[Boost-users] [proto] [ was Re: [spirit2] Trivial parser crashes ]

18 Nov 2008

      Joel de Guzman wrote:
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Joel de Guzman wrote:
...
sm4 wrote:
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Oops - seems formatting got in the way.
Hope this is better:
Ooops, I missed your reply. Its better to post spirit
related questions to spirit's mailing list. I'm re-posting
this there.
Ok, this looks like a bug in handling aliases of the form:
r1 %= r2;
where r1 and r2 are rules with the same type. A quick
workaround is to copy the rhs rule:
r1 %= r2.copy();
Eric,

It seems to be a problem with Spirit2's usage of proto extends.
Here's what's happening:

1) Rule uses proto::extends
2) Rule implements operator %=

On expressions:

     r %= some-expression;

The rule's operator%= kicks in. However, on expressions:

     r %= r2; // r and r2 has the same type

Then proto %= kicks in leaving it as a no-op. Nothing happens.
Hence, the rhs is never "auto-assigned" to the lhs.

I'm not sure how to deal with this. Do you have a hint?
I place the original test code here. Tracing the code, the
first %= works as expected while the second and third becomes
no-ops calling proto operator %=.
...
-----------------------------------------------------------------------------------------
...
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include <iostream>
#include <string>
using namespace boost::spirit;
using namespace boost::spirit::qi;
using namespace boost::spirit::ascii;
using namespace boost::spirit::arg_names;
namespace phoenix = boost::phoenix;
using phoenix::at_c;
struct key_val
{
    std::string key;
    std::string value;
};
BOOST_FUSION_ADAPT_STRUCT(
    key_val,
    (std::string, key)
    (std::string, value)
)
template <typename Iterator>
struct my_grammar : grammar
{
    my_grammar() : my_grammar::base_type(start)
    {
        quoted_string %= lexeme['"' >> +(char_ - '"') >> '"'];
        key %= quoted_string;
        value %= quoted_string;
start =
                key           [at_c<0>(_val) = _1]
             >> ':'
             >> value         [at_c<1>(_val) = _1]
             ;
    }
rule key;
    rule quoted_string;
    rule value;
rule start;
};
int main()
{
    typedef std::string::const_iterator citerator;
    std::string str = "\"x\": \"25\" ";
citerator iter = str.begin();
    citerator end = str.end();
my_grammar<citerator> my_parser;
    key_val kv;
    phrase_parse(iter, end, my_parser, kv, space); // ASSERTs
return 0;
}
-----------------------------------------------------------------------------------------

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net