Question

Is there any performance loss/gain using bitset in place where hand written?

How to build the following using a bitset at runtime

• make all the bits between 2 and 5 as zero i.e., 11110011.

Answer 1

The easiest solution to your second question would be to use another bitset.

void makebitszero(bitset<8>& b) {
  // Everything but bits 3 and 4 (between 2 and 5).
  static const bitset<8> mask = ~bitset<8>(12);
  b &= mask;
}

It takes a bit of math to come up with an expression for mask given two bit positions.

[edit]
Ok, here’s the math. The trick is that (1UL << X) -1 is a sequence of X ones. E.g. 3 => 00000111.
Hence, (1<<5) – (1<<3) = 00011111 – 00000111 -1 + 1 = 00011000 (bits 3 and 4). Thus in code:

template<int i, int j, int N> 
void makeBitsZero(bitset<N>& b) {
  // Everything from bit i up to but not including bit j (i < j)
  static const bitset<N> mask = ~bitset<N>(1UL<<j) - (1UL<<i));
  b &= mask;
}