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();
};
#endif

and a.cpp

#include "a.h"
a::a(){
}

and yet it won’t compile:

g++ a.cpp a.cpp:3: error: ‘a’ has not
been declared

Clearly, I am doing something wrong, but what?

Fixed, I replaced the #ifdef with #ifndef

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.