Design pattern question

[alebcn75]

Hello,

I'm trying to solve the following issue which is puzzling me.
I have a concrete class, say Parser, which provides some basic parsing
functionality (such as reading word by word, line by line, etc)
Then I have a second Parser-derived class, say HtmlParser, which
besides providing the Parser functionality, provides extra features,
such as read tag by tag, attributes, etc..

What I'd like to achieve is a "factory" which would return the proper
class depending on the content type received, so I'd call this factory
like factory.create("html");, etc

The problem I'm facing is that the derived parser will contain more
methods which are not available in the base class.
So how can I achieve a generic behavior without relying on if/else/
switch constructs? So far the only way I see is dynamic casting the
Parser pointer returned by the factory, but I feel this defeats the
whole purpose of a factory.
The other obvious approach would be putting all methods in the base
class, but some of them wouldn't have any sense.. such as reading a
tag from a generic text..

Any hint to where I can find some pointers? Thank you.

 

[Daniel T.]

I'm trying to solve the following issue which is puzzling me.
I have a concrete class, say Parser, which provides some basic parsing
functionality (such as reading word by word, line by line, etc)
Then I have a second Parser-derived class, say HtmlParser, which
besides providing the Parser functionality, provides extra features,
such as read tag by tag, attributes, etc..

What I'd like to achieve is a "factory" which would return the proper
class depending on the content type received, so I'd call this factory
like factory.create("html");, etc

The problem I'm facing is that the derived parser will contain more
methods which are not available in the base class.
So how can I achieve a generic behavior without relying on if/else/
switch constructs? So far the only way I see is dynamic casting the
Parser pointer returned by the factory, but I feel this defeats the
whole purpose of a factory.
The other obvious approach would be putting all methods in the base
class, but some of them wouldn't have any sense.. such as reading a
tag from a generic text..

Any hint to where I can find some pointers? Thank you.

Write the class that uses the parser first. Write it in such a way that
it will work no matter what format the text is in. Then your problem
will be solved.

Think more abstractly.

 

[Fei Liu]

Hello,

I'm trying to solve the following issue which is puzzling me.
I have a concrete class, say Parser, which provides some basic parsing
functionality (such as reading word by word, line by line, etc)
Then I have a second Parser-derived class, say HtmlParser, which
besides providing the Parser functionality, provides extra features,
such as read tag by tag, attributes, etc..

What I'd like to achieve is a "factory" which would return the proper
class depending on the content type received, so I'd call this factory
like factory.create("html");, etc

The problem I'm facing is that the derived parser will contain more
methods which are not available in the base class.
So how can I achieve a generic behavior without relying on if/else/
switch constructs? So far the only way I see is dynamic casting the
Parser pointer returned by the factory, but I feel this defeats the
whole purpose of a factory.
The other obvious approach would be putting all methods in the base
class, but some of them wouldn't have any sense.. such as reading a
tag from a generic text..

Any hint to where I can find some pointers? Thank you.

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_(computer_science)
Know exactly what you want and define your problem. Do you want a lexer
or a parser?

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++.

 

[alebcn75]

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_(computer_science)
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?

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.

 

[Fei Liu]

Hi, thanks a lot for your answer..


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.

 

[Jerry Coffin]

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.

There seems to be a bit of confusion here. EBNF and recursive descent
are orthogonal. EBNF is a language in which you express a grammar --
i.e. you specify the language that will be accepted by the parser.

Recursive descent is a method of implementing a parser. A recursive
descent parser is a top-down parser. As you'd guess when it's expressed
that way, the primary alternative is a bottom-up parser.

Using Boost.spirit, you specify the input in a modified form of EBNF,
and the library produces a recursive descent parser from that input.
Bottom-up parsers are most often produced by yacc, bison, byacc, and
other such parser generator tools.