CreatingarealsingletonclassinDelphi5

Creating a real singleton class in Delphi 5

Abstract:The article describes how to create a class that follows the singleton pattern. The class described will take care of the singleton requirements and effects itself, effectively leaving the programmer to use the class as any others.

Creating a real singleton class in Delphi

by Lasse V錱s鎡her Karlsen, Systems Developer for Cintra Software Engineering

Errata 1. Implementation of TSingleton.NewInstance had the first test switched, the one that checked if the Instance variable was assigned or not. -- Fixed 11-08-2000 - Thanks to the people that emailed me about this -- 2. Overzealous fix for the first errata, the NewInstance method should now work as stated. -- Fixed 11.08.2000 --

A singleton is a class that supports the creation of just one object. Its like your computer -- theres just one keyboard. So if you were writing Delphi code that simulated your computer, you would want just one object instance to deal with keyboard read, write, and control activities.

Articles about singleton classes are rare but there are a few on the Internet. Ive seen some in magazines too. But none of these articles include sample code to create a real singleton class.

By "real" I mean that the class itself enforces the one-instance requirement, instead of leaving that task to the programmer. All of the articles I have seen so far have required the programmer to use the class in a special way to enforce the singleton pattern.

In this article you will see how to create a proper singleton class that includes logic to enforce the one-instance rule.

Note: The conventional approach, in which the one-instance rule is maintained explicitly by the programmer, is not without merit. Real singleton classes like the one I present here effectively hide the details and awareness of the singleton pattern from the programmer. The programmer is relieved of the task of enforcing the pattern -- thats good -- but the programmer may also be unaware of the special nature of the class, which is bad. If you dont know that the class is a singleton class then all sorts of errors can crop up. You have been warned!

Writing the code

Our goal is to write a class that can be used like this:

procedure Test;
var
  s1, s2 : TSingleton;
begin
  s1 := TSingleton.Create;
  s2 := TSingleton.Create;
  // Do something with s1 and s2 here
  s2.Free;
  s1.Free;
end;

(Ive left out the try...finally blocks and other safeguards for simplicitys sake.)

The goal is to make the TSingleton class behave in such a way that both s1 and s2 refer to the same object. Heres what we have to do:

• Instantiate the object the first time Create is called (when s1 is created above)
• Ensure that when another Create is executed (s2 above), the existing object is reused instead of another one created
• Avoid destroying the object when its not the last reference that is destroyed (when s2 is freed)
• Destroy the instance when the last reference is destroyed (when s1 is freed above)

Is there a way to override the creation and destruction of a new object in Delphi? There sure is. In the TObject class (the mother of all objects {pun intended}), there are two methods we can use:

class function NewInstance: TObject; virtual;
procedure FreeInstance; virtual;

NewInstance is responsible for allocating memory to hold a new instance of the class, and FreeInstance is responsible for freeing that memory when the class is through with it.

These methods control what happens when the object is created and when the object is destroyed. If we overwrite this code, we can alter the default behavior to work the say a singleton class requires. Nothing to it.

Tracking instances is a little trickier. We must:

1. Keep track of each existing instance of our class
2. Keep track of how many references there are to this instance
3. Create a new object only when no instance exists
4. Destroy the object when the last reference is removed

To keep track of an existing instance, we will use a global variable. Actually, the variable will ultimately be declared inside the Implementation part of a unit so it wont be a true global variable. But the scope must be sufficient to track all the Create and Free calls. Well call the variable Instance so we know what it refers to.

As for keeping track of how many references exist, we need another variable. We can it inside the class or make it a sort-of global like Instance. Ill opt for the latter way but do what you feel is best. Ill name this variable Ref_Count.

We now have two variables:

var
  Instance  : TSingleton  = nil;
  Ref_Count : Integer     = 0;

I initialize the variables so that initially they dont contain any garbage. I know that the compiler does this automatically, so this is just a readability issue.

Well need to declare the TSingleton class above the variable block, and if you take a look at the example files that you can download at the end of this article youll see that Ive put the declaration in the interface part of the unit so that its visible outside of it.

Heres the declaration of the TSingleton class:

type
  TSingleton = class
  public
    class function NewInstance: TObject; override;
    procedure FreeInstance; override;
    class function RefCount: Integer;
  end;

I added the RefCount function so that we can see that it actually works, and its often handy to be able to read how many references to the object exist. Its not required, however, so you dont have to add it to your singleton classes if you dont need it.

Ok, now for the implementation of the three methods:

procedure TSingleton.FreeInstance;
begin
  Dec( Ref_Count );
  if ( Ref_Count = 0 ) then
  begin
    Instance := nil;
    // Destroy private variables here
    inherited FreeInstance;
  end;
end;

class function TSingleton.NewInstance: TObject;
begin
  if ( not Assigned( Instance ) ) then
  begin
    Instance := inherited NewInstance;
    // Initialize private variables here, like this:
    //   TSingleton(Result).Variable := Value;
  end;
  Result := Instance
  Inc( Ref_Count );
end;

class function TSingleton.RefCount: Integer;
begin
  Result := Ref_Count;
end;

And thats it!

When you call TSingletons constructor, a call is placed to the NewInstance method declared in TObject. This method allocates memory to hold the new object and returns it to the constructor. The constructor uses that memory and eventually returns a pointer to the memory to the code that called the constructor. This pointer is usually stored in a variable while the object is in use.

I have overridden the NewInstance method so it will allocate the memory only if no instance of the class exists. If there is an existing instance, the function simply returns that instance to the constructor so it will be reused