How to order C structure/function declarations which use each other?

Question

I am trying to create a structure by the name "IExampleVtbl" that will hold a pointer to my functions (SetStringPtr,GetStringPtr) and will be a part of another structure "IExample".

But I want to pass the other structure "IExample" as parameter to the functions (SetStringPtr,GetStringPtr).

This is the code:

#include <windows.h>
#include <stdio.h>

typedef struct {
    SetStringPtr *SetString;
    GetStringPtr *GetString;
} IExampleVtbl;

typedef struct {
    IExampleVtbl *lpVtbl;
    DWORD   count;
    char    buffer[80];
} IExample;

typedef long SetStringPtr(IExample *, char *);
typedef long GetStringPtr(IExample *, char *, long);

long SetString(IExample *this, char * str)
{
    ...

    return(0);
}

long GetString(IExample *this, char *buffer, long length)
{
    ...

    return(0);
}

As you can see the first structure need to know about the functions, the functions need to know about the second structure which need to know about the first structure.

How I can solved that?

score 4 · Accepted Answer · answered Feb 25 '17 at 18:44

You can solve the problems by putting things in the following order

typedefs for the structures
typedefs for the function pointers
structure definitions
function definitions

To make that work, you'll need to define the structures with tags:

typedef struct IExampleVtblTag IExampleVtbl;
typedef struct IExampleTag IExample;
typedef long SetStringPtr(IExample *, char *);
typedef long GetStringPtr(IExample *, char *, long);

struct IExampleVtblTag {
    SetStringPtr *SetString;
    GetStringPtr *GetString;
};

struct IExampleTag {
    IExampleVtbl *lpVtbl;
    DWORD   count;
    char    buffer[80];
};

long SetString(IExample *this, char * str)
{
    return(0);
}

long GetString(IExample *this, char *buffer, long length)
{
    return(0);
}

score 1 · Answer 2 · answered Feb 25 '17 at 18:44

You combine a forward declaration with the type-alias definition:

// Forward declaration of the structure IExample
// And at the same time definition of the type-alias IExample
typedef struct IExample IExample;

typedef long SetStringPtr(IExample *, char *);
typedef long GetStringPtr(IExample *, char *, long);

// Now the definition of the structures
typedef struct { ... } IExampleVtbl;

// Because the previous type-alias definition, we need to specify a structure tag
struct IExample { ... };

Aniruddh Dikhit · Answer 3 · 2017-02-25T18:50:42.543

0

typedef for the struct to be typedef'd can precede the definition of struct, so a bit of re-arrangement should make things work here

typedef struct IExample IExample;

typedef long SetStringPtr(IExample *, char *);
typedef long GetStringPtr(IExample *, char *, long);

typedef struct {
    SetStringPtr *SetString;
    GetStringPtr *GetString;
} IExampleVtbl;

struct IExample {
    IExampleVtbl *lpVtbl;
    long   count;
    char    buffer[80];
};

edited Feb 25 '17 at 18:50

answered Feb 25 '17 at 18:45

Aniruddh Dikhit

662
4
10

Symbols with leading underscore followed by an upper-case letter is reserved for the "implementation" (compiler and standard library) in all scopes. And since structure tags live in a separate namespace, they can have the same name as other symbols (like the type-alias) so the underscore isn't needed. – Some programmer dude Feb 25 '17 at 18:46
[What are the rules about using an underscore in a C++ identifier?](http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier) – Richard Chambers Feb 25 '17 at 18:50

How to order C structure/function declarations which use each other?

3 Answers3