C/C++ Bit Array or Bit Vector

I think you’ve got yourself confused between arrays and numbers, specifically what it means to manipulate binary numbers.

I’ll go about this by example. Say you have a number of error messages and you want to return them in a return value from a function. Now, you might label your errors 1,2,3,4… which makes sense to your mind, but then how do you, given just one number, work out which errors have occured?

Now, try labelling the errors 1,2,4,8,16… increasing powers of two, basically. Why does this work? Well, when you work base 2 you are manipulating a number like 00000000 where each digit corresponds to a power of 2 multiplied by its position from the right. So let’s say errors 1, 4 and 8 occur. Well, then that could be represented as 00001101. In reverse, the first digit = 1*2^0, the third digit 1*2^2 and the fourth digit 1*2^3. Adding them all up gives you 13.

Now, we are able to test if such an error has occured by applying a bitmask. By example, if you wanted to work out if error 8 has occured, use the bit representation of 8 = 00001000. Now, in order to extract whether or not that error has occured, use a binary and like so:

  00001101
& 00001000
= 00001000

I’m sure you know how an and works or can deduce it from the above – working digit-wise, if any two digits are both 1, the result is 1, else it is 0.

Now, in C:

int func(...)
{
    int retval = 0;

    if ( sometestthatmeans an error )
    {
        retval += 1;
    }


    if ( sometestthatmeans an error )
    {
        retval += 2;
    }
    return retval
}

int anotherfunc(...)
{
    uint8_t x = func(...)

    /* binary and with 8 and shift 3 plaes to the right
     * so that the resultant expression is either 1 or 0 */
    if ( ( ( x & 0x08 ) >> 3 ) == 1 )
    {
        /* that error occurred */
    }
}

Now, to practicalities. When memory was sparse and protocols didn’t have the luxury of verbose xml etc, it was common to delimit a field as being so many bits wide. In that field, you assign various bits (flags, powers of 2) to a certain meaning and apply binary operations to deduce if they are set, then operate on these.

I should also add that binary operations are close in idea to the underlying electronics of a computer. Imagine if the bit fields corresponded to the output of various circuits (carrying current or not). By using enough combinations of said circuits, you make… a computer.