Question

I perform some calculations, based on the result, I would like to either use a short int or int for some type of data for the remaining program. Can (/How can) this be done sensibly in C or C++? I don’t really care about the amount of memory used (i.e., 2 or 4 bytes), my primary aim is to access generic arrays as if they contained data of this type. I would like to avoid code such as the following:

char s[128];
if (result of preliminary calculations was A)
  *((int*) s) = 50;
else
  *((short int*) s) = 50;

to set the first 4 or 2 bytes of s. A conditional global typedef would be ideal:

if (result of preliminary calculations was A)
  typedef int mytype;
else 
  typedef short int mytype;

I am not that familiar with C++ class templates (yet). Do they apply to my problem? Would I have to change the declarations throughout my program (to myclass< > and myclass< >*)?

Many thanks!

Frank

Edit: The values may not always be aligned. I.e, a int can start at position 21. Thanks for the answers.

Answer 1

For plain C, you could do this using function pointers:

static union { s_int[32]; s_short[64]; s_char[128]; } s;

static void set_s_int(int i, int n)
{
    s.s_int[i] = n;
}

static int get_s_int(int i)
{
    return s.s_int[i];
}

static void set_s_short(int i, int n)
{
    s.s_short[i] = n;
}

static int get_s_short(int i)
{
    return s.s_short[i];
}

static void (*set_s)(int, int);
static int (*get_s)(int);

Set them once based on the preliminary calculations:

if (result of preliminary calculations was A)
{
    set_s = set_s_int;
    get_s = get_s_int;
}
else
{
    set_s = set_s_short;
    get_s = get_s_short;
}

Then just use the function pointers in the rest of the program:

set_s(0, 50);   /* Set entry 0 in array to 50 */

Your file writing function can directly reference s or s.s_char depending on how it works.