qt - How signal and slots are implemented under the hood?

Question

This question is already asked in this forum but I don't understand the concept.

I was reading around and it seems that signal and slots are implemented using function pointers i.e the signal is one big function which inside it calls all connected slots (function pointers). Is this correct? And what is the role of the generated moc files in the whole story? I don't understand how the signal function knows which slots to call i.e which slots are connected to this signal.

Thanks for your time

Answer 1

Qt implements these things in a way that resembles interpreted languages. I.e. it constructs symbol tables that map signal names to function pointers, maintains them and looks up the function pointer by function name where needed.

Each time you emit a signal, i.e. write

emit something();

you actually call the something() function, which it automatically generated by meta object compiler and placed into a *.moc file. Within this function it's checked what slots this signal is connected to at the moment, and appropriate slot functions (which you implemented in your own sources) are sequentially called via the symbol tables (in the way described above). And emit, like other Qt-specific keywords, are just discarded by C++ preprocessor after *.moc were generated. Indeed, in one of the Qt headers (qobjectdefs.h), there exist such lines:

#define slots 
#define signals protected
#define emit

Connection function (connect) just modifies the symbol tables maintained within *.moc files, and the arguments passed to it (with SIGNAL() and `SLOT macros) are also preprocessed to match the tables.

That's the general idea. In his or her another answer, ジョージ supplies us with links to trolltech mailing list and to another SO question on this topic.