A Basic C question, declarations in cyclic dependency

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I was doing COM in plain C[^] tutorial. I have the following header file:

C++

#ifndef EXAMPLE_H
#define EXAMPLE_H

#define BUF_SIZE 80

typedef long SetStrPtr(IExample *, char *);
typedef long GetStrPtr(IExample *, char *, long);

typedef struct {
    SetStrPtr * SetString;
    GetStrPtr * GetString;
} IExampleVtbl;

typedef struct {
    IExampleVtbl * lpVtbl;
    DWORD          count;
    char           buffer[BUF_SIZE];
} IExample;
#endif

Now this gives me a compile error. Because..

1. to declare the structure IExample, I need to first declare IExampleVtbl
2. ..which needs the declaration of SetStrPtr and GetStrPtr
3. ..which needs the declaration of IExample

Now as you can see, it's a cyclic dependency. How to get through this?

Posted 16-Feb-12 1:14am

krumia

Updated 16-Feb-12 1:15am

v2

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2 solutions

Solution 1

It's nasty, but the only way to do it is you change the IExample pointers to void pointers:

C#

typedef long SetStrPtr(void *, char *);
typedef long GetStrPtr(void *, char *, long);

It will work perfectly happily because all pointers are the same length, but it won't be strongly typed.

Posted 16-Feb-12 1:41am

OriginalGriff

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krumia 16-Feb-12 11:56am

Thanks! But signor Pallini had a strongly typed answer :)

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CPallini · Accepted Answer · 2012-02-16T02:07:00

Solution 2

try this:

C

#ifndef EXAMPLE_H
#define EXAMPLE_H
 
#define BUF_SIZE 80

struct _IExample;
typedef struct _IExample IExample;

typedef long SetStrPtr(IExample *, char *);
typedef long GetStrPtr(IExample *, char *, long);

 
typedef struct {
    SetStrPtr * SetString;
    GetStrPtr * GetString;
} IExampleVtbl;
 
struct _IExample{
    IExampleVtbl * lpVtbl;
    DWORD          count;
    char           buffer[BUF_SIZE];
};

#endif

Posted 16-Feb-12 2:07am

CPallini

Updated 16-Feb-12 9:33am

v2

Comments

krumia 16-Feb-12 11:52am

Yay! worked! Grazie signor Pallini. ;) 5ed!

But, there should be a slight change. typedef struct _Example IExample; should be typedef struct _IExample IExample;

CPallini 16-Feb-12 15:34pm

Grazie!
Sorry for the mistake, now it should be fixed.