[fpc-pascal] Cast, inheritance, etc

[Marcos Douglas]

Hi,

See the code below.
My question is: Why the cast worked, considering the c variable is a
TAClass instance, not a TBClass.

-------------

program Project1;

{$mode objfpc}{$H+}

uses
  Classes, SysUtils;

type
  TAClass = class
  protected
    FInfoA: string;
  public
    constructor Create;
  end;

  TBClass = class(TAClass)
  protected
    FInfoB: string;
  public
    function GetInfoA: string;
    function GetInfoB: string;
  end;

{ TAClass }

constructor TAClass.Create;
begin
  FInfoA := 'TAClass info';
end;

{ TBClass }

function TBClass.GetInfoA: string;
begin
  Result := FInfoA;
end;

function TBClass.GetInfoB: string;
begin
  Result := FInfoB;
end;

var
  c: TAClass;
  s: string;

begin
  c := TAClass.Create;
  try
    // The c is a TAClass, not a TBClass instance!
    // but worked... WHY?!
    s := TBClass(c).GetInfoA;
    writeln(s);

    // Now I got an AV... ok for me.
    writeln(TBClass(c).GetInfoB)
  finally
    c.Free;
  end;

  writeln('Done');

end.

Marcos Douglas