Using class methods to detect Memory-Leaks

Problem/Question/Abstract:

When you dinamically create objects at runtime, you need to check that you free those objects too. Class methods can help in the process.

Answer:

Class Methods aply to the class level, in other words you don�t need an instance to call the method

I wish we could define class objects as well, but they doesn�t exist in Object Pascal, so we will do a trick, we are going to define a variable in the implementation section of the unit, this variable will hold the number of instances the class will have in a moment in time. Object Oriented purist might claim about it, but it works, nobody is perfect (not even Delphi!).

For example say you need to create instances of a class named TFoo, so you create the following Unit.

We will define two class procedures: AddInstance(to increse the counter of instances) and ReleaseInstance(to decrese the number of instances), these are called in the constructor and the destructor acordingly. Finally we define a class function NumOfInstances which returns the actual number of instances.

Add a Initilialization and a Finalization section to the Unit, in the Finalization section ask if the number of instances is <> 0, if this is the case you known that you didin�t destroy all the objects that you created.

unit U_Foo;

interface

uses
  Classes, Windows, SysUtils;

type
  TFoo = class
  private
    class procedure AddInstance;
    class procedure ReleaseInstance;
  public
    constructor Create;
    destructor Destroy; override;
    class function NumOfInstances: Integer;
  end;

implementation

var
  TFoo_Instances: Integer = 0;

  { TFoo }

class procedure TFoo.AddInstance;
begin
  Inc(TFoo_Instances);
end; //end of TFoo.AddInstance

constructor TFoo.Create;
begin
  AddInstance;
end; //end of TFoo.Create

destructor TFoo.Destroy;
begin
  ReleaseInstance;
  inherited;
end; //end of TFoo.Destroy

class function TFoo.NumOfInstances: Integer;
begin
  Result := TFoo_Instances;
end; //end of TFoo.NumOfInstances

class procedure TFoo.ReleaseInstance;
begin
  Dec(TFoo_Instances);
end; //end of TFoo.ReleaseInstance

initialization

finalization

  if TFoo_Instances <> 0 then
    MessageBox(0,
      PChar(Format('%d instances of TFoo active', [TFoo_Instances])),
      'Warning', MB_OK or MB_ICONWARNING);

end.