what are parsing generators?

I am writing a C++ documentation tool for my Bsc final year project
and I am getting a bit confused with all this lex and yacc talk!


I have read about lexers, grammars and parsing and I am starting to
understand (a little) about what goes on. Then I find out that you
can use programs to do it all for you! If anyone could help me
understand this subject a little better, I would appreciate it. I
have some questions to give you an idea of what I am babbling about:


1) In order to learn parsing I am reading COMPILER books. The
question is, how much of this do I need to know to write a doctool?
The tool will insert comments above functions, classes and variables,
and also generate skeliton HTML documentation like KDOC and Doxygen.


2) Should I learn lex and yacc and make use of them, or should I do it
myself? I am gonna do it myself anyway (requirement of the
project)!!, but I'm interested to know whether it is better to use
lex/yacc or not.


3) What are parser trees used for? I understand that lexers break
code into symbols. I understand that the parser uses the symbols and
a grammar to generate a tree. But what can I do with this tree? I
will probably learn this with further research but a brief explanation
would be nice :)


Note that I am only interested in C++, and I understand that lex and
yacc can produce output for any language, so maybe they arn't
appropriate.


Thanx in advance for any help - as you can see from these lame
questions, I haven't got a clue :(

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