Is this scenario a good use for polymorphic functions

[Angus]

I am developing a server which receives a range of different
messages. There are about 12 different message types so I thought
that a good idea would be to devise a class for each message type.
Then in my base class I for example have a pure virtual function
called eg PerformAction. Then in each message class I implement
PerformAction.

The base class works out what the type of message is and then calls
the correct inherited class function. Eg if the function were a blah
message which simply responds with the message "Blah" then the base
class would call the Blah class PerformAction function which would
maybe create a blah message response.

Does this seem like a reasonable design in this scenario?

[Daniel T.]

Angus <an*********@gm ail.comwrote:

I am developing a server which receives a range of different
messages. There are about 12 different message types so I thought
that a good idea would be to devise a class for each message type.
Then in my base class I for example have a pure virtual function
called eg PerformAction. Then in each message class I implement
PerformAction.

The base class works out what the type of message is and then calls
the correct inherited class function. Eg if the function were a blah
message which simply responds with the message "Blah" then the base
class would call the Blah class PerformAction function which would
maybe create a blah message response.

Does this seem like a reasonable design in this scenario?

I will give you a heuristic... If making a base class and two or more
subclasses will allow you to remove one or more switch or if statements,
then go ahead and use polymorphism, otherwise don't.

[James Kanze]

On Sep 18, 5:11 pm, Angus <anguscom...@gm ail.comwrote:

I am developing a server which receives a range of different
messages. There are about 12 different message types so I thought
that a good idea would be to devise a class for each message type.
Then in my base class I for example have a pure virtual function
called eg PerformAction. Then in each message class I implement
PerformAction.

The base class works out what the type of message is and then calls
the correct inherited class function. Eg if the function were a blah
message which simply responds with the message "Blah" then the base
class would call the Blah class PerformAction function which would
maybe create a blah message response.

Does this seem like a reasonable design in this scenario?

Not per se. If the base class has to work out the message type,
and call a different function, then there's no point in
derivation.

If it's possible to determine the action from some information
at the beginning of the buffer, then it would be reasonable to
use that information to create a derived class of the correct
type; in this case, the base class doesn't need to work out
anything, it just calls its virtual "doAction" function. You
still need to work out which type of object to create, but this
can typically be done by looking up a factory in a table or map,
and calling the factory function for the actual construction.

In other cases, it might make more sense to have a class with
the message itself, and use the strategy pattern for the action;
the base class then determines the correct strategy to
instantiate (again, by means of table lookup and a factory).

--
James Kanze (GABI Software) email:ja******* **@gmail.com
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