C++ idiom when using a C library which expects a function pointer? [duplicate]

Question

I am using a C library in a C++ application. The C SDK has functions that take a callback function pointer as an argument. The signature of these functions is usually like:

typedef int (* Func) (type1 c, type2 d);

I have my code structured using classes in C++. However, I can't pass any member functions to this function as callback because it doesn't accept int (MyClass::*)(type1 c, type2 d) and only accepts int (*)(type1 c, type2 d).

I'm getting around this by defining all my callbacks as static in the various classes and then passing them to the C library which then works.

I am still new to C++ so I'm not sure if this is the right solution? The code works, but I'd love to hear if I'm doing this wrong.

Answer 1

On most platforms your code is ever likely to encounter, your solution is fine. However, strictly speaking, there can be a platform where the C calling convention and C++ calling convention differ. On this platform, your code would not work.

The 100% correct solution (as opposed to the 99.9% correct one which you use) is to define a function with C language linkage as the callback. If you need this function to have member access to your class, it can then call a static member function in your class:

class MyClass
{
  static int secret;
public:
  static void callback() { secret = 42; }
};

extern "C" void callback() { MyClass::callback(); }

---

Note that it is considered good practice for authors of callback registration points (that is, authors of the library which will call you back) to provide a way for the user to associated void* data with the callback, which will be passed in as they call you. If your library allows that, you can use this void *user_data to pass in a pointer to your C++ class. With such a nicely-designed callback library, it could look like this:

class MyClass
{
  int secret;

public:
  void setSecret() { secret = 42; }
};

extern "C" void callback(void *userData) { static_cast<MyClass*>(userData)->setSecret(); }

MyClass mc;

int main()
{
  cLibrary_registerCallback(&callback, &mc);
}