Of course O can do that. The point is that I want to do it many times over, in different places in my code.
What I want to do is to spend the time and effort in understanding and debugging the process, and then encapsulating it in a single function call. Then completely forget about the details and use that single function call.
What is the advantage in not having user-defined functions? None, zilch, nada.
What thing you cannot do? I have used capnproto, generated c++ files and passed the result to the input of targets perfectly ok.