Generate Bitmask

Question

Given two integers; size and offset, how would I go about generating a bitmask with the following properties;

MSD             LSD
1111 1111 0000 1111
          ^  ^
size = 4 -|  |
             |  
offset = 4  -|

Answer 1

Since the question is tagged as c++, I'm going to provide an STL based solution:

bitset<NUM_BITS> bs(0);
bs = ((bs.flip() << size).flip() << offset).flip();

Same code split into several lines:

bitset<NUM_BITS> bs(0);
bs.flip();
bs <<= size;
bs.flip();
bs <<= offset;
bs.flip();

Performance considerations are up to the readers.