Calling c++ object in callback function of c library

Question

I am using a C library, which uses callback functions.

Is there any way I can access calling object of C++ class?

Edit:

I am using c-client lib. Which have function mm_log.

 void mm_log(char* string, long err_flag)

which is getting internally called by library. I want to check on which Imap stream it is getting called.

More Info you can download library from ftp://ftp.cac.washington.edu/imap

Answer 1

All (good) C library functions that want a callback have a void* user_data pointer as part of the function and the callback parameter. You just pass a pointer to your object as this to the function and it just gets passed back to you in the callback. Example:

typedef void (*callback)(void*);

void dumb_api_call(callback cb, void* user_data){
  cb(user_data);
}

struct Foo{};

void my_callback(void* my_data){
  Foo* my_foo = static_cast<Foo*>(my_data);
}

int main(){
  Foo my_foo;
  dumb_api_call(my_callback, &my_foo);
}

Answer 2

If mm_log is a function which you are implementing and the library is calling (which is a terrible way for a library to do callbacks, by the way), then there is no way you can get it to reference a member function in your class.

What you could do is use a global variable which you set to point to your object before invoking the library (and clear after) and then use it within mm_log to invoke the desired method. This is nasty and dangerous but can work.

If you have more than one thread then exercise extreme caution - or find a better library.

Answer 3

Code is important for such a question. But without seeing any of your code, I can still give you a blanket statement:)

You'd have to wrap your C++ object with global functions that access a plain-old-struct, and export those with:

extern "C"

There are a plenty of caveats, but this is the gist of it.

See this FAQ: http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html