c define arrays in struct with different sizes

Question

I need to define a struct for two types of objects. Both have exactly the same data structure and perform same tasks (member methods).

The ONLY difference is that the a

Answer 1

Even with a union, the struct will be as big as the largest of the two arrays.

Do one of these:

1. Ignore the overhead of the biggest array size. (Use one array or a union)
2. Create a separate struct for each type.
3. Dynamically allocate the array with malloc.

Answer 2

I would make matr a flexible array at the end of your struct. Then, I would stick the arr array into the last row of matr.

typedef struct {
    int size;
    int var1, var2;
    int matr[];
} s;

static inline int size_ok_s (int size) {
    switch (size) {
    case SIZE_A:
    case SIZE_B:
        return 1;
    default:
        break;
    }
    return 0;
}

s * create_s (int size) {
    s *x = 0;
    if (size_ok_s(size)) {
        x = malloc(sizeof(*x) + sizeof(int[size+1]));
        if (x) x->size = size;
    }
    return x;
}

To achieve a uniform interface, you can use a macro:

#define s_matr(x) ((int (*)[(x)->size])(size_ok_s((x)->size) ? (x)->matr : 0))
#define s_arr(x)  (s_matr(x)[(x)->size])

So, to access the i^th row and j^th column of s *foo's matr, and its k^th element of arr:

s *foo = create_s(SIZE_A);
/* ... */
    s_matr(foo)[i][j] = 0;
    s_arr(foo)[k] = 0;

Flexible array members are a new feature of C.99 described in §6.7.2.1 ¶16. Prior to C.99, C programmers often used what was known as the struct hack:

typedef struct {
    int size;
    int var1, var2;
    int matr[1];
} s;

s * create_s (int size) {
    s *x = 0;
    if (size_ok_s(size)) {
        x = malloc(sizeof(*x) + sizeof(int[size]));
        if (x) x->size = size;
    }
    return x;
}

This is a hack since in C.89-90, indexing the matr array with a value greater than 0 is technically accessing an object beyond its boundary. However, it was a common practice, and widely portable. C.99 formally sanctioned the mechanism with the flexible array member, although it requires the syntax of not specifying a size in the array declaration.

Answer 3

Have the arrays be pointers and allocate to them (the appropriate size) as needed. Only downsize is access to the 2D array will be a little clumsy.