Re: [boost] [dynamic_bitset] Endianess and Adaption Vs Copying

Subject: Re: [boost] [dynamic_bitset] Endianess and Adaption Vs Copying
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2012-05-03 13:22:28

Joel wrote:
> dynamic_bitset provides a constructor that takes raw data as input.  If 
> the source for that input has the bits ordered consecutively (which would
> allow one to change the block type without changing the ordering of the bits)
> then the constructor will not function properly.

Perhaps you could post a short example of some code that you think 
would fail?

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) { ...

So you can do this:

const char* data = ......;  // From a "platform independent" binary file.
dynamic_bitset<unsigned int>  b(data,data+size);

This probably doesn't do what you want, but not for endianness reasons; 
it seems to initialise each int-sized block with one byte of data, 
leaving the other 24 bits unset.  If you write this:

const char* data = ......;  // From a "platform independent" binary file.
dynamic_bitset<unsigned char>  b(data,data+size);

then I believe it will always work.  If you write this:

const char* data = ......;  // From a "platform independent" binary file.
const int* data_i = static_cast<const int*>(data);
dynamic_bitset<int>  b(data_i,data_i+size);

then you get endianness problems.  But you would have the same trouble 
if you tried to initialise e.g. a vector<int> in that way.

> 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?)

Regards,  Phil.