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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.-

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