Delphi/pascal: overloading a constructor with a different prototype

Question

I\'m trying to create a child class of TForm with

1. a special constructor for certain cases, and
2. a default constructor that will maintain compatibilit

Answer 1

There's a really easy way to avoid this. Give your new constructor a different name. Unlike some other popular languages, Delphi has named constructors; you don't have to call them Create. You could call your new one CreateWithDataset and not interfere with the virtual Create constructor at all.

TfrmEndoscopistSearch = class(TForm)
  /// original constructor kept for compatibility
  constructor Create(AOwner: TComponent); override;
  /// additional constructor allows for a caller-defined base data set
  constructor CreateWithDataset(AOwner: TComponent; ADataSet: TDataSet; ACaption: string = '');
end;

In fact, unless you're instantiating this class polymorphically, you don't even need the original constructor. You could declare your new one like this:

TfrmEndoscopistSearch = class(TForm)
  /// additional constructor allows for a caller-defined base data set
  constructor Create(AOwner: TComponent; ADataSet: TDataSet; ACaption: string = ''); reintroduce;
end;

Attempting to call the one-argument constructor directly on TfrmEndoscopistSearch would yield a compilation error.

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(Creating it polymorphically would generally involve using Application.CreateForm:

Application.CreateForm(TfrmEndoscopistSearch, frmEndoscopistSearch);

That always calls the one-argument virtual constructor introduced in TComponent. Unless it's your main form, you don't need to do that. I've written about my feelings on Application.CreateForm before.)