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From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-17 12:51:37

Maurizio Vitale wrote:
> Eric Niebler <eric_at_[hidden]> writes:
>> Maurizio, deep_copy() alone will not work here. The user is creating a
>> lambda expression and assigning it directly to function<>:
>> function<int(int&,int&)> f(_0 + _1);
>> function<> knows nothing about proto expressions, and doesn't know to
>> call proto::deep_copy().
> Rats! (apologizes to Dan, I didn't intend to mislead)
> But now I'd appreciate you entering "grade school mode" and do some explaining.
> 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.

> - generators are used to wrap expressions as they are built bottom-up

Generators are used whenever you want to customize the result of proto's
operator overloads.

> So, assuming that the expression is properly wrapped in lambda_expr<> why the result
> of calling proto::expr on the whole expression would be different from using the generator?

Sorry, I don't understand the question.

> And why this would tell function<> more about proto expressions?

It doesn't.

> What am I missing?

Maybe you're not clear on how generators work. (Not surprising, it isn't
documented yet.) By default, proto's operator overloads create
proto::expr<> objects in which everything (terminals and non-terminals)
are held by reference. Generators give you a hook where you can
customize the operators' return types. The operator overload passes the
generator the new node (bare proto::expr<>, sub-nodes held by reference)
and the generator is free to do whatever it wants with it. It can wrap
it in a domain-specific wrapper, deep_copy it, or whatever.

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.


Eric Niebler
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