I'm trying to build a generic ancestor for composite controls. The initial idea looked something like this:
type
TCompositeControl<TControl1: TControl; TControl2: TControl> = class(TWinControl)
private
FControl1,
FControl2: TControl;
public
constructor Create(AOwner: TComponent); override;
end;
TLabelAndEdit = TCompositeControl<TLabel, TEdit>; // simple example for illustration only
constructor TCompositeControl<TControl1,TControl2>.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
FControl1 := TControl1.Create(Self);
FControl2 := TControl2.Create(Self);
end;
As you might already be aware, this will trigger compiler error E2568: Can't create new instance without CONSTRUCTOR constraint in type parameter declaration. Adding the constructor
constraint doesn't help however as it implies a parameter-less constructor.
Casting the templates to TControl
makes the code compilable:
...
FControl1 := TControl(TControl1).Create(Self);
...
...but it results in an Access Violation at runtime.
One hack that would probably work is invoking the constructor via RTTI, but I would consider that a rather dirty solution.
Another hack that essentially works is using class type variables as intermediates:
type
TControlClass = class of TControl;
constructor TCompositeControl<TControl1,TControl2>.Create(AOwner: TComponent);
var
lCtrlClass1,
lCtrlClass2: TControlClass;
begin
inherited Create(AOwner);
lCtrlClass1 := TControl1;
FControl1 := lCtrlClass1.Create(Self);
lCtrlClass2 := TControl2;
FControl2 := lCtrlClass2.Create(Self);
end;
Is there a cleaner solution? Also, can somebody explain to me why the classtype-constraint does not suffice for invoking the virtual constructor on the type parameter directly?
Your typecast is bad: TControl(TControl1).Create(Self)
. That tells the compiler that TControl1
is an instance of TControl
, but we know that it's not an instance. It's a class reference. Type-cast it to the class-reference type instead:
FControl1 := TControlClass(TControl1).Create(Self);
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With