I did this kind of thing some time ago, but have forgotten the details.

Let us say you have something like TBaseType(Variable).Method, you want to let TBaseType be a variable so you can have some code like this.

procedure (value:someType)
var
  variableType: TClass; //not quite sure here
  variableType := TypeOf(value);

then further on you execute something like

variableType(Variable).Method, rather than TBaseType(Variable).Method because you can't hard code TBaseType because can't tell what it will be at runtime. I remember doing something like that with Delphi of old, probably Delphi 3 or Delphi 7

Can someone help me with this, with some use cases as well?

Thanks

Like so:

procedure Test;
var
  SomeType: TClass;
  SomeObject: TObject;
begin
  SomeType := TStringList;
  SomeObject := SomeType.Create;
  (SomeObject as TStrings).Add('Hello World');
  ShowMessage((SomeObject as TStrings)[0]);
  SomeObject.Free;
end;

It can be implemented with RTTI or variants perhaps.

No, if all your types are classes descending from the same root you can do it is like this:

type
  TBaseType = class;
    procedure somemethod();
  end;
  TExtendedType = class(TBaseType);
  TBaseTypeClass = class of TBaseType;
procedure (value: TBaseType; type_: TBaseTypeClass)
begin
  value := type_.Create();
  value.somemethod();
end;

And you can call your procedure as:

proc(somevar, TExtendedType);

For other types you need to use generics.

In our previous episode, Frank Church said:

There is typeinfo in Delphi/fpc, but that is for RTTI, and quite different

The above is a bit strange in Pascals with unit system, since it potentially creates a var of a type that might not be in the scope.

So I think it is not even possible to implement this in a straight way.