Question

I have a list of checkpoints and then a run a function. I originally built this list in that function, but now I have to build it outside. The problem is that I cannot include checkpoint.h in the class that implements that function because checkpoint.h returns a structure of the type of that class. The initial list was declare in class.c globally. How can I transfer the list created outside into class so I can use it?

So I have this header, turing_machine.h:

#ifndef __TURING_MACHINE__ 
#define __TURING_MACHINE__

#include "tape.h"
#include "alphabet.h"
#include "symbol_table.h"

...

#endif

and the checkpoint.h header defining the checkpoint_list class:

#ifndef __CHECKPOINT_H__
#define __CHECKPOINT_H__

#include "turing_machine.h"

...

#endif

So I want to send to a function from turing_machine.h a list of structures checkpoint but I can’t modify anything because that’s how the classes must stay.

I have also turing_machine.c:

#include "turing_machine.h"
#include "checkpoint.h"
#include "symbol_table.h"
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

checkpoint_list *c;

So at the beginning I created in turing_machine that list, c, but now I must create it outside and I have to initialize that list c but I don’t know how. I hope this is more clear.

I used the term class wrong; I have just .c and .h files.

Answer 1

Reading between the lines, somewhat, I think your trouble is that you have ‘mutually referencing’ structures.

The way to work around this is with an incomplete type definition:

typedef struct checkpoint_list checkpoint_list;

You can then use that inside turing_machine.h:

#ifndef TURING_MACHINE_H_INCLUDED
#define TURING_MACHINE_H_INCLUDED

#include "tape.h"
#include "alphabet.h"
#include "symbol_table.h"

typedef struct checkpoint_list checkpoint_list;

typedef struct turing_machine
{
    ...
} turing_machine;

extern checkpoint_list *tm_function(turing_machine *);
extern turing_machine  *tm_create(const char *); 

#endif

And, inside checkpoint.h, you can write:

#ifndef CHECKPOINT_H_INCLUDED
#define CHECKPOINT_H_INCLUDED

#include "turing_machine.h"

/* No typedef here in checkpoint.h */
struct checkpoint_list
{
    ...
};

extern checkpoint_list *cp_function(const char *);
extern turing_machine  *cp_machine(checkpoint_list *);

#endif

This technique is recognized and defined by the C standard (C90, let alone C99 or C11).

Note that I’ve also renamed the include guards; names that start with double underscore are reserved for ‘the implementation’ (meaning the C compiler and its libraries), and you should not invent and use such names in your own code.