/**
*  header1.h
*/

#ifndef HEADER1_H
#define HEADER1_H

// Here we include header2.h
#include <header2.h>

class someClass {
   someOtherClass* object;
};

#endif

/**
*  header2.h
*/

#ifndef HEADER2_H
#define HEADER2_H

// Here we include header1.h
#include <header1.h>

class someOtherClass {
   someClass* object;
};

#endif

/**
*   At this time there is no circular dependency. The compiler only
*   sees the post-processed source files. Here is the source file before processing:
*/

/**
*   source.c
*/
#include <header1.h>

/**
*   We dont care about the rest of the file. So, source.c just has a single #include line
*   to the preprocessor to include header1.h. When building, we first run source.c in
*   the preprocessor, so source.c is modified by the preprocessor to something like:
*/

/**
*   source.c
*/

#define HEADER1_H

#define HEADER2_H

class someOtherClass {
   someClass* object;
};

class someClass {
   someOtherClass* object;
};

#endif

/**
*  The above is what the compiler sees. But wait! when we get to someOtherClass, the
*  someClass class isnt defined yet. So we reach the "someClass* object" line, we get
*  compiler errors do to an unknown class being used, most commonly "missing ';'" errors
*/