Re: [Boost-users] [Proto] Turnign grammar into primitive transform : _default is being naughty with me :(

Subject: Re: [Boost-users] [Proto] Turnign grammar into primitive transform : _default is being naughty with me :(
From: Eric Niebler (eric_at_[hidden])
Date: 2010-03-17 21:36:52

On 3/18/2010 7:58 AM, joel falcou wrote:
> Here is link to the working.cpp and not_working.cpp stand alone source
> file for inspection.
> test were done on 1.42 release.
>
> http://www.lri.fr/~falcou/tmp/working.cpp
> http://www.lri.fr/~falcou/tmp/not_working.cpp
>
> The error is :
>
> http://www.lri.fr/~falcou/tmp/errors.txt
>
> thanks for any pointer on this issue which, I think, is a stupid mistake
> on my side.

Hi Joel,

This isn't a stupid mistake on your side; it's a bug in Proto. To 
explain: all transforms including _default take their expression 
parameter as ...

   template<class E>
   ... operator()(E &e)

That is, the expression must either be an lvalue or a const-qualified 
rvalue. It's done this way to reduce the number of overloads and improve 
compile times.

All the temporary expression objects Proto creates are const-qualified 
rvalues so they can be passed to transforms. Well, *almost* all, the 
exception being functional::make_expr and functional::unpack_expr. 
That's an oversight. I just fixed it on trunk.

To work around your problem, you need to change line 109 of 
not_working.cpp to first save the result of unpack to a temporary and 
then pass that to rec.

HTH,

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