Thread: nested structure help

Hello,
I have a structure that I would like to reference different structures using the same member. for example:

typedef struct
{
word32 menuItemEnabledFlag : 1;
word32 updateImmediatelyFlag : 1;
word32 absolutePositionFlag : 1;
word32 octorectFlag : 1;
word32 verticalTextFlag : 1;
word32 TimeoutFlag : 1;
word32 spare : 26; //spare bits

byte backgroundColor;
byte foregroundColor;
byte borderColor;
word16 xLeftPos;
word16 yTopPos;
word16 minXLength;
word16 minYLength;
byte displayField;
word32* field;
}tMENU_ITEM;

typedef struct
{
word32* firstGraphic;
word32* lastGraphic;
word32* current;
byte size;
}tGRAPHIC_FIELD;

tGRAPHIC_FIELD graphic;

typedef struct
{
word16 lowLimit;
word16 highLimit;
word16 current;
}tNUMERIC_FIELD;

tNUMERIC_FIELD numeric;

I would like the word32* field pointer to point to either a tGRAPHIC_FIELD structure or a tNUMERIC_FIELD structure, optionally. When I initialize the field member I use either:

(word32*) &graphic,

or

(word32*) &numeric,

If I want to access a member (like current, for example) of the tGRAPHIC_FIELD structure I can use a cast, like so, to get access to the member:

(mItem is a pointer to a instance of tMENU_ITEM)

fieldPtr = (*(tGRAPHIC_FIELD*)mItem->field).current;

The problem is this, I need a generic accessor to the nested structure and thus the cast is causing a problem. How can I get around this? Thanks. Please note that the "sub" structures are numerous and conditional statements wouldn't work.

Well if you want a cast-less solution, you need to do this

Code:

typedef struct 
{ 
    word16 lowLimit; 
    word16 highLimit; 
    word16 current; 
} tNUMERIC_FIELD; 

typedef struct 
{ 
    word32* firstGraphic; 
    word32* lastGraphic; 
    word32* current; 
    byte size; 
} tGRAPHIC_FIELD; 

typedef struct 
{ 
// etc
    byte displayField; 
    union {
        tGRAPHIC_FIELD *gf;
        tNUMERIC_FIELD *nf; 
    } field;
}tMENU_ITEM; 

tGRAPHIC_FIELD graphic; 
tNUMERIC_FIELD numeric;

Then you can do things like

Code:

tMENU_ITEM thing;
thing.field.gf = &graphic;
thing.field.gf->size = 23;

> I need a generic accessor to the nested structure
Your only other choice is to make field a void* type, but then you have to cast.