Should I use Qt signal/slot mechanisms over traditional callbacks?

Question

A senior developer in my team used traditional C-style callbacks in our Qt application instead of using Qt signal/slot mechanisms.

My first reflex would be to replace his code and use Qt signal/slot instead.

Is there any good reasons to use callbacks in a Qt application/library?

Thanks.

Answer 1

I think the better approach would be to embrace the framework you are using and use signal/slots.

That being said, if the code in question works, and is not ugly or causing problems, then you may be best to leave it alone.

Consulting the Signal/Slot documentation describes why the Signal/Slot approach is better:

Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.

Do be aware of the following though:

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide

The speed probably doesn't matter for most cases, but there may be some extreme cases of repeated calling that makes a difference.

Answer 2

You should distinguish between callbacks that are synchronous and used for returning either gradual results or tuples (multiple items, like pair<> but more) and callbacks that are asynchronous loose coupling. Signals/slots should usually be used for the latter. For the former, it can make sense either way depending on whether it is a major function of your application (use signals/slots) or the interface of a non-GUI library that could be more portable (use straight callbacks for plain C/C++ portable code).

Generally, I have concise programming design rules that tend to point to whatever is cleanest and least code / interface complexity. For the tuple return case, I'll often use QMap rather than creating yet another class definition or signal/slot or callback method. This works well for bundling and passing parameters also, especially for things that need to evolve in different parts of the system.