Re: Design pattern question

From:
Fei Liu <feiliu@aepnetworks.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 05 Jun 2007 13:30:24 -0400
Message-ID:
<f446ie$v39$1@aioe.org>
alebcn75@gmail.com wrote:

Hi, thanks a lot for your answer..

Ok there are a couple issues revealed here:
1) From your description, you are writing a lexical analyzer (scanner
and tokenizer), do not confuse a parser with a lexical analyzer. Start
from here:http://en.wikipedia.org/wiki/Semantic_analysis_%28computer_science%29
Know exactly what you want and define your problem. Do you want a lexer
or a parser?


Yes, thanks for letting me know. I didn't have enough knowledge on the
separation of these processes.
Actually, it seems I need both of them..

The idea is basically being able to interpret a http header, and also
other protocols, say rtsp to put an example.
Both have similarities, but there are some things specific to each
protocol.
Am I wrong if I say that in most cases the analyzing and parsing is
done in the same place?


They work this way: a parser is fed by a lexer. Usually a token
signifies a start of a syntactic block that can be analyzed by a parser.
  They are not done in the same place (I assume you meant lexical and
syntactic analysis)

Check EBNF and Boost::spirit. In your case I don't think you need a
recursive descent parser. A simple EBNF parser is most likely adequate.

My idea is simply being able to get a stream of data and then build a
data structure holding the different headers,
with its fields and attributes.. From what I think, I don't think I
need to make a lex analyzer and then a parser.. I could do
obth things at the time?

2) A factor method works best when parallel hierarchies exists, you can
formulate your hierarchies into a lexer hierarchy and a token hierarchy.
Then a lexer class can create a token class through covariance and
virtual methods:
class lexer{
        public:
                virtual token * create_token(const attr & at){
                        return new token(at);
                }
                virtual ~lexer(){};};

class html : virtual public lexer {
        public:
                html_token * create_token(const attr & at){
                        return new html_token(at);
                }
        }

};

class token {
        public:
                token(const attr & at){ // construct a token
                }};

class html_token : virtual public token{
        pubilc:
                html_token(const attr & at){ // construct a html token
                }};

3) There is nothing wrong with if/else/switch constructs. In fact IMO
that's only way to can initialize objects dynamically within the realm
of C++. You can use platform specific feature, i.e. dynamic library and
name resolution, to facilitate a more generic solution but that does not
have anything to do with C++.


Thanks. I definetly need to give this more thought, or learn existing
examples to see how they do this.

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