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deriving Python classes from a C++ class

Started by Ezbez Jan 9, 2007 at 6:15 AM 4 replies 1k views

Original Post

Jan 9, 2007 at 6:15 AM

I know that this is possible, and I can do this. What I'd like to know is whether C++ will call the Python derived class's function if you call a virtual function in C++? Here's my example:

//C++
class Base
{
    virtual void Run()
    {
     std::cout << "Base class.";
     }
}

//Python, assuming that I've got the C++ class 'into Python' with boost::Python already
class Derived(Base):
    Run():
         print "Derived."

Then let's say that I've got: Base* ptr = SomeFactoryFunction(); //<- returns an instance of Derived() ptr->Run() Would that call Base::Run() or Derived::Run()? Thanks in advanced.

mikeman

Jan 9, 2007 at 6:20 AM

It's all in the Boost.Python documentation: link. Scroll down to "Class Virtual Functions" and "Virtual Functions with Default Implementations".

Personally, I just use Pyste to generate automatically the Boost.Python code from header files.

Ezbez

Jan 9, 2007 at 6:31 AM

Alright, I understand that I can make classes in Python that derive from C++, but I'm not sure whether virtual functions called in C++ would use the derived class's function instead of the virtual function. That is, in my example, would it print 'Base' or 'Derived'?

mikeman

Jan 9, 2007 at 6:36 AM

That's the question you asked the first time, before I gave you link. Of course it will print "Derived", as expected. That's the whole point of these 2 paragraphs in the link.

Ezbez

Jan 9, 2007 at 2:17 PM

Quote:
Original post by mikeman
That's the question you asked the first time, before I gave you link. Of course it will print "Derived", as expected. That's the whole point of these 2 paragraphs in the link.

I know it's the same, I was clarifying since I didn't see the answer in your link or post.

But, thanks for the answer and sorry for the confusion. I find this pretty mind-boggling for some reason. Run-time made classes being used in a language that only has classes made at compile-time seems very difficult to me, so I just had to check.

Vorpy

Jan 9, 2007 at 4:11 PM

I think the "trick" is that the hooks for python objects to override C++ base class methods are created at compile time. Boost creates a derived class that contains a pointer to the python object and overrides all the virtual methods to check if the python object implements them. It's adding a layer of python style polymorphism on top of C++'s less flexible polymorphism.

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