How to Start Creating My Own Classes with Delphi?

Question

I posted a question a few days ago, and the answers told me to create my own classes.

I\'m an old-school programmer from the pre-OOP days my programming is well structur

Answer 1

To me, this does not appear to be a science. It appears to be an art of how to best structure your information into objects.

Well... Yeah. There really aren't a lot of formal requirements. It's really just a set of tools to help you organize your ideas, and eliminate a lot of duplication along the way.

Then an OOP advocator will probably tell me to make this a class. Logically, I would think this would be a class inherited from the generic TList.

Actually, the whole point of generic containers is that you don't have to make a new container class for each type of object. Instead, you'd make a new content class and then create a TList.

Think of a class instance as a pointers to a record.

Now: why use a class when you could use a pointer to a record? A couple reasons:

• encapsulation: You can hide some aspects of the implementation with the private keyword so that other developers (including your future self) know not to depend on implementation details that may change or that just aren't important to understanding the concept.
• polymorphism: You can avoid a lot of special dispatch logic by giving each of your records a set of pointers to functions. Then, rather than having a large case statement where you do different things for each type of object, you loop through your list and send each object the same message, then it follows the function pointer to decide what to do.
• inheritance: As you start making records with pointers to functions and procedures, you find that you often have cases where you need a new function-dispatch record that's very much like one you already have, except you need to change one or two of the procedures. Subclassing is just a handy way to make that happen.

So in your other post, you indicated that your overall program looks like this:

procedure PrintIndiEntry(JumpID: string);
  var PeopleIncluded : TList<...>;
begin      
   PeopleIncluded := result_of_some_loop;
   DoSomeProcess(PeopleIncluded);
end;

It's not clear to me what Indi or JumpID mean, so I'm going to pretend that your company does skydiving weddings, and that Indi means "individual" and JumpID is a primary key in a database, indicating a flight where all those individuals are in the wedding party and scheduled to jump out of the same plane... And it's vitally important to know their Relationship to the happy couple so that you can give them the right color parachute.

Obviously, that isn't going to match your domain exactly, but since you're asking a general question here, the details don't really matter.

What the people in the other post were trying to tell you (my guess anyway) wasn't to replace your list with a class, but to replace the JumpID with one.

In other words, rather than passing JumpID to a procedure and using that to fetch the list of people from a database, you create a Jump class.

And if your JumpID actually indicates a jump as in goto, then you'd probably actually a bunch of classes that all subclass the same thing, and override the same method in different ways.

In fact, let's assume that you do some parties that aren't weddings, and in that case, you don't need the Relationships, but only a simple list of people:

type TPassenger = record
   FirstName, LastName: string;
end;

type TJump = class
  private
    JumpID   : string;
    manifest : TList< TPassenger >;
  public
    constructor Init( JumpID: string );
    function GetManifest( ) : TList< TPassenger >;
    procedure PrintManifest( ); virtual;
end;

So now PrintManifest() does the job of your PrintIndyEntry(), but instead of calculating the list inline, it calls Self.GetManifest().

Now maybe your database doesn't change much, and your TJump instance is always short lived, so you decide to just populate Self.manifest in the constructor. In that case, GetManifest() just returns that list.

Or maybe your database changes frequently, or the TJump sticks around long enough that the database may change underneath it. In that case, GetManifest() rebuilds the list each time it's called... Or perhaps you add another private value indicating the last time you queried, and only update after the information expires.

The point is that PrintManifest doesn't have to care how GetManifest works, because you've hidden that information away.

Of course, in Delphi, you could have done the same thing with a unit, hiding a list of cached passenger lists in your implementation section.

But clasess bring a little more to the table, when it comes time to implement the wedding-party-specific features:

type TWeddingGuest = record
  public
    passenger    : TPassenger;
    Relationship : string;
end;

type TWeddingJump = class ( TJump )
  private
    procedure GetWeddingManifest( ) : TList< TWeddingGuest >;
    procedure PrintManifest( ); override;
end;

So here, the TWeddingJump inherits the Init and GetManifest from the TJump, but it also adds a GetWeddingManifest( );, and it's going to override the behavior of PrintManifest() with some custom implementation. (You know it's doing this because of the override marker here, which corresponds to the virtual marker in TJump.

