c++ - Using part of a variable as bool

Question

Let's say memory is precious, and I have a class with a uint32_t member variable ui and I know that the values will stay below 1 million. The class also hase some bool members.

1. Does it make sense to use the highest (highest 2,3,..) bit(s) of ui in order to save memory, since bool is 1 byte?

2. If it does make sense, what is the most efficient way to get the highest (leftmost?) bit (or 2nd)? I read a few old threads and there seems to be disagreement about using inline ASM or some sort of shift.

Answer 1

It's a bit dangerous to use part of the bits as bool. The thing is that the way the numbers are kept in binary, makes it harder to maintain that keeping mechanism correct.

Negative numbers are kept as a complement of positive. Check this for more explanation. You may assign number to be 10 and then setting bool bit from false to true, and the number may turn out to become huge negative number as a result.

As for getting if n-th bit is 0 or 1 you can use this, where 0-th bit is the right most:

int nth_bit(int a, int n){
    return a & (1 << n);
}

It will return 0 or 1 identifying the n-th bit.