Re: Design pattern question

Fei Liu <>
Tue, 05 Jun 2007 10:15:57 -0400
<f43r5s$tp1$> wrote:


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

class token {
        token(const attr & at){ // construct a token
class html_token : virtual public token{
        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++.

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