[boost] Re: [spirit] rule<> and copying

From: Eric Niebler (eric_at_[hidden])
Date: 2004-06-07 10:44:06

Thorsten Ottosen wrote:

> 
> maybe
> 
> rule<> a, b;
>  a = parser >> that >> refers >> to >> b;
>  b = parser >> that >> refers >> to >> a;
>  a = a.clone();
> 
> ?
> 

No. operator= should not do deep copy; otherwise, you cannot use rule<> 
objects in generic code like STL containers.

I have a new idea. Deep copy can be triggered by assigning to a dummy 
member:

rule<> a, b;
a.rule = parser >> that >> refers >> to >> b;
b.rule = parser >> that >> refers >> to >> a;

This allows:
a = b; // shallow copy, creates alias
a.rule = b.rule; // deep copy, modifies 'a' and all its aliases

xpressive now works this way. It's heavier syntactically, but far less 
confusing then using a special deep-assign operator.

> | I don't know. Another question is: would spirit's users accept any
> | amount of syntactic/conceptual overhead to have a rule with standard
> | copy semantics and that was able to track its references and avoid
> | cyclic dependencies?
> 
> I'm not an expert spirit user, but the current semantics already carries some
> conceptual overhead with it. Anything that can reduce surprises would help. And
> I can replace this code
> 
> template< int >
> rule<>& some_rule(...)
> {
>    static rule<> r = ...;
>    return r;
> }
> 
> rule<> r1 = some_rule<1>(...)
> rule<> r2 = some_rule<2>(...)
> 
> with
> 
> rule<> some_rule(...)
> {
>    return ...;
> }
> 
> rule<> r1 = some_rule(...)
> rule<> r2 = some_rule(...)
> 

Yes, that is the idea. You would be able to return a rule by value, even 
if it refers to other local/temporary rules.

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