I did not understand the functioning of #ifndef ?

Question

I did not understand the functioning of #ifndef in infinite inclusions. Someone will be able to explain to me more (where it is necessary to put the # of the two files to avoid inclusion for example).

Answer 1

The # is to indicate preprocessor commands, in order to help create the proper environment for the program.

I the specific case of circular inclusion, you can create your own flags to indicate that you've already got what you need.

file1.h:

#define file1
#include <stdio.h>
#include file3.h
<header definitions here>

file2.h:

#include file1.h

<more header stuff>

file3.h:

#ifndef file3

#define file3
#include file1.h
#include <stdlib.h>

more useful declarations...

#endif

mainfile.cpp:

#include file1.h
#include file2.h
#include file3.h

The #ifndef statement protects you from going back and forth between the header 1 and 3 files, while allowing you to just list out the known dependencies without having to know and account for the circular references in the actual code files.

It's a good defensive technique, and something to keep in mind as your get into more complicated projects.

Did that clarify anything?