Wrapping C++ class API for C consumption

Question

I have a set of related C++ classes which must be wrapped and exported from a DLL in such a way that it can be easily consumed by C / FFI libraries. I'm looking for some "best practices" for doing this. For example, how to create and free objects, how to handle base classes, alternative solutions, etc...

Some basic guidelines I have so far is to convert methods into simple functions with an extra void* argument representing the 'this' pointer, including any destructors. Constructors can retain their original argument list, but must return a pointer representing the object. All memory should be handled via the same set of process-wide allocation and free routines, and should be hot-swappable in a sense, either via macros or otherwise.

Answer 1

Foreach public method you need a C function.
You also need an opaque pointer to represent your class in the C code.
It is simpler to just use a void* though you could build a struct that contains a void* and other information (For example if you wanted to support arrays?).

Fred.h --------------------------------  #ifdef  __cplusplus class Fred {     public:     Fred(int x,int y);     int doStuff(int p); }; #endif  // // C Interface. typedef void*   CFred;  // // Need an explicit constructor and destructor. extern "C" CFred  newCFred(int x,int y); extern "C" void   delCFred(CFred);  // // Each public method. Takes an opaque reference to the object // that was returned from the above constructor plus the methods parameters. extern "C" int    doStuffCFred(CFred,int p); 

The the implementation is trivial.
Convert the opaque pointer to a Fred and then call the method.

CFred.cpp --------------------------------  // Functions implemented in a cpp file. // But note that they were declared above as extern "C" this gives them // C linkage and thus are available from a C lib. CFred newCFred(int x,int y) {     return reinterpret_cast<void*>(new Fred(x,y)); }  void delCFred(CFred fred) {     delete reinterpret_cast<Fred*>(fred); }  int doStuffCFred(CFred fred,int p) {     return reinterpret_cast<Fred*>(fred)->doStuff(p); }