Mixing C++ code from different compilers

Question

Suppose I have two projects that I would like to link together:

• A C++ library compiled with Visual C++ to a DLL file.
• A C++ executable compiled with C++ Builder that uses the classes in the library.

I realize that there is no standard C++ ABI and that any attempts to directly link these two C++ projects together will fail. What is a good, automated way of creating a compatibility layer that allows me to accomplish this?

For example, conceivably the C++ library could expose itself via a C interface. Then the executable would have some C++ classes that wrap the C interface exposed by the C++ library. Since there is a standard ABI for C, it would work.

The only question is how to automatically create the C interface and C++ wrapper classes - manually maintaining this would not be an option. The SWIG project looks promising, but unfortunately, C++ is not one of the exits of SWIG listed on their web site. Is there a way to do what I want with SWIG? Or is there another project other than SWIG that would help me with this task?

Or am I going about this the wrong way?

Edit: The core C++ library is intended to be cross-platform. The executable, obviously, is Windows-specific. I don't want to pollute the core library to the extent that it becomes impossible to compile it on other platforms.

Answer 1

If it only has to run on Windows, I would expose the classes as COM objects. They'll still be in a DLL and they can be used by any language which understands COM.

Answer 2

The "standard" way of doing this, in Windows, is to use COM objects. So, that is certainly a good option to look at. In Linux systems, the module interaction model (e.g., executable-DLL interaction) is very different, and ABIs exist for C++.

If you would want to do this manually (create your own COM-like library), it can be a lot of work with many little tricky issues to take seriously. You'll need a cross-module RTTI system, you'll need an interface query/definition protocol, some mechanism to manage memory across modules, etc. Beyond that, to "automate" it, you will probably need a combination of MACROs and template meta-functions.

One cross-platform option that I would highly recommend that you consider or at least look at is using Boost.Python and the Python language as the "glue" between your modules. The Boost.Python library basically does the whole "automated exporting / importing of classes", but it exports your C++ classes and functions as Python classes and functions. And, it is completely non-intrusive and cross-platform, so this is really an ideal example of automated exporting. So, you might consider using Python to write your high-level glue-code, or using Python as an intermediate between the C++ modules, or even reworking the Boost.Python library to use only the "automated exporting" mechanisms to export to whatever interface system you design or use.

I'm sure there a plenty other similar libraries out there. But the number one question is, of course, do you really need this? You might be using a bazooka to kill a fly.