incomplete type

You might end up with a circular include, which means that by the time the code is pre-processed that the type definition comes after the function declaration or in some cases implementation.

This is often caused by so called "global" or "master" headers, which include lots of your own headers.


Read this article.

I used the:

#ifndef HELLO_H
#define HELLO_H
...
#endif // HELLO_H


magic, so I don´t think that´s the case.

Include guards prevent multiple definition errors, but do not guarantee that circular inclusions will work properly. Again, read the article rip-off posted.