[boost] Re: [Spirit-general] Crash using spirit to parse simple grammar...

From: Michael Burbidge (mburbidg_at_[hidden])
Date: 2003-09-08 09:56:58

Great, thanks. I was puzzled for a while as to why the example 
ast_calc.cpp worked since it didn't declare an indirection rule as the 
root rule of the parser. The reason is because the start() method is 
declared to return a  rule<ScannerT, parser_context, 
parser_tag<expressionID> > const&, instead of a rule<ScannerT> const& 
like I was declaring.

Bottom like is that I can fix my problem either as you describe or 
declare the start() method to return the right type so that a temporary 
object does not have to be created.

Thanks for the help!

Michael-

On Sunday, September 7, 2003, at 12:35 AM, Hartmut Kaiser wrote:

> Michael Burbidge wrote:
>
>> The following is a simple program that uses spirit to parse a trivial
>> grammar. Of course my real grammar is not so simple, but I've
>> narrowed
>> it down to a very small example. When I run the program it crashes
>> during the call to parse. I've tried this program on Windows and
>> Macintosh and using the spirit in boost 1.30.2 and the latest spirit
>> 1.7.0. They all crash. This code was derived from the ast
>> calc example.
>>
>> If I change the line:
>>
>> rule<ScannerT, parser_context, parser_tag<escaped_expr_id> >
>> escaped_expr;
>>
>> to:
>>
>> rule<ScannerT > escaped_expr;
>>
>> it doesn't crash. But since I'm trying to  build a abstract
>> syntax tree
>> I really need the rule id. Can anyone tell me how I might change my
>> simple program so that it doesn't crash?
>
> My compiler spits out a warning about returning a reference to a
> temporary object here:
>
>     rule<ScannerT, parser_context, parser_tag<escaped_expr_id> >
>         escaped_expr;
>     rule<ScannerT> const& start() const
>     { return escaped_expr; }    // <<--- HERE
>
> which is the root of your problem. The compiler has to implicitely
> construct a temporary of the type rule<ScannerT> to fit your type
> conversion needs. To solve this problem you'd need to insert another
> indirection rule at the top of your parser hierarchy:
>
>     template <typename ScannerT>
>     struct definition
>     {
>         definition(expr_grammar const& self)
>         {
>             start_rule = escaped_expr;
>             escaped_expr
>                =    ch_p('{') >> ch_p('}')
>                ;
>         }
>
>         rule<ScannerT, parser_context, parser_tag<escaped_expr_id> >
> escaped_expr;
>         rule<ScannerT> start_rule;
>         rule<ScannerT> const& start() const { return start_rule; }
>     };
>
>
> HTH
> Regards Hartmut
>
>
>
>
>
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