Even the simplest class doesn't compile, "has not been declared

Question

OK, this is really annoying. I have the (almost) simplest class possible. Two files: a.cpp and a.h a.h:

#ifdef A_H
#define A_H
class a{
  public:
    a();
};
#en         


        

Answer 1

Its because

 #ifdef A_H

needs to be

 #ifndef A_H

notice the "n", as in if NOT defined.

The former will only compile the code if A_H is defined, which it isn't since you only define it on the next line.