[fpc-pascal] Understanding virtual methods

[Xiangrong Fang]

Hi All,

I am reading this document:
http://www.freepascal.org/docs-html/ref/refsu29.html   and doing an
experiment with the following code:

program project1;
{$mode objfpc}{$H+}
type
  TBase = class
    constructor Create; virtual;
  end;
  TDerived = class(TBase)
    constructor Create; override;
  end;

var
  c1, c2: TBase;
  c3: TDerived;

constructor TDerived.Create;
begin
  WriteLn('Entering TDerived.Create');
  inherited Create;
  WriteLn('Leaving TDerived.Create');
end;

constructor TBase.Create;
begin
  WriteLn('Entering TBase.Create');
  Writeln('Leaving TBase.Create');
end;

begin
  WriteLn('Creating a TBase and assigning to TBase variable...');
  c1 := TBase.Create;
  WriteLn('Creating a TDerived and assigning to TBase variable...');
  c2 := TDerived.Create;
  WriteLn('Creating a TDerived and assigning to TDerived variable...');
  c3 := TDerived.Create;
end.


The problem is, it makes NO DIFFERENCE at all in the following cases:

CASE 1:

TBase.Create;
TDerived.Create;

CASE 2:

TBase.Create; virtual;
TDerived.Create; virtual;

CASE 3:

TBase.Create; virtual;
TDerived.Create; override;

According to the document, "inherited" cannot be used in non-virtual
methods, and it is wrong to use virtual in sub-class. But my test shows the
contrary.   BTW, I am running Linux on 64bit platform.

Regards,
Shannon
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