Redefinition of classes (#include headers)

What is the best way to avoid the: 'class' type redefinition errors? This comes from anytime I include a header for a file that has already been included somewhere else. First of all, why is this a problem, and second, what a quick and easy way to fix this.

thanks

-from looking at previous post it looks like my answer has something to do with either "using namespace std" or "#ifndef"

for example, a main.h file

Code:

#ifndef _MAIN_H
#define _MAIN_H

//...............code here

#endif

First of all, why is this a problem

You can't declare or define things twice.

and second, what a quick and easy way to fix this.

Use inclusion guards, as EvBladeRunnervE has shown.

To add to ' EvBladeRunnervE' post...

Code:

#ifndef _MAIN_H
#define _MAIN_H

#ifndef _STDIO_H
#include <stdio.h>
#endif 

#ifndef _STDLIB_H
#include <stdlib.h>
#endif 

//...............code here

#endif // !_MAIN_H