[Boost-users] Re: Spirit help (again)

From: Jeffrey Holle (jeff.holle_at_[hidden])
Date: 2004-11-17 12:26:04

Its because the State object is held by reference.
The const parameter forces the grammar object to be stateless.  This is 
by design.

Cory Nelson wrote:
> I'm just trying to make a simple parser like uint_p.  From the source
> I've dug through it doesn't look like those use grammar classes though
> it's very possible I'm wrong.
> 
> If the object passed through definition() is const, how would state be updated?
> 
> 
> On Tue, 16 Nov 2004 21:59:24 -0500, Jeffrey Holle
> <jeff.holle_at_[hidden]> wrote:
> 
>>Not sure where you are stuck at, given your description, but I'll give
>>it a try.
>>
>>In order to hook semantic actions into your grammar:
>> 1.  You should create a grammar object that houses your grammar.
>> 2.  You should use functor action operators.
>>
>>The first provides a way to expose application objects to the grammar in
>>a maintainable way.
>>
>>For instance:
>>
>>struct My_grammar : public grammar<My_grammar>
>>{
>>  My_grammar(State& state) : state_(state) {}
>>  template <typename ScannerT>
>>  struct definition
>>  {
>>    definition(My_grammar const& self)
>>    {
>>    // define grammar here
>>    rule<ScannerT> outerRule;
>>    rule<ScannerT> const&
>>    start() const { return outerRule; }
>>  };
>>  State& state_;
>>}
>>
>>In the object example, the action objects can get mutable access to the
>>State object to record what needs recording.
>>
>>The second "should" involves the actual action operators.  These can be
>>either functions or functors.  I sugguest using only the later in that
>>they can be used like:
>>
>>struct captureSA
>>{
>>  captureSA(State *pState) : m_pState(pState) {}
>>  void operator()(iterator_t first, const iterator_t& last) const;
>>  State *m_pState;
>>};
>>
>>This functor can be attached to a rule like (note: within the body of
>>definition above):
>>
>>  outerRule[captureSA(self.state)]
>>
>>There is one caviot to this.  The signature of the operator() method
>>changes to:
>>  void operator()(const char& ch)
>>when the rule matches single characters.
>>Sometimes its necessary to define both methods in an action functor.
>>
>>Hope that helps.
>>
>>Cory Nelson wrote:
>>
>>>Okay, I'm trying to learn Spirit and I've got the grammar down.  I've
>>>tried looking at the documentation but still can not find out how to
>>>make a custom parser (that will let me use the [action] operator).
>>>Could someone give an example of a pascal string parser (one byte for
>>>length, string follows) to get me started?
>>>
>>
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> 
> 
>