But now, suppose that PrintManifest is actually a rather complicated procedure, and you don't want to duplicate all that code when all you want to do is add one column in the header, and another column in the body listing the relationship field. You can do that like so:

type TJump = class
   // ... same as earlier, but add:
   procedure PrintManfestHeader(); virtual;
   procedure PrintManfiestRow(passenger:TPassenger); virtual;
end;
type TWeddingJump = class (TJump)
   // ... same as earlier, but:
   // * remove the PrintManifest override
   // * add:
   procedure PrintManfestHeader(); override;
   procedure PrintManfiestRow(passenger:TPassenger); override;

end;

Now, you want to do this:

procedure TJump.PrintManifest( )
   var passenger: TPassenger;
begin;
   // ...
   Self.PrintManifestHeader();
   for guest in Self.GetManifest() do begin
      Self.PrintManifestRow();
   end;
   // ...
end;

But you can't, yet, because GetManifest() returns TList< TPassenger >; and for TWeddingJump, you need it to return TList< TWeddingGuest >.

Well, how can you handle that?

In your original code, you have this:

IndiPtr: pointer

Pointer to what? My guess is that, just like this example, you have different types of individual, and you need them to do different things, so you just use a generic pointer, and let it point to different kinds of records, and hope you cast it to the right thing later. But classes give you several better ways to solve this problem:

• You could make TPassenger a class and add a GetRelationship() method. This would eliminate the need for TWeddingGuest, but it means that GetRelationship method is always around, even when you're not talking about weddings.
• You could add a GetRelationship(guest:TPassenger) in the TWeddingGuest class, and just call that inside TWeddingGuest.PrintManifestRow().

But suppose you have to query a database to populate that information. With the two methods above, you're issuing a new query for each passenger, and that might bog down your database. You really want to fetch everything in one pass, in GetManifest().

So, instead, you apply inheritance again:

type TPassenger = class
  public
    firstname, lastname: string;
end;
type TWeddingGuest = class (TPassenger)
  public
    relationship: string;
end;

Because GetManifest() returns a list of passengers, and all wedding guests are passengers, you can now do this:

type TWeddingJump = class (TJump)
  // ... same as before, but:
  // replace: procedure GetWeddingManfiest...
  // with:
  procedure GetManifest( ) : TList; override;
  // (remember to add the corresponding 'virtual' in TJump)
end;

And now, you fill in the details for TWeddingJump.PrintManifestRow, and the same version of PrintManifest works for both TJump and TWeddingJump.

There's still one problem: we declared PrintManifestRow(passenger:TPassenger) but we're actually passing in a TWeddingGuest. This is legal, because TWeddingGuest is a subclass of TPassenger... But we need to get at the .relationship field, and TPassenger doesn't have that field.

How can the compiler trust that inside a TWeddingJump, you're always going to pass in a TWeddingGuest rather than just an ordinary TPassenger? You have to assure it that the relationship field is actually there.

You can't just declare it as TWeddingJupmp.(passenger:TWeddingGuest) because by subclassing, you're basically promising to do all the things the parent class can do, and the parent class can handle any TPassenger.

So you could go back to checking the type by hand and casting it, just like an untyped pointer, but again, there are better ways to handle this:

• Polymorphism approach: move the PrintManifestRow() method to the TPassenger class (removing the passenger:TPassenger parameter, as this is now the implicit parameter Self), override that method in TWeddingGuest, and then just have TJump.PrintManifest call passenger.PrintManifestRow().
• Generic class approach: make TJump itself a generic class (type TJump = class), and instead of having GetManifest() return a TList, you have it return TList. Likewise, PrintManifestRow(passenger:TPassenger) becomes PrintManifestRow(passenger:T);. Now you can say: TWeddingJump = class(TJump) and now you're free to declare the overridden version as PrintManifestRow(passenger:TWeddingGuest).

Anyway, that's way more than I expected to write about all this. I hope it helped. :)