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delphidelphi-2007

Object inheritance involving enumerated types

Delphi 2007, moving to Delphi XE over the next year.

Our product makes extensive use of a third-party component. We don't use the component directly, but instead use a custom descendant of it, to which we've added quite a lot of extra behavior (the custom descendant component was developed several years ago by developers who have since have retired).

In the source unit of the third-party Parent class, some enumerated types are declared, which control various operations of the component:

TSpecialKind = (skAlpha, skBeta, skGamma);
TSpecialKinds = set of TSpecialKind;

In our descendant class, we want to add new behavior, which would require expanding the selection of enumerated types. Essentially, we want this:

TSpecialKind = (skAlpha, skBeta, skGamma, skDelta, skEpsilon);
TSpecialKinds = set of TSpecialKind;

Obviously, we want to avoid editing the third-party code. Is it valid to simply redeclare the enumerated type, repeating the original values and adding our new ones, in our own descendent unit? Will it have any effect on existing code?

Edit: Example scenario to (hopefully) clarify. Say you've got a (parent) component for ordering vehicle parts. The parent unit has an enumerated type Tvkind for vehicle kind, with values vkCar and vkCycle defined. These values are used, among other things, to indicate how many wheels the vehicle has, 4 or 2.

Now, in your descendent component, you want to be able to handle 3-wheeled vehicles as well. Extending the Tvkind enumerated type to include a new value vkTrike seems like the obvious approach. But what if you don't have access to or don't want to modify the parent component code?

Solution

  • I don't believe that you can reasonably expect to make the change that you want without modifying the original component.

    Let's take your vehicle kind example and delve a bit deeper. I expect that the original component will have code like this:

    case Kind of
vkCar:
  CarMethod;
vkCycle:
  CycleMethod;
end;

    Now, suppose you introduce an enumerated type with an extra enumeration

    TExtendedVehicleKind = (vkCar, vkCycle, vkTrike);

    If the case statement above runs, with ExtendedKind equal to vkTrike, no method will be called.

    Now, perhaps the behaviour that you want from the original control can be achieved by setting Kind to vkCar or vkCycle when ExtendedKind is vkTrike. But that seems unlikely to me. Only you can know for sure, because only you have the code, and know what your actual problem is.