Delphi RTTI dynamic cast

Question

Let's say that I have:

type
 TClassA = class
  function prova: integer; virtual;
  function provaSuA: integer; virtual;
 end;

type
 TClassB = class(TClassA)
  function prova: integer; override;
  function provaSuB: integer; virtual;
 end;

Then I use this code:

procedure TForm1.Button1Click(Sender: TObject);
var a: TClassA;
    b: TClassB;
begin
 Memo1.Clear;

 a := TClassB.Create;
 try

  b := ?? //dynamic_cast on C++

  Memo1.Lines.Add(a.prova.ToString);
  Memo1.Lines.Add(b.provaSuB.ToString);

 finally
  a.Free;
 end;

end;

I am using polymorphism and the static type of a is TClassA but the dynamic type is TClassB . Of course I can only call on a methods that are declared on TClassA (or that are overridden in TClassB ).

If I want to use a and have access to ALL methods in TClassB in C++ I'd use a dynamic_cast that is (together with typeid) in C++ RTTI. How can I use Delphi's RTTI to do that?

Answer 1

Delphi "RTTI" has a slightly different meaning from C++'s one. To replicate what dynamic_cast does, you need to follow one of the two patterns:

• pattern 1: if the cast fails, I want an exception : this is achieved, by using the as operator which performs the proper type-checking and then casts the object when possible. In case of failure, a ClassCastException is thrown.

  b:= a as TClassB;

• pattern 2: if the cast fails, I want to handle it manually : this is achieved by using the is operator which strictly performs the test about the cast. Then you are need to manually cast (look comments in code) on success:

  if (a is TClassB) then begin // this is a cast, however, in this context, you are simply interpreting // the memory pointed by a as a class of type ClassB. You are not using any operator. b:= TClassB(a); end else begin // here you know that a cannot be cast to TClassB, therefore you can gracefully take // proper action here, without catching any exception end;