Re: [Boost-users] Need boost's help with poor design

Hi Robert, On 3/29/08, Robert Dailey wrote:

On Fri, Mar 28, 2008 at 4:10 PM, Asif Lodhi wrote: X...............................................................................................Right now there is no possible way to have a factory that returns the original type of the packet, since what is being passed in (the ID) is not known at compile time, as I've already stated.

I have just re-read the whole thread. Downcasting the return value of a factory method really seems strange to me. I think, here is what you need to do: 1) Create a protected "packet_id" data member in your base "Packet" class, as Steven said earlier. 2) Create an abstract virtual function in your base class - for example: virtual const int &PacketTypeID()=0; // NO NEED to try finding a factory method that returns the original type of the // packet. The above virtual function and its over-ridden variants in each child // packet class will correctly return the original packet type - that is, the "ID" integer // of the packet - if that's what you mean by the original packet type. 3) In your factory method, add code to store the "ID" integer of the packet received in the packet_id protected data member. 4) Over-ride the virtual function specified in (2) in each child class of Packet. and just return the packet_id. 5) Use a simple template-based Observer to implement event dispatch instead of using a look-up based function pointer mechanism - no need to use MPL. 5) NO NEED TO DOWNCAST the return value of your factory method. Your factory method should have a reference/pointer to the base Packet class as the return type. Just store this return value in a variable of the "base Packet type" instead of downcasting to the concrete PakcetType. If you store the reference/pointer of the base Packet type (that your factory method returns) in a variable of the BASE PacketType (which will actuall a reference to the concrete Packet type if your factory method is really a factory method) then the built-in polymorphism will correctly call the implementation of the PacketTypeID() function defined in the corresponding concrete child Packet class. 6) Instead of a look-up table of function pointers you should define a virtual function for signal dispatch in the base class and over-ride the same in each child Packet class to enable execution of the correct code. 7) If there are too many Packet types then you can do the whole thing using SIMPLE templates, instead of resorting to MPL, so that the appropriate concrete instances of each concrete [child] Packet class (and their virtual functions for returning the packet_id and dispatching) automatically get generated at compile time. You said that the packet type ID is not known at compile time - I wonder how, then, you are mapping IDs with the Packet types. Your reply seems to suggest that, each time you get a certain type of Packet, it can have a different ID. If that's not the case then I think the above strategy will do. -- Best regards, Asif