[fpc-pascal] Extending an enumeration

Thierry Coq tcoq at free.fr
Wed Jul 21 08:55:58 CEST 2010

On 18/07/2010 20:56, Mark Morgan Lloyd wrote:
> Yes, I was thinking that. However if the basic class was say a 
> round-robin scheduler with phases rrQuiescent and rrInitialised and 
> the descendant was say an HP comms protocol handler with additional 
> phases hpReceivingPadding, hpReceivedSync and so on it would seem to 
> be questionable practice to have to define all possible enumerated 
> values in the basic class.
In this case, enumeration cannot be extended. If you look at it from an 
analysis point of view, you're implementing a state machine with an 
enumeration. If your state machine is extendable instead of fixed, and 
you want to add substates to it, then a possible implementation is to 
define a state class:

TSchedulerPhase = Class(TObject);

where the basic phases are defined as variables in the base class, and 
initialized in the initialization of the base module:
    spQuiescent: TSchedulerPhase;
    spInitialized: TSchedulerPhase;
   spQuiescent := TSchedulerPhase.Create;

The descendent class can add other states, and compute other transitions.
This is a classic implementation of the state design pattern.

I hope this helps,

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