How to use C++ classes with ctypes?

Question

I'm just getting started with ctypes and would like to use a C++ class that I have exported in a dll file from within python using ctypes. So lets say my C++ code looks something like this:

class MyClass {   public:     int test(); ... 

I would know create a .dll file that contains this class and then load the .dll file in python using ctypes. Now how would I create an Object of type MyClass and call its test function? Is that even possible with ctypes? Alternatively I would consider using SWIG or Boost.Python but ctypes seems like the easiest option for small projects.

Answer 1

Besides Boost.Python(which is probably a more friendly solution for larger projects that require one-to-one mapping of C++ classes to python classes), you could provide on the C++ side a C interface. It's one solution of many so it has its own trade offs, but I will present it for the benefit of those who aren't familiar with the technique. For full disclosure, with this approach one wouldn't be interfacing C++ to python, but C++ to C to Python. Below I included an example that meets your requirements to show you the general idea of the extern "c" facility of C++ compilers.

//YourFile.cpp (compiled into a .dll or .so file) #include <new> //For std::nothrow //Either include a header defining your class, or define it here.   extern "C"  //Tells the compile to use C-linkage for the next scope. {     //Note: The interface this linkage region needs to use C only.       void * CreateInstanceOfClass( void )     {         // Note: Inside the function body, I can use C++.          return new(std::nothrow) MyClass;     }      //Thanks Chris.      void DeleteInstanceOfClass (void *ptr)     {          delete(std::nothrow) ptr;      }      int CallMemberTest(void *ptr)     {          // Note: A downside here is the lack of type safety.          // You could always internally(in the C++ library) save a reference to all          // pointers created of type MyClass and verify it is an element in that         //structure.          //         // Per comments with Andre, we should avoid throwing exceptions.           try         {             MyClass * ref = reinterpret_cast<MyClass *>(ptr);             return ref->Test();         }         catch(...)         {            return -1; //assuming -1 is an error condition.          }     }  } //End C linkage scope. 

You can compile this code with

gcc -shared -o test.so test.cpp #creates test.so in your current working directory. 

In your python code you could do something like this (interactive prompt from 2.7 shown):

>>> from ctypes import cdll >>> stdc=cdll.LoadLibrary("libc.so.6") # or similar to load c library >>> stdcpp=cdll.LoadLibrary("libstdc++.so.6") # or similar to load c++ library >>> myLib=cdll.LoadLibrary("/path/to/test.so") >>> spam = myLib.CreateInstanceOfClass() >>> spam [outputs the pointer address of the element] >>> value=CallMemberTest(spam) [does whatever Test does to the spam reference of the object]  

I'm sure Boost.Python does something similar under the hood, but perhaps understanding the lower levels concepts is helpful. I would be more excited about this method if you were attempting to access functionality of a C++ library and a one-to-one mapping was not required.

For more information on C/C++ interaction check out this page from Sun: http://dsc.sun.com/solaris/articles/mixing.html#cpp_from_c