Creating a real singleton class in Delphi

by Lasse Vegsfther Karlsen, Systems Developer for Cintra Software Engineering

A singleton is a class that supports the creation of just one object. It's like your computer -- there's 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. I've 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 -- that's good -- but the programmer may also be unaware of the special nature of the class, which is bad. If you don't 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;

The goal is to make the TSingleton class behave in such a way that both s1 and s2 refer to the same object. Here's 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 it's 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)

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

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. I'll opt for the latter way but do what you feel is best. I'll name this variable Ref_Count.

We now have two variables:

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

We'll 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 you'll see that I've put the declaration in the interface part of the unit so that it's visible outside of it.

Here's the declaration of the TSingleton class:

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

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;

When you call TSingleton's 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.

If we call the constructor three times, an object is created only the first time. The other two calls simply reuse the first object. The reference count variable let us know that we have three references to the single instance.

When the program calls the destructor, a call to FreeInstance is placed to free the memory allocated in the constructor. This method, too, is overridden so that the object is destroyed only when the last reference is removed.

If you intend to use a singleton in a multithreaded program, treat the object as you would any variable you share between threads. Because that's just what you do: share it between the threads. So you must take special care when changing data.

Simplicity itself

As you can see, creating a singleton class doesn't require much effort, just the right knowledge and a few lines of code. My code works fine in Delphi 5. The technique will probably work fine with older versions of Delphi, but I haven't tested it so I don't make any guarantees.

Before I give you the file to play with, let me give you a few words of warning.

• Don't descend from a singleton class. The reason for that is that there is only one instance and reference count variable. If you derive two classes from TSingleton, only one object will be created. The other class will reuse that object, which will be an instance of a different class. Don't go down that road!
• You can't make singleton components for the simple reason of ownership. A component is owned by a form and one component can't be owned by several other components.
• Remember that the constructor and destructor get called for each new reference as they are created and destroyed. Don't initialize private variables in the constructor and don't free them in the destructor. Instead, build that code into the NewInstance and the FreeInstance methods, as shown in the comments.
• The order of adjusting Ref_Count in the two methods in conjunction with the rest of the code in those two methods is critical. It has to do with proper creation and destruction when something goes wrong. If your initialization code raises an exception, the order of doing things like shown above will make sure that the class is destroyed properly. Alter this code at your peril!

Link to file: CodeCentral entry 15083.

I'm sure you can find a few places where a singleton class comes in handy and now you have the tools to create your own! If you want to get in touch with me, my email is lasse@cintra.no.

Happy programming!