Two good articles on exporting C++ classes from a DLL from Eli Bendersky and Alex Blekhman. I've researched this before and found varying degrees of intelligibility. These seem to be fairly complete yet concise.

The words or the language, as they are written or spoken, do not seem to play any role in my mechanism of thought. The psychical entities which seem to serve as elements in thought are certain signs and more or less clear images which can be 'voluntarily' reproduced and combined …

This is perhaps the simplest, slickest way to wrap C++ in a C interface I've ever seen, courtesy of Matthieu Brucher. The key concept is the forward declaration of the Manager type, which is valid for both C++ and C. Any C that attaches to this interface doesn't have to …