Probably there is no right or wrong answer to this but I thought to ask
to put my mind at rest. Ok lets say you have a object hierarchy (eg.
the Glyph in Lexi from GOF book) and you want to use the visitor
pattern. So we place an accept method in the the base class glyph and
procede to create the visitor hierarchy. The accept signature will look
like this
void Glyph::Accept(V isitor& v);
The Visitor hierarchy will have a Vistor base class that will have a
visit method for each different type of Glyph so say if we have an
AssignnentNode and VariableNode there will be two methods
void visit(Assignnen tNode& n);
void visit(VariableN ode & n);
and in each concrete visitor we implement these methods to behave as we
wish. So far so good...
Imagine you think of 20 different types of operations that you like to
perform on your object structure and u like to use the Visitor pattern
to do this. So you create 20 different classes each implementing the
visitor interface.
Should we inherit all these 20 classes from our visitor base class? If
so this means a single Accept method in the object structure is
sufficent to serve all of these operations.
Should we create different and separate visitor hierarchies each with
its own accept method but grouping related operations together?
Should we go half way and create these groups but all inherit from a
common visitor base class. This approach also will require a single
accept method.
Currently I am leaning towards the last approach but I am worried that
having a single visitor hierarchy has tied 20 operations together (some
related while others unrelated). The hierarchy is more bushy and deeper
but it still
Whats the general feeling of the developer community on this topic?
17  2609 
I did not completly understand what you said.
I am not able to understand the class hierarchy.
but I am able to understand that you want to overload the visit
function 20 times. That is not really needed and it is very painful.
Use templates instead of that.
If you know templates, you will understand need for reusability,
especially when you want to overlaod several hundred times. You will
create only one function and you use it for any kind of datatype
(including user-defined)
Murali Krishna wrote: I did not completly understand what you said.
I am not able to understand the class hierarchy.
Please quote the message you are answering to and do read about the
visitor pattern.
Jonathan
Merlin wrote: Should we go half way and create these groups but all inherit from a
common visitor base class. This approach also will require a single
accept method.
Currently I am leaning towards the last approach but I am worried
that having a single visitor hierarchy has tied 20 operations
together (some related while others unrelated). The hierarchy is more
bushy and deeper but it still
Whats the general feeling of the developer community on this topic?
I'm not sure what is the problem here and what you are asking about.
If I understand it correctly, you try to have different visitors
implementing different algorithms. Am I right?
If I am, then I'd try to combine Visitor pattern with Strategy pattern.
I'm not sure about possible redundancy here so may be someone else
would like to comment it.
In other words, if you have GoF in front of you take a look at the
Strategy (page 315) and there are two classes in sample: Composition and
Compositor. My idea is to make Composition a Visitor whish uses
different strategies. Does it make sens to anybody? I'm curious too :-)
Cheers
--
Mateusz Łoskot http://mateusz.loskot.net
Hi Jonathan,
Thanks for that. Any way I am new to this Google group. I wanted to
Quote. Frankly, I dont know how to do that. Help me how to quote. Next
time I'll start quoting.
Any way did my answer gave any solution?
Firstly thank you for your replies guys but I think maybe you are
unfamiliar with the visitor pattern.
Let me rephrase my question.
Is it a good idea to have just a single Accept method in the Glyph
class for all the visitor classes even if there are 100's? Or should we
group these
visitors logically and introduce different Accept methods in the Glyph
class. I dont see whats not clear about this question... maybe I need
some sleep!
Regards
Murali Krishna wrote: Hi Jonathan,
Thanks for that. Any way I am new to this Google group. I wanted to
Quote. Frankly, I dont know how to do that. Help me how to quote. Next
time I'll start quoting.
Any way did my answer gave any solution?
It's not a Google group, it's a Usenet group. Google groups is just one
of the ways of reading and posting.
Instead of clicking Google's "Reply" link at the bottom of the message,
click "Show Options" at the top of the message then use the "Reply"
link revealed there. You will find the message you are replying to is
automatically quoted for you. Remember to post your message below or
inline with what you are replying to (but not above) and to snip
unnecessary context.
I posted this message from Google Groups and quoted you without a
problem.
Gavin Deane
Merlin wrote: Firstly thank you for your replies guys but I think maybe you are
unfamiliar with the visitor pattern.
I'm pretty familiar with Visitor but may be you don't give us clear
picture of the problem. I think both are possible :-)
Let me rephrase my question.
OK, let's try again.
Is it a good idea to have just a single Accept method in the Glyph
class for all the visitor classes even if there are 100's? Or should
we group these visitors logically and introduce different Accept
methods in the Glyph class.
Hard to say because I don't know your structure much.
So, I'll ask additional question.
Can all those Visitors be unified in terms of common interface?
May be you can solve your problem by inheritance from common Visitor
interface. Then all Visitors would be used in unified way (common
interfaace) but they could still distinguish in terms of implementation.
In such case you need only one Accept function which takes Visitor by
it's interface (pointer to Visitor base class).
Please, not I said "may be", so try to fit my idea (if you like) to your
problem becasue you know it (the problem) best.
What I'm sure in 99% about is that having class with 100 member
functions is definitely not a good idea. Would you like to be a client
of such class? I would not ;-)
Cheers
--
Mateusz Łoskot http://mateusz.loskot.net
Mateusz Loskot wrote: What I'm sure in 99% about is that having class with 100 member
functions is definitely not a good idea. Would you like to be a client
of such class? I would not ;-)
Ever used std::string? ;-)
Gavin Deane
Hi Mateusz
You will never be the client of a class with 100 member functions. You
keep forgetting that the visitor base class will have as many visit
methods as there are nodes in the Glyph hierarchy, eg Row, Column,
Graphics, etc look in GOF. In other words only a few methods will be in
Visitor. BUT there will be 100 derived classes overriding these few
methods in different ways, each specific to the type of operation being
carried out.
When the time comes to use one these visitors, you only create an
OperationalVisi tor that you need and then apply it to your glyph.
Now back to my original problem. 3 options exist
1) Have one visitorBase with 100 derived concrete visitors. One accept
method in Glyph
2) Have one visitorBase with the 100 concrete visitors grouped in
smaller hierarchies that derive from visitorBase. One accept method in
Glyph
3) Have a family of visitors each with its own baseclass and a
corresponding accept method in glyph(ie multiple accept methods)
The common interface exists for all 100 visitors. It consists of the
visit methods for the various different types of nodes for the glyph
hierarchy.
Thanks
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