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Posted by jpluimers on 2018/05/16
Generics and constraints in Delphi is still a bit of pain.
A while ago, I needed a factory for threads. It was more convoluted than I meant it to be, so I had a short chat with Stefan Glienke.
He came up with an example similar to the one below. I tore it apart so you can on each line see what the compiler dislikes.
This fails, but was kind of how I wanted it to be:
type
TFactory = record
class function Manufacture<T: TBaseClass, constructor>(parameter definitions): T;
end;
class function Manufacture<T: TBaseClass, constructor>(parameter definitions): T;
begin
Result := T.Create(parameter values);
end;Which means I need to update Delphi Constraints in Generics – RAD Studio XE documentation wiki
The factory:
type TComponentFactory = class class function CreateComponent<T: TComponent>(AOwner: TComponent): T; static; end; class function TComponentFactory.CreateComponent<T>(AOwner: TComponent): T; var ComponentClass: TComponentClass; Component: TComponent; begin ComponentClass := T; Component := ComponentClass.Create(AOwner); // you can't do `T.Create(AOwner)` Result := T(Component); // you can't do `Result := ComponentClass.Create(AOwner);` end;
The usage:
var Component: TButton; Owner: TComponent; begin Owner := Application.MainForm; Component := TComponentFactory<TButton>.CreateComponent(Owner); Component.Parent := Owner; end;
Full source example is at: After a discussion with Stefan Glienke: a pattern for a factory, this one for components, but can be uses for anything having a base class like a TThread descendant.
In the mean time, Spring4D has added the [Archive.is] Spring4D: TActivator record with many CreateInstance methods that allow you to create instances of classes for which you just have type information.
–jeroen
This fails:
type
TFactory = record
class function Manufacture<T: TBaseClass, constructor>(parameter definitions): T;
end;
class function Manufacture<T: TBaseClass, constructor>(parameter definitions): T;
begin
Result := T.Create(parameter values);
end;
| type | |
| TComponentFactory = class | |
| class function CreateComponent<T: TComponent>(AOwner: TComponent): T; static; | |
| end; | |
| class function TComponentFactory.CreateComponent<T>(AOwner: TComponent): T; | |
| var | |
| ComponentClass: TComponentClass; | |
| Component: TComponent; | |
| begin | |
| ComponentClass := T; | |
| Component := ComponentClass.Create(AOwner); // you can't do `T.Create(AOwner)` | |
| Result := T(Component); // you can't do `Result := ComponentClass.Create(AOwner);` | |
| end; |
| var | |
| Component: TButton; | |
| Owner: TComponent; | |
| begin | |
| Owner := Application.MainForm; | |
| Component := TComponentFactory<TButton>.CreateComponent(Owner); | |
| Component.Parent := Owner; | |
| end; |
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