Callback functions when using a class

Question

This question is somewhat similar to How to pass non-static class member to callback?, but there are a few differences, especially since a lot of my uses have "interesting" additional complexities. I'd like to understand better my options in using them, since I have not gained enough understanding from other answers (and I need to write both the callback and the function that calls it).

I have a number of libraries that produce "events" that I'd like to be able to write separate classes or functions to respond to (so I don't need to customize every library every time I develop something). The annoying part is that I've long-since discovered the this pointer that's hidden in class method calls, which prevents me from passing currentgui.leftbutton as a function to menu.leftbuttonpressed(void (*callbackfuncptr)(void)).

I'm also using the ESP8266 web server library, which needs a callback in the context of the server object that can call the server as a global to actually get the data sent to it! (Unless the server passes its own this as a parameter to the callback, which I don't know since the documentation consists almost solely of a small number of "examples" which don't show the majority of the actual features of the web server, rather than a real explanation.)

I could use some variant of a pointer to a class method, but the syntax for that is eye-gouging and I'm using more than one class to provide callbacks to (especially for the task scheduler), so this would require customizing the library to handle pointers to every class I use again. I can't use polymorphism, either, since some classes contain multiple scheduler callbacks to be done on different times/conditions.

How do I handle these callbacks? My current method is to write a function that wraps the method's function, but that relies on a specific instance of that method, so it can't be reused. It's also really awkward since I'm now dealing with multiple functions/methods that are now very similar.

I would also like to be able to have a class, that when passed a web server object, registers itself as a callback to a particular page event (I want to write a network connection manager similar to the WiFiManager library, but with the ability to integrate with an existing webpage. I have all the code, but it currently can't be nicely integrated into a standalone class because of this issue). It needs to store a pointer to the server object to get the data once the callback's called, but it also needs to be attached to the callback of the class that holds that particular pointer AND the other handler functions once the data's retrieved. There is already a library that does something similar, but I can't figure out at all how this works, since I was under the impression that this is not possible. Can someone help me understand how the following class operates? https://github.com/esp8266/Arduino/tree/master/libraries/ESP8266HTTPUpdateServer/src

Answer 1

The ESP8266WebServer uses callback functions to handle HTTP request by your code. This callback functions can't be members of a class. But you can use anonymous functions to implement this callbacks. And if you like you can implement them as 'redirects' to functions of your objects.

server.on("/leftbutton", []() {
  currentgui.leftbutton(server); 
}

Note: this anonymous functions are called lambda functions and they can do some tricks with parameters too

Answer 2

They call the same instance of the class' member functions to get data back.

Yes, but if they eventually call the class' member function to get data, other than static data, then the need to know what instance of the class to get the data from.

I'd like to know how to get a class I wrote (nothing special about it, but how do I connect it?) to register itself as a callback so IT gets data and I don't need a bunch of disconnected non-class functions around

If function X (not a member of this class) needs to be able to access the data from instance A or instance B then clearly X needs to know which instance it is looking at (the this pointer). This is not a syntax thing, you need to have a way of solving this problem.

In fact, when you say "a class I wrote" then that would also have instance data (its own this) so now you are trying to connect two instances of two classes.

How many server objects do you have? If just one in this particular case you might be able to fudge your way around it by storing its this somewhere, or making some glue routines.