Re: Is this scenario a good use for polymorphic functions
Angus wrote:
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?
No.
If the base class is aware of all message types and can know what
function to call for each of them, you don't need the derived types
at all. Just let the single message class do its job.
Here is the scenario where you'd need derived classes and polymorphic
behaviour:
Your "processor" class knows nothing about the types, but it can
register itself in the message dispatcher registry. When registering
the "processor" says what type of message it processes. The
dispatcher then creates a map of instances for each type:
map<message_type, processor_base*> registry;
and any time a message comes the dispatcher gets the type and calls
the processor using the pointer it obtains by looking up the message
type in the registry.
The difference here is (a) the processors are not known ahead of
time, they come from some plug-ins that may or may not exist, and
(b) the number of message types is not necessarily static, it can
change from run to run, when new processors are developed to serve
new message types.
V
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