Question

I noticed that qmake passes all C++ preprocessor flags to the moc utility. I tried moc without these flags on one file and it produced identical results. My question is: would there be cases in which moc would need these flags?

Answer 1

moc creates a CPP file which manages the SLOTS and SIGNALS of the class.

if any of those are under preprocessor rules, MOC needs to know about it.
for example:

class A {
 Q_OBJECT
public slots:
#ifdef ENABLE_SLOT1
  void slot1();
#endif;
  void slot2();
};

if you run moc without a “-DENABLE_SLOT1” parameter then the generated CPP file will not have any idea that it need to generate code handling slot1().