[C++-sig] Inheritance problem

[Erik Türke]

On 10/14/11 16:48, Holger Joukl wrote:
> Hi,
>
>   >  Ok lets say my BaseClass has a member function called init( vector4 ):
>> class Base
>> {
>> public:
>>       void init( vector4&vec ) { //doWhatEver }
>>       //a lot of other functions
>> };
>>
>> Unfortunetaly i can not expose this init function directly to python so
>> i am writing a BaseWrapper
> Why's that? Can't you expose vector4 to Python?
>
>
>> So when i am exposing Base and Derived like:
>>
>>
>> BOOST_PYTHON_MODULE( my_module )
>> {
>>
>>       class_<Base, BaseWrapper>( "Base", init<>() )
>>       .def("init",&BaseWrapper::_init)
>>       ;
>>       class_<Derived, bases<Base>  >( "Derived", init<>() );
>> }
>>
>> I want to have all functions for objects of Derived that are available
>> in Base.
>> The thing is, that e.g. ipython recognizes the functions.
>> So in ipython, when i have an object of type Derived with tab completion
>> i see the functions from Base.
>> But when i try to call them i always get this "signature" error.
> I think the problem is that the Derived class doesn't actually have any
> inheritance
> relationship with BaseWrapper, i.e.
>          Base
>          /   \
>         /     \
>        /       \
> BaseWrapper  Derived
>
> So in an example like this
>
> // file cppcode.hpp
>
> #include<iostream>
>
> class Base
> {
> protected:
>      int m_area;
> public:
>      Base() : m_area(0) {}
>      void init(int area) {
>          m_area = area;
>      }
>      virtual void print() { std::cout<<  "hello Base "<<  m_area<<
> std::endl; }
>
> };
>
>
> class Derived : public Base
> {
> public:
>       virtual void print() { std::cout<<  "hello Derived "<<  m_area<<
> std::endl; }
>
>
> };
>
> // only to show callback-into-python-overrides necessities
> void callback(Base&  base) {
>      base.print();
> }
>
>
> // file wrap.cpp
>
> #include<boost/python.hpp>
> #include "cppcode.hpp"
>
> namespace bp = boost::python;
>
>
> class BaseWrapper : public Base, public bp::wrapper<Base>
> {
>   public:
>      void _init(int x, int y) {
>          init(x * y);
>      }
> };
>
>
> BOOST_PYTHON_MODULE(cppcode)
> {
>      bp::class_<BaseWrapper, boost::noncopyable>("Base")
>          .def("init",&BaseWrapper::_init)
>          .def("printIt",&Base::print)
>       ;
>      bp::class_<Derived, bp::bases<Base>  >("Derived");
>      bp::def("callback",&callback);
> };
>
>
> #!/apps/local/gcc/4.5.1/bin/python2.7
>
> # file test.py
>
> import cppcode
>
> print "--->  base"
> base = cppcode.Base()
> base.printIt()
> base.init(3, 4)
> base.printIt()
>
>
> print "--->  derived"
> derived = cppcode.Derived()
> derived.printIt()
> derived.init(3, 4)
> derived.printIt()
> cppcode.callback(derived)
>
> class PythonDerived(cppcode.Base):
>      def printIt(self):
>          print "hello PythonDerived"
>
> print "--->  python derived"
> pyderived = PythonDerived()
> pyderived.printIt()
> cppcode.callback(pyderived)
>
> I run into this error when trying to call .init() on the Derived object:
>
> $ python2.7 ./test.py
> --->  base
> hello Base 0
> hello Base 12
> --->  derived
> hello Derived 0
> Traceback (most recent call last):
>    File "./test.py", line 23, in<module>
>      derived.init(3, 4)
> Boost.Python.ArgumentError: Python argument types in
>      Base.init(Derived, int, int)
> did not match C++ signature:
>      init(BaseWrapper {lvalue}, int, int)
>
>
> Which makes sense since Derived does not inherit from BaseWrapper.
>
>> So i do not know how to use those callback approach you suggested.
>> Especially if you are using function overloading. And additionally, this
>> would mean, that i have to write such a callback function for each
>> function in my base class as a global function.
> Never mind the callback, I might have just confused you. The callback is
> only for showing
> that you'd need a Wrapper class if you want to inherit in Python and be
> able to call back
> from C++ into Python and actually call methods overridden in Python.
>
>> One thing i have to mention is, that it is perfectly working if i omit
>> the BaseWrapper class. So if the functions of Base can be exposed
>> without using a wrapper class:
>> [...]
>> ...works. But unfortunately not with the BaseWrapper Class :-(
> Because now you don't have the problem that Derived has no inheritance
> relationship with BaseWrapper.
>
> Maybe you can just use a free function:
>
> // file wrap.cpp
>
> #include<boost/python.hpp>
> #include "cppcode.hpp"
>
> namespace bp = boost::python;
>
>
> void _init(Base&  base, int x, int y) {
>          base.init(x * y);
> }
>
>
> BOOST_PYTHON_MODULE(cppcode)
> {
>      bp::class_<Base>("Base")
>          .def("init",&_init)
>          .def("printIt",&Base::print)
>       ;
>      bp::class_<Derived, bp::bases<Base>  >("Derived");
>      bp::def("callback",&callback);
> };
>
>
>
> # file test.py
>
>
> import os
> import sys
>
>
> import cppcode
>
> print "--->  base"
> base = cppcode.Base()
> base.printIt()
> base.init(3, 4)
> base.printIt()
>
>
> print "--->  derived"
> derived = cppcode.Derived()
> derived.printIt()
> derived.init(3, 4)
> derived.printIt()
> cppcode.callback(derived)
>
> class PythonDerived(cppcode.Base):
>      def printIt(self):
>          print "hello PythonDerived"
>
> print "--->  python derived"
> pyderived = PythonDerived()
> pyderived.printIt()
> # to make this invoke PythonDerived.printIt() you need a wrapper class
> cppcode.callback(pyderived)
>
> ===>
>
> $ python2.7 ./test.py
> --->  base
> hello Base 0
> hello Base 12
> --->  derived
> hello Derived 0
> hello Derived 12
> hello Derived 12
> --->  python derived
> hello PythonDerived
> hello Base 0
> Holger
>
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Hi Holger,

well, ok, i understand now what the problem is. Many thanks for your help!

Best Regards!

-- 
Erik Türke
Department of Neurophysics
Max-Planck-Institute for Human Cognitive and Brain Sciences
Stephanstrasse 1A
04103 Leipzig
Germany
Tel: +49 341 99 40-2440
Email: tuerke at cbs.mpg.de
www.cbs.mpg.de