Only a minority of the Delphi developers knows that every Delphi developer uses a Factory pattern (delphi.about.com has an example in "regular" Delphi), but then implemented using virtual Create constructors.
So: time to shed some light on that :-)
Virtual constructors are to classes like virtual methods are like object instances.
The whole idea of the factory pattern is that you decouple the logic that determines what kind (in this case "class") of thing (in this case "object instance") to create from the actual creation.
It works like this using virtual Create constructors:
TComponent has a virtual Create constructor so, which can be overridden by any descending class:
type
TComponent = class(TPersistent, ...)
constructor Create(AOwner: TComponent); virtual;
...
end;
For instance the TDirectoryListBox.Create constructor overrides it:
type
TDirectoryListBox = class(...)
constructor Create(AOwner: TComponent); override;
...
end;
You can store a class reference (the class analogy to an object instance reference) in a variable of type 'class type'. For component classes, there is a predefined type TComponentClass in the Classes unit:
type
TComponentClass = class of TComponent;
When you have a variable (or parameter) of type TComponentClass, you can do polymorphic construction, which is very very similar to the factory pattern:
var
ClassToCreate: TComponentClass;
...
procedure SomeMethodInSomeUnit;
begin
ClassToCreate := TButton;
end;
...
procedure AnotherMethodInAnotherUnit;
var
CreatedComponent: TComponent;
begin
CreatedComponent := ClassToCreate.Create(Application);
...
end;
The Delphi RTL uses this for instance here:
Result := TComponentClass(FindClass(ReadStr)).Create(nil);
and here:
// create another instance of this kind of grid
SubGrid := TCustomDBGrid(TComponentClass(Self.ClassType).Create(Self));
The first use in the Delphi RTL is how the whole creation process works of forms, datamodules, frames and components that are being read from a DFM file.
The form (datamodule/frame/...) classes actually have a (published) list of components that are on the form (datamodule/frame/...). That list includes for each component the instance name and the class reference.
When reading the DFM files, the Delphi RTL then:
- finds about the components instance name,
- uses that name to find the underlying class reference,
- then uses the class reference to dynamically create the correct the correct object
A regular Delphi developer usually never sees that happen, but without it, the whole Delphi RAD experience would not exist.
Allen Bauer (the Chief Scientist at Embarcadero), wrote a short blog article about this topic as well.
There is also a SO question about where virtual constructors are being used.
Let me know if that was enough light on the virtual Create constructor topic :-)
--jeroen