How do I count the number of zero bits in an integer?

Question

How would i go about finding the number of \'zero\' bits in C++. Suppose I have an integer;

int value = 276; 

For which I have the bits 100010

Answer 1

I'm surprised no one has mentioned this one:

int num_zero_bits = __builtin_popcount(~num);

This will give the number of zero bits in num when used with GCC.