Can you find this Delphi coding error (part 4)?

October 30, 2016

Can you spot the error in the following Delphi code?

unit Worker;

interface

uses
  Classes, WorkUnit;

type
  TBaseClass = class
  private
    FList : TList;
  public
    constructor Create;
    destructor Destroy; override;
    procedure Process; virtual;
    property List : TList read FList write FList;
  end;
  
  TChildClass = class(TBaseClass)
  private
    FChildList : TList;
  public
    constructor Create;
    destructor Destroy; override;
    procedure Process; override;
  end;

implementation

constructor TBaseClass.Create;
begin
  inherited Create;
  FList := TList.Create;
end;

destructor TBaseClass.Destroy;
begin
  FList.Free;
  inherited Destroy;
end;

procedure TBaseClass.Process;
begin
  WorkUnit.DoSomeWork(Self);
end;

constructor TChildClass.Create;
begin
  inherited Create;
  FChildList := TList.Create;
end;

destructor TChildClass.Destroy;
begin
  FChildList.Free;
end;

procedure TChildClass.Process;
begin
  WorkUnit.DoSomeOtherWork(Self);
end;

end.

This was quite easy, right?

Solution: The destructor in TChildClass does not call its inherited destructor. The object will not be destroyed properly which at least will result in a memory leak. There should be a last line "inherited Destroy;" in TChildClass.Destroy.

Not so hard to spot! But imagine finding this problem among thousands of lines of code spread over hundreds of units! And consider also a code base that is evolving all the time, so the code must constantly be rechecked.

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