c++ - Most vexing parse

Question

I got the code from here.

class Timer {
 public:
  Timer();
};

class TimeKeeper {
 public:
  TimeKeeper(const Timer& t);

  int get_time()
  {
      return 1;
  }
};

int main() {
  TimeKeeper time_keeper(Timer());
  return time_keeper.get_time();
}

From the looks of it, it should get compile error due to the line:

TimeKeeper time_keeper(Timer());

But it only happens if return time_keeper.get_time(); is present.

Why would this line even matter, the compiler would spot ambiguity on time_keeper(Timer() ) construction.

Answer 1

This is due to the fact that TimeKeeper time_keeper(Timer()); is interpreted as a function declaration and not as a variable definition. This, by itself, is not an error, but when you try to access the get_time() member of time_keeper (which is a function, not a TimeKeeper instance), your compiler fails.

This is how your compiler view the code:

int main() {
  // time_keeper gets interpreted as a function declaration with a function argument.
  // This is definitely *not* what we expect, but from the compiler POV it's okay.
  TimeKeeper time_keeper(Timer (*unnamed_fn_arg)());

  // Compiler complains: time_keeper is function, how on earth do you expect me to call
  // one of its members? It doesn't have member functions!
  return time_keeper.get_time();
}