bitwise operations on vector<bool>

Question

Hi!

what's the best way to perform bitwise operations on vector<bool>?

as i understand, vector<bool> is a specialisation that uses one bit per boolean. I chose vector<bool> for memory saving reasons. I know that there are some problems with vector<bool> but for my needs it's apropriate.

now - what's the most performant way of aplying bitwise operations to whole such vectors?

if i do it in a for loop and readout each single bool and store it back, the way I understand it a lot more operations are performed inside in order to access the actual values.

thanks!

Answer 1

Ignoring the title of your question, lets answer this question, instead:

what's the best way to perform bitwise operations on vector?

The best way is to define your vector as vector<unsigned char> (or vector<uint32_t>, or whichever other integer type you choose), and do your bitwise operations how you normally would for an array of unsigned integers. Things will be much faster this way, and there will be no hidden mechanism.

You can use division (or bitwise operators, if you're slick) to resolve which array index you need to operate against, and for-loops to apply bitwise operations larger than a single element.

Here's a related question: http://stackoverflow.com/questions/3956898/bit-twiddling-a-lot-of-bits-in-c/3956920#3956920

You will basically be doing these same operations, if and when you decide to wrap vector<unsigned some-int-type> with your own operators.

Answer 2

If the number of bits are fixed at compile time, you would be much better off using std::bitset (GNU bitset docs)

If not, (i.e. number of bits varies at runtime), then you should see and can use boost dynamic_bitset. (Boost docs)

In both of these, it is extremely easy to do all the bitwise operations.