typedefs and header files

**csprite** · 12-04-2004

Hi all!

I was wondering if there was a way to defer typedef declaration untill a later time? Now what I'm trying to do is create a general linked list .c & .h file of all the definitions and functions of a list such as listempty, traverselist, createlist etc. However, I want to keep it as general as possible including the data type of the list untill I need it for one of my projects.

Now I've read a textbook on this subject and it's say's I can declare the datatype just before including the header file of the general list like so:

typedef MyDataType GeneralDataType;
#include "general_list.h"

where: GeneralDataType is undefined and part of a structure in general_list.h and MyDataType is a clearly defined datatype such as char, int or some other structure.

The problem I'm having is that general_list.c complains that I haven't defined what GeneralDataType is even though I have defined it in the header file of a main program.

Is there a work around or preprocessor command I'm missing? Any help would be greatly appreciated.

**Thantos** · 12-04-2004

That is because general_list.c and your main program source file are compiled independtly of each other. As for a work around I can't think of any off the top of my head but I'm sure there is one.

**Codeplug** · 12-04-2004

>>[b]...general_list.c complains that I haven't defined what GeneralDataType is...[b]
>>...I have defined it in the header file...
Did you #include the header within general_list.c?

>>Is there a work around...
Get what you have working first. Once you understand how this method works, we can talk about other methods that have different advantages/disadvantages.

gg

**Sebastiani** · 12-04-2004

>> Is there a work around or preprocessor command I'm missing?

you could put the .c code into .h file to simplify things and just use a define:

#define GeneralDataType struct customer
#include "general_list.h"

**Salem** · 12-04-2004

Well sounds like a job for void

Code:

struct node {
    void *data;
    struct node *next;
};

struct node *insertAtHead ( struct node *head, void *data ) {
    struct node *new = malloc( sizeof *new );
    new->data = data;
    new->next = head;
    return new;
}

Then you can do things like

Code:

struct node *list1 = NULL;
struct node *list2 = NULL;
list1 = insertAtHead( list1, &myint );
list2 = insertAtHead( list2, &mych );

But you do need to allocate space for your data in this example, though no doubt a more sophisticated implementation could take care of that.

The list implementation only ever has to deal with a void* pointer to the thing you want to store in the list, so it could be anything you want when you use the list.

**csprite** · 12-04-2004

Originally Posted by Thantos

That is because general_list.c and your main program source file are compiled independtly of each other.

Ah I see. I always thought that if you included the necessary file headers into your program then they would be compiled succinctly but I guess it's too much to ask c to compile first and ask questions later about the datatype.

Originally Posted by Sebastiani

you could put the .c code into .h file to simplify things and just use a define:

No,I don't want to do that since I want to modularize & encapsulate my code so that if others want to use my code, I can just give them the object and header file.

Originally Posted by Salem

Well sounds like a job for void

I was thinking of implementing it this way and it seems I have no choice given the fact that I or anyone else can't predict what future datatypes will be stored in this general list. The only problem I have with this is the example you gave of the list being allowed to store any data type in the list which is kind of like a mixed bag of goodies waiting to be spilled all over the table. Segmentation faults anyone? But this is generally acceptable I guess since this is c and c generally assumes the programmers know what their doing.

**Salem** · 12-04-2004

It seems to me what you're trying to do is implement C++ templates in C

**Prelude** · 12-04-2004

>I guess it's too much to ask c to compile first and ask questions later about the datatype.
Yes, it is.

If you want dynamic typing, might I suggest Perl or Python?

>I or anyone else can't predict what future datatypes will be stored in this general list.
That's why they call it a general list.

>Segmentation faults anyone?
It's not nearly the issue you make it out to be. I can only think of a few times where there was any question what type of content I had in a data structure. And the question was answered with a quick check before I tried to do anything stupid. Pointers to void generally don't cause a problem unless the user programmer is incredibly stupid, very inexperienced, or has malicious intent.

>since this is c and c generally assumes the programmers know what their doing.
So does C++, whose template feature is what you seem to be in need of.

**anonytmouse** · 12-04-2004

One typical solution is to require that the first member of a struct added to a list is the control data struct. Here is an example from a WinAPI linked list implementation:

Code:

typedef struct _PROGRAM_ITEM {
    SLIST_ENTRY ItemEntry; /* Control data is first member of struct. */
    ULONG Signature;  /* Any user data follows. */
} PROGRAM_ITEM, *PPROGRAM_ITEM;

int main(void)
{
    ULONG Count;
    PSLIST_ENTRY FirstEntry, ListEntry;
    SLIST_HEADER ListHead;
    PPROGRAM_ITEM ProgramItem;

    // Initialize the list header.
    InitializeSListHead(&ListHead);

    // Insert 10 items into the list.
    for( Count = 1; Count <= 10; Count += 1 )
    {
        ProgramItem = (PPROGRAM_ITEM)malloc(sizeof(*ProgramItem)); /* User takes care of memory allocation. */
        ProgramItem->Signature = Count;
        FirstEntry = InterlockedPushEntrySList(&ListHead, 
                       &ProgramItem->ItemEntry); /* Your list functions simply manipulate the control data, you don't know the actual size of the struct. */
    }

    // Remove 10 items from the list.
    for( Count = 10; Count >= 1; Count -= 1 )
    {
        ProgramItem = (PPROGRAM_ITEM) InterlockedPopEntrySList(&ListHead);
        printf("Signature is %u\n", ProgramItem->Signature);
        free(ProgramItem);
    }

    ...

Of course if you need the actual size of the data struct, for sorting or other reasons, you can pass that in too.

>> So does C++, whose template feature is what you seem to be in need of. <<

It seems csprite could get crude template functionality by including the code, as suggested previously. Another option is to use a project level define. Neither of these options allow different data types to be used in the same file.

**Thantos** · 12-04-2004

void main( )

My eyes my eyes!

**quzah** · 12-04-2004

Yeah I just handed out some candy for that one.

Quzah.

**Thantos** · 12-04-2004

Well in all fairness anonytmouse was just showing us how MS does it which we all know is crap code

**Prelude** · 12-04-2004

>anonytmouse
>Join Date: Dec 2002
>Posts: 1,175
>
>void main( )
Aroo? My math may not be the best, but you should know better. Let the beatings begin!

**sand_man** · 12-05-2004

*sand_man beats anonytmouse with a whacking stick

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