Question

I’ve added a class variables to the base class of a deep class hierarchy. It’s an integer intended to count number of instances created per class type. But I’ve run into a problem.

Given the example:

  TBaseClass = class
  private
    class var fCreated: integer;
  public
    class function NewInstance: TObject; override;  
  end;

  TDescendant = class(TBaseClass)
  end;

  ...

  class function TBaseClass.NewInstance: TObject;
  begin
    result := inherited NewInstance;
    inc(fCreated);
  end;

I assumed that I can use the class var to store the number of instances created per class, but this does not seem to be the case.

Inspecting TBaseClass.fCreated returns same value as TDescendant.fCreated, changing one via inspector changes the other, so it behaves as if fCreated is a single global var.

I expected fCreated to be maintained per class type, isn’t that the point ? What am I missing ?

Answer 1

You are missing nothing. Your analysis of how class variables work is correct. A class var is nothing more than a global variable that is scoped by the class.

One simple solution for you is to use a dictionary to count the instances. A more hacky approach is to use a trick of mine that Hallvard Vassbotn blogged about, which (ab)uses the VMT to store class-specific fields. You can read all about it here.