Re: [Boost-users] [Proto] Functions returning an expression

Subject: Re: [Boost-users] [Proto] Functions returning an expression
From: Eric Niebler (eric_at_[hidden])
Date: 2008-11-14 15:10:29

Joel Falcou wrote:
> Eric Niebler a écrit :
>> Why don't you want to use make_expr? That's what it's for. You build 
>> the return type with proto::result_of::make_expr, and call 
>> proto::make_expr() from with f().
>>
> I have nothing against make_expr for building a single node of the 
> expression.
> I have tons of such function combining long chains of call and i don't 
> thin chaing large amount of make_expr is good.
> 
> Is there a way to use BOOST_TYPEOF with proto ?
> So maybe i can write :
> 
> BOOST_TYPEOF( (vec_expr<A>()+vec_expr<B>())/(vec_expr<A>()-vec_expr<B>())
> f( ... ) {}

You could use BOOST_TYPEOF, but you need to be careful about lifetime 
management. The resulting expression will have references to temporary 
objects, and returning it without deep-copying it first will cause 
dangling references.

You could get some mileage from the operator metafunctions. Eg:

template<typename A, typename B>
vec_expr<
   typename proto::divides<
     vec_expr<
       typename proto::plus<
         vec_expr<A> const &,
         vec_expr<B> const &
       >::type
     >,
     vec_expr<
       typename proto::minus<
         vec_expr<A> const &,
         vec_expr<B> const &
       >::type
     >
   >::type
 > const
f(vec_expr<A> const &a, vec_expr<B> const &b)
{
   vec_expr<
     typename proto::divides<
       vec_expr<
         typename proto::plus<
           vec_expr<A> const &,
           vec_expr<B> const &
         >::type
       >,
       vec_expr<
         typename proto::minus<
           vec_expr<A> const &,
           vec_expr<B> const &
         >::type
       >
     >::type
   > that = {{{{a,b}},{{a,b}}}};  // assumes vec_expr uses
                                  // BOOST_PROTO_EXTENDS
   return that;
}

This way you can be very explicit about what nodes are stored by 
reference and which are by value. I don't see this as better than 
make_expr, though, which is IMO your best bet.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com