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

Subject: Re: [Boost-users] [Proto] Functions returning an expression
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2008-11-14 15:14:50

Eric Niebler a écrit :
> 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.
>
OK, i've just skimmed through my files and well, the solution can't be 
other thign than make_expr due to this lifetime problem.
Thanks for the head up though

-- 
___________________________________________
Joel Falcou - Assistant Professor
PARALL Team - LRI - Universite Paris Sud XI
Tel : [phone]