Proto for Boost mailing list page: Re: [proto] Transform result_of nightmare and preserving terminal identity

Subject: Re: [proto] Transform result_of nightmare and preserving terminal identity
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2012-10-16 14:56:12

On 16/10/2012 07:24 a.m., Bart Janssens wrote:
> On Sun, Oct 14, 2012 at 1:20 AM, Agustín K-ballo Bergé
> <kaballo86_at_[hidden]> wrote:
>> When deep copying an expression the 'identity' of the terminals is lost, as
>> each reference to a terminal results in a new hold by value terminal.
>> Consequently, expressions with side effects (like assignment) no longer
>> evaluate correctly. Is it possible to customize the expression 'semantics'
>> so that when a deep copy is made terminals are copied only the first time
>> they are seen, and subsequent uses refer to that first copy?
>
> Hi Agustín,
>
> Re-reading your question after Eric's nudge, I think this is similar
> to a question I asked a while ago. Eric proposed replacing the
> standard deep copy with a transform like this:
>
> struct DeepCopy :
>    boost::proto::or_
>    <
>      boost::proto::when // exceptions for terminals that need to be
> stored by value
>      <
>        boost::proto::or_
>        <
>          boost::proto::terminal<const double&>,
>          boost::proto::terminal<const int&>
>        >,
>        boost::proto::_make_terminal(boost::proto::_byval(boost::proto::_value))
>      >,
>      boost::proto::terminal<boost::proto::_>,
>      boost::proto::nary_expr<boost::proto::_, boost::proto::vararg<
> boost::proto::when<DeepCopy, boost::proto::_byval(DeepCopy)> > >
>    >
> {
> };
>
> This can then be called like DeepCopy()(your_expr). The when<>
> template in here has some exceptions for terminals that I did want to
> keep by value, so if you remove that all terminals should be stored by
> reference in the copy. Concretely in the above example, const doubles
> and ints are stored by value in the copy, all other types including
> non-const doubles and ints are kept by reference. Hope this helps!
>
> Cheers,
>

This is quite smart! If I can combine this with my domain's `as_child` 
then it is exactly what I need, I haven't even considered the 
possibility of using transforms there. Thank you for your help.

Agustín K-ballo Bergé.-
http://fusionfenix.com