Subject: Re: [boost] [dynamic_bitset] Endianess and Adaption Vs Copying
From: Joel (jdy_at_[hidden])
Date: 2012-05-03 19:46:56
Phil Endecott <spam_from_boost_dev <at> chezphil.org> writes:
> Perhaps you could post a short example of some code that you think
> would fail?
If a C library created an internal bit vector where the bits were packed
tightly and in order from left-to-right (low memory to high memory),
dynamic_bitset would be able to correctly map the vector on a big-endian machine
but would mess it up on a little-endian machine.
The order that the bits are packed is completely arbitrary. This library
has chosen to pack the bits from least-significant to most-significant within
blocks and left-to-right across blocks.
> I was a bit surprised to find that it has this:
>
> template <typename Block>
> class dynamic_bitset {
> template <typename BlockInputIter>
> dynamic_bitset(BlockInputIter first, BlockInputIter last) { ...
That didn't surprise me, other than I wish it had a comment giving the
expected bit-ordering.
> So you can do this:
>
> const char* data = ......; // From a "platform independent" binary file.
> dynamic_bitset<unsigned int> b(data,data+size);
Should be
const uint32_t data = .....; in that case!
>
> Joel say:
> > At minimum, the bit-ordering imposed by dynamic-bitset should be
> > explicitly documented.
>
> The BIT ordering? No, bit ordering is always the same. You never need
> to re-order the bits within a byte.
>
> (What am I missing?)
Bit ordering is completely arbitrary. If the constructor for reference in
dynamic_bitset.hpp is changed from:
// the one and only non-copy ctor
reference(block_type & b, block_type pos)
:m_block(b),
m_mask( (assert(pos < bits_per_block),
block_type(1) << pos )
)
{ }
to:
// the one and only non-copy ctor
reference(block_type & b, block_type pos)
:m_block(b),
m_mask( (assert(pos < bits_per_block),
((block_type(1) << (bits_per_block-1)) >> pos ))
)
{ }
and the ordering would be reversed. Now on little-endian machines, the ordering
nicely goes always left-to-right.
Joel