Re: [Boost-users] [mpl] Newbie question - working with sets

From: David Abrahams (dave_at_[hidden])
Date: 2007-09-09 14:33:12

on Fri Sep 07 2007, Dan Posluns <dan-AT-danposluns.com> wrote:

> The Create() method would iterate over the subset, tally up the
> sizeof() for each class, and then perform a single allocation for
> data_buffer_for_properties and proceed to use placement new to
> instantiate each class in the subset. (The destructor would then
> clean up all of that ugliness.)

This sounds like a good job for Boost.Fusion

> The which_properties_are_used member would be an efficient list
> manager that can be used to lookup the existence of properties,
> indexing the types in the set using the order<> operator.
>
> Unfortunately, I haven't been able to get very far with set<>. I tried 
> the following:
>
> typedef fold<
>         PropertiesSubset,
>         int_<0>,
>         plus< _1, sizeof_<_2> >
>     >::type TotalSize;
>
> cout << TotalSize::value;
>
> ... and it gives me nothing like what I'm looking for. 

Huh... That won't give you anything useful anyway, because it doesn't
account for the alignment requirement of the different types.

I suggest using Fusion to build a tuple of the properties you're
interested in.

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