[Boost-users] dynamic_bitset, bitset, etc

Subject: [Boost-users] dynamic_bitset, bitset, etc
From: Zachary Turner (divisortheory_at_[hidden])
Date: 2009-06-02 14:05:37

I find that I frequently want a class that provides the capabilities of a
boost::dynamic_bitset, but that i want to be able to load the values all at
once from a huge block of binary data.  The exact use case is that I'm
reading block data off of a disk, and in most modern file systems there's a
very large chunk of data at the beginning that is just a series of bits that
specify whether or not particular blocks are allocated.  So I issue an I/O
to read a chunk of this bitmap, then I just want to tell the class "do a
huge memcpy and use this block of data as your internal buffer".  Or better
yet, member functions that a) give direct access to the internal buffer and
b) tell me how many bytes the internal buffer is, and c) let me resize the
internal buffer to hold x bits.

There's vector<bool>, std::bitset, boost::dynamic_bitset, and
boost::bitset.  I understand why vector<bool> can't support this, but is
there some fundamental reason why this seemingly simple operation is not
supported on any available implementation of a bit set anywhere?  Or is it
supported and I'm just overlooking the function necessary to do it?  I see
there's a constructor that takes a BlockIterator, but I want it to
ultimately boil down to a memcpy, not a for loop.