How can the subclass constructor be called from the parent class? - delphi

Question

Is there a way to invoke Create of the subclass from the parent class? Below there is this Duplicate method in which I want the constructor of the subclass to be invoked instead, so that the test at the bottom succeeds.
type
IBla<T> = interface(IInvokable)
['{34E812BF-D021-422A-A051-A492F25534C4}']
function GetIntFromIface(): Integer;
function Duplicate(): IBla<T>;
end;
TClassA<T> = class(TInterfacedObject, IBla<T>)
protected
function GetInt(): Integer; virtual;
public
function GetIntFromIface(): Integer;
function Duplicate(): IBla<T>;
end;
TClassB = class(TClassA<Integer>, IBla<Integer>)
protected
function GetInt(): Integer; override;
end;
function TClassA<T>.Duplicate: IBla<T>;
begin
Exit(TClassA<T>.Create());
end;
function TClassA<T>.GetInt: Integer;
begin
Exit(1);
end;
function TClassA<T>.GetIntFromIface: Integer;
begin
Exit(GetInt());
end;
function TClassB.GetInt: Integer;
begin
Exit(2);
end;
procedure TestRandomStuff.Test123;
var
o1, o2: IBla<Integer>;
begin
o1 := TClassB.Create();
o2 := o1.Duplicate();
Assert.AreEqual(o2.GetIntFromIface, 2);
end;

Answer 1

You can do this using RTTI:
uses
System.Rtti;
....
function TClassA<T>.Duplicate: IBla<T>;
var
ctx: TRttiContext;
typ: TRttiType;
mthd: TRttiMethod;
inst: TValue;
begin
typ := ctx.GetType(ClassInfo);
mthd := typ.GetMethod('Create');
inst := mthd.Invoke((typ as TRttiInstanceType).MetaclassType, []);
inst.AsObject.GetInterface(IBla<T>, Result);
end;
There is quite probably a cleaner way to invoke a constructor using RTTI (I know next to nothing about RTTI in Delphi), so you might do well to read around that topic rather than taking the above as being the canonical way to do this.
Of course, this assumes that all subclasses use a parameterless constructor defined in TObject. That might be rather limiting. I would not be surprised if you found yourself having to re-think the design in a more fundamental manner.
If none of your subclasses implement constructors then you could make it even simpler, and not use RTTI at all:
function TClassA<T>.Duplicate: IBla<T>;
begin
ClassType.Create.GetInterface(IBla<T>, Result);
end;
But be aware that this calls the constructor defined in TObject and will not call any constructor defined in a subclass.

Answer 2

This seems to work:
function TClassA<T>.Duplicate: IBla<T>;
begin
//Exit(TClassA<T>.Create());
Exit( ClassType.Create as TClassA<T> );
end;
The subtlety is that ClassType.Create will create (in this case) a TClassB and the original creates a TClassA< integer > which the compiler sees as different to TClassB, and hence calls TClassA< T >.GetInt rather than TClassB.GetInt.
Edit
But be aware that this calls the constructor defined in TObject and will not call any constructor defined in a subclass. (With thanks to David H)
However, here is solution that overcomes that restriction too:
interface
type
IBla<T> = interface(IInvokable)
['{34E812BF-D021-422A-A051-A492F25534C4}']
function GetIntFromIface(): Integer;
function Duplicate(): IBla<T>;
end;
TClassA<T> = class(TInterfacedObject, IBla<T>)
protected
function GetInt(): Integer; virtual;
public
constructor Create; virtual;
function GetIntFromIface(): Integer;
function Duplicate(): IBla<T>;
end;
//TClassB = class(TClassA<Integer>)
TClassB = class(TClassA<Integer>, IBla<Integer>)
protected
function GetInt(): Integer; override;
public
constructor Create; override;
function Duplicate(): IBla<Integer>;
end;
procedure Test123;
implementation
constructor TClassA<T>.Create;
begin
inherited Create;
end;
function TClassA<T>.Duplicate: IBla<T>;
begin
Exit(TClassA<T>.Create());
end;
function TClassA<T>.GetInt: Integer;
begin
Exit(1);
end;
function TClassA<T>.GetIntFromIface: Integer;
begin
Exit(GetInt());
end;
constructor TClassB.Create;
begin
inherited Create;
end;
function TClassB.Duplicate: IBla<Integer>;
begin
Result := TClassB.Create;
end;
function TClassB.GetInt: Integer;
begin
Exit(2);
end;
procedure Test123;
var
o1, o2: IBla<Integer>;
begin
o1 := TClassB.Create();
o2 := o1.Duplicate();
Assert( o2.GetIntFromIface = 2);
end;