How to copy data from one class into another class - class

Question

how to copy data from one class into a second class using operator overloading with DELPHI ?
my dummy app goes like this :
type
TClass_A = class
a: String;
end;
TClass_B = class(TClass_A)
b: String;
end;
implementation
procedure TForm1.Button1Click(Sender: TObject);
var
a: TClass_A;
b: TClass_B;
begin
a := TClass_A.Create;
b := TClass_B.create;
b := a; // <<-- What code should be here? Can I overload := operator?
end;

Answer 1

The assignment operator cannot be overloaded in Delphi.
You will need to introduce a method to perform the copying. An example of how this might be done is TPersistent.Assign. It would be perfectly reasonable for you to derive from TPersistent and override the virtual Assign method to implement the desired functionality.

Answer 2

Here is a small example of how to do this with new style RTTI (D2010 and higher).
Please note that this is a very basic example which only copies fields (not properties) and works best with basic classes (do not use this with TComponent), the fields in the destination object must be the same type. FYI, there are better examples out there :)
uses
Rtti,...
function CopyObject(const FromObj, ToObj: TObject): Boolean;
var
Ctx : TRTTIContext;
FromObjType : TRttiType;
ToObjType : TRttiType;
FromField : TRttiField;
ToField : TRttiField;
begin
Result := False;
FromObjType := Ctx.GetType(FromObj.ClassInfo);
ToObjType := Ctx.GetType(ToObj.ClassInfo);
for FromField in FromObjType.GetFields do
begin
ToField := ToObjType.GetField(FromField.Name);
if Assigned(ToField) then
begin
if ToField.FieldType = FromField.FieldType then
ToField.SetValue(ToObj, FromField.GetValue(FromObj));
Result := True;
end;
end;
end;