Re: [Boost-users] [mpl] Hierarchy Generation

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-02-28 12:19:38

AMDG

Scott Meyers wrote:
> Steven Watanabe wrote:
>   
>> template<class Sequence>
>> struct MakeFeatures {
>>     typedef Features<typename copy<Sequence, back_inserter<vector0<> > 
>>  >::type> type;
>> };
>>     
>
> I'm trying to understand the code you posted, but I'm having trouble, probably 
> because I'm missing some basic concepts.  I hope people here won't mind helping 
> me with something I'm sure is basic:  why do we need to make a copy of Sequence 
> here?  I've tried replacing the above typedef with
>
>    typedef Features<Sequence> type;
>
> and everything breaks, so it's clear that the copy is there for a reason, but 
> what is it?
>   

The copy is to guarantee uniqueness.  If you have a
Features<mpl::list<...> > this will not be the same as
a Features<mpl::vector<...> >.  The mpl::copy<>
makes sure that Features<> always holds a type built
by push_back on vector0<>.  Thus, MakeFeatures<mpl::list<int, char, 
double> >::type
is the same as MakeFeatures<mpl::vector<int, char double> >::type.
Does that help?

In Christ,
Steven Watanabe