Thread: Multiple read types

A struct/union?
http://c-faq.com/struct/taggedunion.html

I would do something along the lines of this:

Code:

#define FEAT_BOOLEAN 0
#define FEAT_BYTE 1
#define FEAT_WORD 2
#define FEAT_DWORD 3
#define FEAT_INT64 4

typedef struct tagFeatureData {
    void* pAnyData;
    unsigned int Type;
} FeatureData;

void multi_read(int feature_id, FeatureData* fd) {
    switch(feature_id) {
        case 1:
        {
            bool whatever = false;
            fd->pAnyData = (void*)&whatever;
            fd->Type = FEAT_BOOLEAN;
            break;
        }
        /* ... */
    }
}

I'm not quite sure about that cast though. I'm more used to C++'s templates.

Edit: I think Dave Sinkula's idea is better.

Code:

typedef struct tagFeatureData {
    enum { BOOLEAN, BYTE, WORD } code;
    union {
        bool dat_boolean;
        BYTE dat_byte;
        WORD dat_word;
        /* ... */
    };
} FeatureData;

FeatureData* multi_read(int feature_id) {
    // You may want to check that line, I've never used malloc()
    FeatureData* fd = (FeatureData*)malloc(sizeof(FeatureData));
    if(fd != NULL) {
        switch(feature_id) {
            case 1:
                fd->dat_code =  BOOLEAN;
                fd->dat_boolean = 1;
                break;
        }
        return fd;
    }
}

int main() {
    FeatureData* fd = multi_read(1);
    if(fd->code == BOOLEAN)
        printf("%d", fd->dat_boolean);
    free(fd);
}

Okie, I realise there's one setback for the above idea:-

Code:

FeatureData* fd = multi_read(1);
if(fd->code == BOOLEAN)
    printf("%d", fd->dat_boolean);

Since the multi_read() returns the struct, I still need an extra line of checking to determine what data type is in use for that feature. I could use this idea with some design change though to my solution.

Still, I would like to simply achieve the initial design having the multi_read() returning the value directly, with whatever checking/processing done in multi_read(), so the caller just invoke the function and can either use the value read directly or store it elsewhere. e.g.

Code:

boolean result;
dword value;
if( multi_read(MY_NEW_FEATURE_ENABLE) == TRUE)
{
  value = multi_read(NEW_FEATURE_X_VALUE);
}

Any ideas?

Edit: I'm not being picky, but it will be nice if the caller of multi_read() need not do anything additional beside just calling the function and getting their value. Like I said, its a design choice, but just wondering if this is achievable in C.
Another constraint I have is that I cannot use memory related function calls e.g. malloc, realloc etc.