C header file loops

I think the problem here is not the missing include guard but the fact that the two structures need each other in their definition. So it's a type define hann and egg problem.

The way to solve these in C or C++ is to do forward declarations on the type. If you tell the compiler that element is a structure of some sort, the compiler is able to generate a pointer to it.

E.g.

Inside tree.h:

// tell the compiler that element is a structure typedef:
typedef struct element_ element;

typedef struct tree_ tree;
struct tree_
{
    tree *first_child;
    tree *next_sibling;
    int tag;

    // now you can declare pointers to the structure.
    element *obj;
};

That way you don't have to include element.h inside tree.h anymore.

You should also put include-guards around your header-files as well.

Crucial observation here is that the element doesn't need to know the structure of tree, since it only holds a pointer to it. The same for the tree. All each needs to know is that there exists a type with the relevant name, not what's in it.

So in tree.h, instead of:

#include "element.h"

do:

typedef struct element_ element;

This "declares" the types "element" and "struct element_" (says they exist), but doesn't "define" them (say what they are). All you need to store a pointer-to-blah is that blah is declared, not that it is defined. Only if you want to deference it (for example to read the members) do you need the definition. Code in your ".c" file needs to do that, but in this case your headers don't.

Some people create a single header file which forward-declares all the types in a cluster of headers, and then each header includes that, instead of working out which types it really needs. That's neither essential nor completely stupid.

The answers about include guards are wrong - they're a good idea in general, and you should read about them and get yourself some, but they don't solve your problem in particular.