Wrapping C/C++ Problem

[Andrew Shap]

Hi,

I have a question regarding wrapping/linking protocols between C and C++. I'm working on existing C++ code that I have to wrap into a C object file for use in a different environment, namely I have a main C program that has to be able to link to the C++ generated .o effectively. Also, I have a C header file that specifies the interface struct between the old environment and the new one that my C main() will have to utilize. I've created a .cc, which will generate the .o, that includes other required C++ headers and the C header interface, specifying C linkage for the interface.h with 'extern "C"'.

In order to do this I created wrapper functions to virtual C++ member functions (in other .hh's) by passing void* as a parameter to the wrapper, dereferencing the pointer and calling the member function within the wrapper, and then specifying C linkage as above. I need to create a 'constructor' wrapper function (in my interface.h) that returns a void* to a C++ class instance. With this I can instantiate and then dereference the pointer in my C main, allowing access to C++ class instances; however, because I can't include C++ headers in the interface.h, I can't find a way to declare the instance of a specific C++ class type. Likewise I need to build a 'destructor' wrapper function which will deallocate the instance of this class but I run into the same problem.

My other question is that the existing code has an analogous C++ struct with exactly the same members as the interface.h struct, although it is given in a C++ header. The other .hh's require this C++ type to be passed as function parameters. The only idea I had would be to use static_cast or reinterpret_cast<c++_type>c_type in functions that require the C++ type as a parameter. I am not familiar with the implementation details of these casts and don't know if it will work, granted I can't debug this without the other fix. Is there another way to implement this I am not thinking of?

Any help would be greatly appreciated.

Thanks,

Andrew

[kbw]

You shouldn't add the C++ implementation header file to the interface header file. Think of your implementation being C++, but the interface being C. As such, the header file that the C program sees, it pure C. No one needs to know that it uses a C++ implementation. All you need to provide is a set of functions and your handle, in your case void*, but you could expose that as an int.

Take for example, take file handling. There's open, close, read, write, seek, tell, all accessed via a handle. You never see the implementation, just functions and a handle.

You have to decide where this struct lives. Is it an interface record or an implementation detail? If it's an interface record, define it as a C struct in the interface header file; otherwise hide it in the implementation.

[Andrew Shap]

Ah yeah it would be an implementation detail so I'll prototype the function in the header and then just define it in my .cc so that the object file I link to the main C program will contain the function definition.

Thanks for the help! I think I'm good from here.