C++, callbacks, objects and best practice

Question

I'm looking for a solution to my problem. I found a workaround but I feel there should be a prettier way of doing this. Here's my problem: I have an IPC library written in C. When message is received, it calls a callback function. I made a wrapper class on this library. When the message is received, the wrapper class should trigger a signal (I'm using Qt yes). As you know, member function can't be a callback, so in my case, I wrote that callback outside the class. Now I have to emit a signal from the callback function, the only way I know is to make a public method for the wrapper class which will emit the signal, but I don't want to have that method public, and I can't access protected/private members outside the class. Anyone had similar problem? How did you solve that?

Thanks

Answer 1

如果希望成员函数为“回调”，则可以在类中将该函数声明为“静态”。

Answer 2

You can declare your outside funktion as a friend of that class and therefore wont need to make the method public.

Here a link to a tutorial.

Postet as an answer as requested in comments.

Answer 3

Well, a member function can be a callback, with some additional work. This is something which is called "delegate". You could try boost::bind / boost::mem_fn . Maybe some other libraries will be more convenient/helpful, eg, boost.lambda (I didn't try it).

Answer 4

The std::mem_fun does the same trick as Vlad pointed. Making friends considered not a good design, since you are exposing all your class internals to be potentially accessible without even calling a member.

Static method will not be able to emit a singal, but it can access another static member pointing to your class. You could create a special "caller" class that way, storing a single static pointer to your Qt object.

You could have a global class pointer, but it's even worse than having a friend. You could make your class singleton, but it is similar to global variable in its tradeoffs, so doesn't worth it just for one use.