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

From: Larry Evans (cppljevans_at_[hidden])
Date: 2004-06-06 14:05:09

On 06/06/2004 11:12 AM, Eric Niebler wrote:
[snip]
> In addition, I have solved the cyclic/dangling reference problem. A 
> regex can refer to other regexes or itself and it tracks changes to its 
> referenced regexes. It lets you copy regex objects freely, even putting 
> them in containers, and they keep their references alive. It works with 
> nothing more sophisticated than shared_ptr and weak_ptr -- no garbage 
> collection is necessary. No cycles are created ever, so no cycles need 
> to be collected.
> 
> There are only two questions:
>   (1) is the rule-as-pointer metaphor acceptable? and
>   (2) can we find a palatable syntax?
> 
> The syntax issue is this: if we take assign to be shallow (creating 
> aliases) we need a different syntax for deep assign. I am using *= to 
> mean deep-assign. Perhaps there is a better solution.
> 
The grammar_pipeline/{base,eff}/productions avoids the cycle problem
by just using indexes into a vector of "grammar expressions".  IOW,
a non-terminal expression is create with nt_exp(unsigned nt_id), where
nt_id is an index into a vector of sole_ptr smart pointers
to the grammar expression defining the rhs of the non-terminal.
You might think this would lead to a "semi-dangling pointer" (a nt_exp
whose index points to a deleted vector of sole_ptr's); however,
deletion of the sole_ptr's delete's the rhs.  Hence, the only
way to create such a semi-dangling pointer would be to create
a nt_exp somewhere other than on the rhs of a non-terminal.  There's
that danger; however, I don't know that it's that significant.
If one want's to do that, he/she must clone the rhs.

Anyway, I may discover this won't work in the future, but right now
it looks OK to me.