c - Access struct members as if they are a single array?

Question

I have two structures, with values that should compute a pondered average, like this simplified version:

typedef struct
{
  int v_move, v_read, v_suck, v_flush, v_nop, v_call;
} values;

typedef struct
{
  int qtt_move, qtt_read, qtt_suck, qtd_flush, qtd_nop, qtt_call;
} quantities;

And then I use them to calculate:

average = v_move*qtt_move + v_read*qtt_read + v_suck*qtt_suck + v_flush*qtd_flush + v_nop*qtd_nop + v_call*qtt_call;

Every now and them I need to include another variable. Now, for instance, I need to include v_clean and qtt_clean. I can't change the structures to arrays:

typedef struct
{
    int v[6];
} values;
typedef struct
{
    int qtt[6];
} quantities;

That would simplify a lot my work, but they are part of an API that need the variable names to be clear.

So, I'm looking for a way to access the members of that structures, maybe using sizeof(), so I can treat them as an array, but still keep the API unchangeable. It is guaranteed that all values are int, but I can't guarantee the size of an int.

Writing the question came to my mind... Can a union do the job? Is there another clever way to automatize the task of adding another member?

Thanks, Beco

Answer 1

What you are trying to do is not possible to do in any elegant way. It is not possible to reliably access consecutive struct members as an array. The currently accepted answer is a hack, not a solution.

The proper solution would be to switch to an array, regardless of how much work it is going to require. If you use enum constants for array indexing (as @digEmAll suggested in his now-deleted answer), the names and the code will be as clear as what you have now.

If you still don't want to or can't switch to an array, the only more-or-less acceptable way to do what you are trying to do is to create an "index-array" or "map-array" (see below). C++ has a dedicated language feature that helps one to implement it elegantly - pointers-to-members. In C you are forced to emulate that C++ feature using offsetof macro

static const size_t values_offsets[] = { 
  offsetof(values, v_move),
  offsetof(values, v_read),
  offsetof(values, v_suck),
  /* and so on */
};

static const size_t quantities_offsets[] = { 
  offsetof(quantities, qtt_move),
  offsetof(quantities, qtt_read),
  offsetof(quantities, qtt_suck),
  /* and so on */
};

And if now you are given

values v;
quantities q;

and index

int i;

you can generate the pointers to individual fields as

int *pvalue = (int *) ((char *) &v + values_offsets[i]);
int *pquantity = (int *) ((char *) &q + quantities_offsets[i]);

*pvalue += *pquantity;

Of course, you can now iterate over i in any way you want. This is also far from being elegant, but at least it bears some degree of reliability and validity, as opposed to any ugly hack. The whole thing can be made to look more elegantly by wrapping the repetitive pieces into appropriately named functions/macros.