin.__init__(self)
>           self.o = make_plugin('libmy-plugin.so')
>           self.set_parameter = self.o.set_parameter
>
> But that to me is not a clean solution.  It makes it hard to inherit
> From my_extended_plugin.  (not possible to invoke
> my_extended_plugin.set_parameter correctly.)
>
> What I really want is something like this;
>
> (case 2, dso_plugin inherits plugin)
>
>   class my_extended_plugin(dso_plugin):
>       def __init__(self, ...):
>           dso_plugin.__init__(self, 'libmy-plygin.so')
>
> Where dso_plugin.__init__ would "replace" the held pointer for 'self'
> with the one that make_plugin('libmy-plugin.so') returns.
>
> Is it possible to do this in anyway, or must I reside to case 1?

I think you can take another approach - mix between first one and the second
+ automatic redirection:

class dso_plugin(plugin):
      def __init__(self, ...):
          plugin.__init__(self)
          self.__o = make_plugin('libmy-plugin.so')

      def __getattr__( self, name ):
          return getattr( self.__o, name )

Now you can derive your plug-ins from the dso_plugin.

P.S.
I am not sure about exact __getattr__ implementation.


-- 
Roman Yakovenko
C++ Python language binding
http://www.language-binding.net/