How do I get the "lost" bit from a bit shift?

Question

I want to bit shift a variable and store the bit that's shifted out in a boolean.

Something like:

unsigned int i = 1;
bool b = rshift(&i); // i now equals 0 and b is set to true

How can this be accomplished?

Answer 1

You have to capture the bit before the shift:

bool rshift(unsigned int* p) {
    bool ret = (*p) & 1;
    *p >>= 1;
    return ret;
}

Answer 2

To do this for any shift, and any type, reverse the operation (to check whether you lost something).

template <typename T> bool rshift(T& val) {
  T const original = val;
  return ((val >>= 1) << 1) != original;
}
template <typename T> bool lshift(T& val) {
  T const original = val;
  return ((val <<= 1) >> 1) != original;
}

Answer 3

You don't. You have to test it before the shift:

bool
rshift( unsigned& i )
{
    bool results = (i & 0x01) != 0;
    i >>= 1;
    return results;
}

(This is simple for a right shift. For a left shift, you have to know the number of bits in the word.)