Nick Hodges

Did you know….

program Project27;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  TOuterClass = class
  private
    type
      TNestedClass = class
      public
        procedure DoThis;
      end;
  private
    FNestedClass: TNestedClass;
  public
    constructor Create;
    destructor Destroy; override;
    property NestedClass: TNestedClass read FNestedClass;
  end;

{ TOuterClass }

constructor TOuterClass.Create;
begin
  FNestedClass := TNestedClass.Create;
end;

destructor TOuterClass.Destroy;
begin
  FNestedClass.Free;
  inherited;
end;

{ TOuterClass.TNestedClass }

procedure TOuterClass.TNestedClass.DoThis;
begin
  Writeln(‘Doing this‘);
end;

var
  OuterClass: TOuterClass;

begin
  try
    OuterClass := TOuterClass.Create;
    try
      OuterClass.NestedClass.DoThis;
    finally
      OuterClass.Free;
    end;
    Readln;
  except
    on E:Exception do
      Writeln(E.Classname, ‘: ‘, E.Message);
  end;
end.

This entry was posted on Wednesday, October 3rd, 2007 at 4:48 pm and is filed under Did You Know. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

9 Responses to “Did you know….”

1. 1
   M J Marshall Says:
   

   Nested classes are very useful for implementing "local" classes such as Enumerators: Instead of defining TMyClassEnumerator as a public class, you define TMyClass.TEnumerator as a private nested class.

   Besides keeping the namespace a little tidier, it allows you to restrict access to classes that you are forced to declare publicly (such as enumerators).

   October 3rd, 2007 at 9:15 pm
2. 2
   Thomas Mueller Says:
   

   Hi Nick,

   this highlights one of the disadvantages of having to fully declare classes in the interface section of a unit: Since the nested class is private, there is no need for it to be part of a unit’s interface section, it only confuses programmers who use the class.
   This would be much tidier if it were possible to declare anything that’s private in the implementation section only. IIRC there is at least one QC report on this issue.

   twm

   October 4th, 2007 at 2:17 am
3. 3
   Nick Hodges Says:
   

   C Johnson –

   Probably the best example is a collection with a specific collection type. The specific type is meaningless outside the context of the collection itself, so it makes sense to nest it within the collection itself.

   Nick

   October 4th, 2007 at 8:04 am
4. 4
   Rob Kennedy Says:
   

   Users of Delphi 2005 should be aware of QC 11331. The compiler will accept some nested-class code that it shouldn’t accept. The bug was fixed for Delphi 2006, apparently.

   Marshall cites an enumerator as a good candidate for a nested class, but it shouldn’t be a _private_ nested class. It should be public. That lets consumers of the class enumerate its contents. Maybe the "for-in" loop can use a private nested class, but the rest of us can’t, and not all enumerations fit the "for-in" model.

   And Nick, your code isn’t valid. It’s using left single quotation marks instead of apostrophes. Delphi doesn’t support curly quotes, does it?

   October 4th, 2007 at 8:26 am
5. 5
   Nick Hodges Says:
   

   Rob –

   I copied the code right out of the Code Editor and pasted it into a formatting tool. That may have done something to the font.

   Nick

   October 4th, 2007 at 8:44 am
6. 6
   David Clegg Says:
   

   What tool do you use for code formatting, Nick? FWIW, YAPP (http://lingua.codegear.com/yapp/) doesn’t seem to suffer from the same problem with the quotes.

   October 4th, 2007 at 11:41 am
7. 7
   Serg Says:
   

   The code example looks strange for me - TNestedClass type declared as private while property NestedClass: TNestedClass is made public. What difference it makes if I declare TNestedClass type in public section of TOuterClass?

   October 4th, 2007 at 10:24 pm
8. 8
   Nick Hodges Says:
   

   Serg –

   This is just typical information hiding — the actual declaration of the class doesn’t matter and thus isn’t made public. The property exposes the functionality.

   You can, of course, design things totally differently if you like.

   Nick

   October 5th, 2007 at 8:26 am
9. 9
   Nick Hodges Says:
   

   Cleggy –

   I’m using a LiveWriter plugin — unfortunately, I can’t seem to find the name of it.

   I can tweak the formatting, so I’ll give that a look.

   Nick

   October 5th, 2007 at 8:27 am

Leave a Reply