*
fernandez.dan@gmail.com:[color=blue]
> I'm still learning how to use Object Oriented concepts. I'm have a
> basic question dealing with design. I have two classes that deal with
> I/O pertaining to network and usb that inherit from an abstract parent
> with four basic functions: Open, Read, Write, and Close.
>
> // Note alot of the stuff has been left out
> // just to give a basic picture
> class IO
> {
> public:
> virtual int Open(..) =0;
> virtual int Read(..) =0;
> virtual int Write(..) =0;
> virtual int Close(..)=0;
> };
>
> class network : public IO{
> ....
> };
>
> class usb: public IO {
> ...
> };
>
> Now I want to create a class that uses the network or usb to
> read/write. The different ways of doing these and I'm not sure which is
> appropriate. Here my inital thoughts
>
> 1)
> class MyProtocol
> {
> IO* m_io;
>
> public:
> MyProtocol(IO *);
> };
>
> Here I could give my class MyProtocol either Network or Usb and it
> wouldn't care. The other choice
> 2)
> class MyNetworkProtocol: public Network
> {};
>
> class MyUsbProtocol: public Usb
> { };
>
> Here I will create two classes for each one but each class would have
> the same logic.[/color]
Assuming that the same protocol _may_ be used with both 'network' and 'usb'
choice 2 would mean redundant code, which is nearly always Evil (e.g., think
of maintenance, keeping two corresponding implementations in synch).
Also, the inheritance should probably not be public, because a protocol
isn't an 'io': you wouldn't want client code to circumvent the protocol,
calling 'read' and 'write' directly, would you?
However, you could do something like choice 2 as
3)
template< class IoProvider >
class Protocol: private IoProvider
{
...
};
The difference between choices 1 and 3 is that 1 provides _run-time_
selection of the IoProvider, while 3 provides _compile-time_ selection.
Choice 1 is far more flexible. While choice 3 can help you avoid things
like using an inappropriate Protocol/IoProvider combination, catching such
errors at compile time (e.g. a partial specialization with no
implementation) I don't think that's very much of an advantage in practice.
Also, choice 3 is more difficult to implement and debug.
[color=blue]
> These are the two choices that I thought of but I'm not sure which one
> is appropriate. I'm still unsure about the design, I'm open to any
> advice. Thanks[/color]
It may be that also Protocol should be designed as a pluggable class. And
perhaps it might be a good idea to think about protocol stacks, i.e.
protocols plugged into protocols for a few levels. But whether you need
that depends on the application you have in mind, and since generality
nearly always has some cost, both in development time and run-time, it is a
matter of biting nails, throwing dice, soliciting advice, perusing budgets,
checking what others have done, and so on, so as not to overdo generality.
Final word of advice: think about whether Protocol should be directly used
by client code.
Perhaps the design will be a lot cleaner if Protocol is designed as
pluggable into some class X that has a public interface adapted exclusively
to the client code's needs (implementing X might help design Protocol)?
--
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A: Top-posting.
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Question about class design 
