[boost] Re: New dynamic_bitset

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-14 08:12:35

Gennaro Prota <gennaro_prota_at_[hidden]> writes:

> On Wed, 14 Apr 2004 09:34:14 +0200, Gennaro Prota
> <gennaro_prota_at_[hidden]> wrote:
>
>>I also added
>>the following functions: find_first, find_next, get_allocator(),
>>max_size(), intersects() and empty().
>
> The function intersects() was a user request. It returns true if and
> only if the two bitsets have any bit "in common", i.e.:
>
> a.intersects(b) == true <=> An index i < a.size(), b.size() exists
>                             such as a[i] == true and b[i] == true
>
> Logically, this is the same as doing (a & b).any() [except that
> currently a & b requires the two bitsets to have the same size - this
> restriction will likely be removed in the future if we reach a
> consensus on semantics] but is faster because it doesn't require any
> copy and can be done with one pass through the blocks
>
> I was thinking, however: what about returning the index of the first
> bit in common (or npos), instead of true/false?

Ugh,  not bit indices, at least not with *that* name!

      if (intersects(a,b))  // oops
      {

      }

Maybe optional<std::size_t>?

> Secondly, does anyone find confusing that empty() means size() == 0
> rather than "the set is empty"? 

Not at all.  This is not a std::set<> after all.  It's well known that
it has the semantics of an array.  But maybe you should rename it
bit_array or bit_vector instead of dynamic_bitset ;-)

> Someone could be tempted to write e.g.
>
>   if ((a ^ b).empty()) ...
>
> instead of
>
>   if ((a ^ b).none()) ...

IMO 

    if (a ^ b)

should be supported.  But then again, this is making me think of
expression templates ;-)

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com