Question

Following a C tutorial, I’ve encountered a type dependency that I don’t know how to resolve. So far, we had this:

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

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

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

No problem. Now, it’s changing to include a THIS pointer:

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

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

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

The definitions depend on each other in a circular way. The cl.exe compiler shows lots of syntax errors because by the time I use IExample it hasn’t been declared yet. How can I resolve this?

Compile with cl.exe /W4.

Update

Thanks – three moreless equivalent answers at the same time, I could have chosen any one of them.

So, to resume, what choices do you have to make when solving this problem? First, naming convention – append _t to typedefs, or _s (or Struct) to structs? Probably a matter of taste. Then, whether you want to have the forward definition as part of a typedef or as part of a struct. Probably also a matter of taste. Here’s the problem in another form and three ways to solve it:

/* won't compile */
typedef struct {
        B * b;
        C * c;
} A;

typedef struct {
        A * a;
        C * c;
} B;

typedef struct {
        A * a;
        B * b;
} C;

First solution:

struct B_s;
struct C_s;

typedef struct {
        struct B_s * b; // typedef not available yet
        struct C_s * c; // ditto
} A;

typedef struct {
        A * a;
        struct C_s * c; // ditto
} B;

typedef struct {
        A * a;
        B * b;
} C;

Second solution:

typedef struct B_s B;
typedef struct C_s C;

typedef struct {
        B * b;
        C * c;
} A;

// typedef struct ... B => change to:
struct {
        A * a;
        C * c;
} B_s;

// typedef struct ... C => change to:
struct {
        A * a;
        B * b;
} C_s;

And the third (and most symmetrical) solution:

struct A_s;
struct B_s;
struct C_s;

typedef struct A_s A;
typedef struct B_s B;
typedef struct C_s C;

struct {
        B * b;
        C * c;
} A_s;

struct {
        A * a;
        C * c;
} B_s;

struct {
        A * a;
        B * b;
} C_s;

Answer 1

typedef struct IExample_s IExample;

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

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

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