Re: [boost] [Proto] Copying expressions

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-17 14:12:15

Maurizio Vitale wrote:
> Eric Niebler <eric_at_[hidden]> writes:
> 
>> Maurizio Vitale wrote:
> <snip>
>>> My mental model was: 
>>>    - proto::deep_copy traverse the expression top-down replacing references with value.
>> Actually, bottom-up, but yes references are replaced with values.
> 
> It is probably terminology, but the following seems top-down to me: you're asked to deep-copy 
> an expression and you assemble an expression which has the same shape (tag and argcount) of the
> original and as children the result of recursive evaluations of deep-copy on the children.

Just terminology. You recurse all the way down to the leaves, deep-copy 
them and store the results by-value in new parent nodes, all the way 
back up to the root. So the first deep-copy is done to the leaves -- 
hence bottom-up.

<snip>

> I guess my question is what is the difference between:
> 
>   struct lambda_domain : domain<generator<lambda_expression> > {}; 
>   auto expr = deep_copy(_1 + 42);
> 
> and:
> 
>   auto expr_1 = _1 + 42; in the case where the generator you provided is used
> 
> Aren't expr and expr_1 exactly the same?

Exactly the same, yes.

>> How this helps is that as phoenix expressions are being built, the 
>> phoenix generator will ensure that all the nodes are held by-value. 
>> There will never be a need for the user to deep_copy phoenix expressions 
>> explicitly -- it's done piecemeal by phoenix. So function<> is passed an 
>> object that does not need to be deep-copied.
> 
> I see this difference now and understand the reasons behind your suggestion.
> Thanks for the explanation,

No problem.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com
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