Re: [Boost-users] [Boost][mpl] vector_c boolean operations

1 Oct 2005

      On Sep 29, 2005, at 7:45 PM, David Abrahams wrote:
...
"Noel Belcourt"  writes:
...
r = p1 || p1 && p2
That would be equivalent to r = p1.  Why go to all that trouble?  Or
do you mean
r == p1 || p1 && p2
Yes, that's what I meant.
...
Anyway, what does it mean to use the logical && operator on two
sequences?  Do you want elementwise comparison?
Yes, element-wise comparison.
...
Do you want the same
for r == p1?
Yes.
...
The usual meaning is to compare all elements and produce
a scalar boolean, which would then be incompatible with the vector of
bools produced by p1 && p2.  Maybe you mean something like;
mpl::transform<
          mpl::transform >::type
        , mpl::transform >::type
        , mpl::or_<_,_>
...
::type
This looks reasonable.  Transform returns a new sequence produced by 
applying the binary operation to a pair of elements from the input 
sequences.

Let me see if I understand this.

So if I want to perform an element-wise mpl::or operation of two 
sequences (r, p1) and then test the resulting sequence for equivalence 
to the input sequence, something like this should work since as you 
observe below, mpl::equal will compare two sequences and yield a scalar 
boolean.

BOOST_STATIC_ASSERT((
   equal<
	r
        , transform >::type
...
::type::value
));
...
Ugh; do yourself a favor and apply a couple of namespace aliases at
least; the whole point of those placeholders is that they're supposed
to increase readability!
Duly noted.
...
It seems to me that you also have several conceptual problems here.
First, you have perfectly good metafunction parameters here that you
can use; what are the placeholders supposed to be doing?  Placeholders
only have an effect in a context where an MPL placeholder expression
can be used, e.g. in an argument to mpl::apply<...>.  Why not just use
R, P1, and P2 directly?
In this case I could.
...
Second, you seem to be trying to use the equal algorithm, which
operates on sequences, on the results of some invocation of mpl::or_,
which does not return a sequence but a scalar bool-valued MPL integral
constant.
True, I was confused about this point.  So I'll use transform to 
generate a sequence representing the element-wise binary operation and 
then use equal.

Thanks,

-- Noel