Re: [boost] [xpressive] semantic actions: is ref() necessary

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2008-02-06 11:20:54

Eric Niebler skrev:
> Thorsten Ottosen wrote:
>> Hi Eric,
>>
>> My semantic actions might look like
>>
>>   +(name[push_back(ref(causeNames),as<std::string>(_) )]);
>>
>> My first question here is whether ref() is really necessary?

> The issue is one of lifetime management. Consider:
> 
> sregex make_regex( int &i, int j )
> {
>      sregex rx = (some >> regex)[ ref(i) += j ]; // TODO
>      return rx;
> }
> 
> What happens to 'j'? If we follow your suggestion, 'j' should be held by 
> reference. Then when make_regex returns, the resulting regex object is 
> left holding a dangling reference to an int. That's why anything not 
> wrapped in ref() is held *by value* by the regex. So that's why ref() is 
> needed in your case.

Right.

> Have a look at xpressive::local<> and xpressive::reference<> as ways of 
> decorating an object so that xpressive knows how to store it in an action.

Ok, but let me make the question more specific: why would you ever pass
a container by value to push_back()?

>> I also have another question: is it really necessary to write the
>> as<std::string>(_) part of the semantic action?
>>
>> Can this type not be deduced from the template argument to push_back()?
>> In my case it would simply be Container::value_type.
>>
>> If so, we would end up with the really slick looking
>>
>>   +(name[push_back(causeNames)]);
> 
> No, push_back takes two arguments, and I don't want to get into the game 
> of making xpressive semantic actions too clever by a half by trying to 
> guess what you mean, or "know" the semantics of push_back, etc.. 
> However, in a semantic action, _ stands in for a sub_match object, which 
> happens to have an implicit conversion to std::string. So this should work:
> 
> // Untested.
> local<std::vector<std::string> > causeNames;
> sregex rx = +(name[ push_back(causeNames, _) ]);

Ok, that is neat, but I still don't get what you mean when you say

   or "know" the semantics of push_back, etc..

Don't we know the semantics? I thought the whole point was to
call the member function called push_back on a container that supports 
that operation.

-Thorsten