Peganza - Blog

Can you see the Delphi coding error, part 15?

May 7, 2025

Here is the rather artificial code sample. What is the problem?

program TestFreeAndNil;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

type
  TApartment = class
  private
    FKitchen : TObject;
    FBedRoom : TObject;
    FSalon : TObject;
  protected
    function GetSalon : TObject;
  public
    property Kitchen : TObject read FKitchen write FKitchen;
    property BedRoom : TObject read FBedRoom write FBedRoom;
    property Salon : TObject read GetSalon write FSalon;
  end;

  function TApartment.GetSalon : TObject;
  begin
    Result := FSalon;
  end;

var
  Apartment : TApartment;
begin
  Apartment := TApartment.Create;
  Apartment.Kitchen := TObject.Create;
  Apartment.BedRoom := TObject.Create;
  Apartment.Salon := TObject.Create;

  // .. more code here

  FreeAndNil(Apartment.Kitchen);
  FreeAndNil(Apartment.BedRoom);
  FreeAndNil(Apartment.Salon);

  // .. more code following involving
  // Apartment and its fields

end.

FreeAndNil (System.SysUtils) is a convenient method to destroy an object and set it to nil. Of course it is better to just call Free, if the object anyway is going out of scope. But if you need to later use the object again, it can be useful to call FreeAndNil.

But there is a big problem with the above code:

First two calls to FreeAndNil work as expected. The object is destroyed and set to nil. But the last call to FreeAndNil(Apartment.Salon) gives that Apartment.Salon will not be set to nil! The object itself however is destroyed. If you want to check this, just compile the code and run the debugger.

If you call FreeAndNil for a property, make sure that the property has not got a get-function.

But the good news is that in the upcoming May updates of Pascal Analyzer and Pascal Expert, the WARN56-Parameter to FreeAndNil is not an object report section, will report also these situations.