[C++-sig] extending a c++ object by overriding it's virtual functions..

[Kleistereimer]

hi dave,

thankyou for your explanations and sourcecode.

i have some more questions about it thought.



first here is the sourcecode again:


//Python
>>> class Derived(Base):
...     def f(self):
...         return 42
>>> base = Base()
>>> derived = Derived()
>>> base.f()
0
>>> derived.f()
42

//C++ code from your answer
>1: // substitute the module name where "derived" is defined for "__main__"
>2: python::object module = python::import("__main__");
>3: python::object derived = module.attr("derived");
>4: python::object b = derived( /* arguments to __init__ here */ );
>5: int result = b.f();




1)
the mentioned include file <boost/python/import.hpp> is not in boost
1.33.1. where can i get it? is it in cvs? does it suffice if i get this
.hpp file only, or do i need the complete cvs version? for now i used
the c api.. instead of this line.

2)
what does the above code exactly? i think it instantiates a python
'Derived' class to object 'derived', so line 3 has to be:
 3 python::object derived = module.attr("Derived"); //capital 'D'
and at line 5 'f' is called on this instance. this gives me  a compiler
error:
"f is no element of ..::api::object". how to solve this?
i guess it involves something like:
 python::call_method<int>(b.ptr(),"f")
but this fails in arg_to_python.hpp:181
"cannot_convert_raw_PyObject<T*>::to_python_use_handle_instead();"
how to do it corectly?

3)
so this scheme is not 'overwriting c++ virtual functions with python
code' directly, instead it's 'deriving a python class from a c++ class,
overwriting  virtual functions, and then making an instance of this
python class and access it from c++'
right?


this means if i have some heavily used c++ class which needs only some
minor functions tweakable from python, i should better do it like so:


class py_MyClass:
	def Log(self):
		...		

class MyClass
{
	MyClass() {m_PyObject = Instantiate("py_MyClass");} //using line 3 and 4
	~MyClass() {delete m_PyObject;}
	...
	...
	void Log(std::string) {python::call_method<void>(m_PyObject, "Log"); }
};

because this calls into python only if one of the 'pythonized' functions
is needed (which is seldom) but not if other c++ functions are used
(which is extremely often)


did i understood it right?


regards
kl