Re: Factories, handles, and handle wrappers

Maxim Yegorushkin <>
Thu, 17 Dec 2009 19:42:06 +0000
On 17/12/09 18:19, James Kanze wrote:

On Dec 16, 9:50 pm, Maxim Yegorushkin<>

On 16/12/09 13:50, Michael Mol wrote:
  > Let's say you have a factory which returns handles to objects that
  > it allocates. You pass these handles to the factory whenever you
  > want to use the object or destroy it. The factory's
  > "OperateOnObject" class verifies that the handle is within its
  > object collection, and then marshals the OperateOnObject call to
  > the class's internal OperateOnObject method. All external
  > operations on the object instance are required to pass through the
  > Factory in order to guarantee that the object instance is still
  > valid at the time of call.

  > This leads to code roughly like this. (Not using exceptions here,
  > but rather return error codes.)

  > // For the sake of scope, consider this as a global singleton factory.
  > // For the sake of concerns over initialization and destruction,
  > assume that's dealt with in code not shown.
  > class SomeFactory
  > {
  > public:
  > OBJHANDLE CreateObject();
  > ERRCODE DestroyObject(OBJHANDLE);
  > ERRCODE OperateOnObject(OBJHANDLE objHandle, int someArgument);
  > protected:
  > OBJCOLLECTION objCollection;
  > } factoryObj;
  > // In some function, somewhere
  > OBJHANDLE objHandle = factoryObj.CreateObject();
  > factoryObj.OperateOnObject(objHandle, 42);
  > factoryObj.DestroyObject(objHandle);

In other words, you've got:

1) A factory that creates objects.
2) Those objects implement an interface, which is currently belongs to
factory class.
3) You'd also like for the factory to check whether the object reference
is valid.

You can refactor this to simply things.

1) Extract object interface from the factory.

    struct SomeObject {
        // former SomeFactory::OperateOnObject
        virtual ERRCODE Operate(int someArgument) = 0;
        virtual ~SomeObject() = 0;

Using such object now does not require a factory object, i.e. you can
call Operare() directly on the object.

2) Make factory return smart-pointers to SomeObject. The objects it
creates implement SomeObject interface.

    typedef boost::shared_ptr<SomeObject> SomeObjectPtr;

    class SomeFactory {
         SomeObjectPtr createSomeObject();

Now the factory function returns a smart-pointer. This smart-pointer
takes care of destroying the object when it is no longer used. No
manual object destruction required.

Except that boost::smart_ptr won't necessarily work here---it will
render destruction non-deterministic,

I would say that the destruction is perfectly deterministic -- when the
last copy of the shared_ptr<> referencing that particular object is
destroyed. (Although, in practice, I normally prefer factories to return
auto_ptr<> to emphasize the fact that the ownership of a newly created
object is being transferred).

 > and will cause objects to "leak" as soon as there are any cycles.

It is interesting that you mention non-deterministic destruction and
leaks due to cycles with regards to one subject. To me it looks like
leaks due to cycles are an inherent problem of precisely deterministic
destruction, when some form of reference counting is used. For example,
Perl and Python use deterministic destruction and in these languages
leaks are possible due to cycles. Java, on the other hand, offers
non-deterministic destruction which does not suffer from cycles (but
suffers from non-determinism, which to me is a bigger evil than cycles).

Of course, his solution won't work either, since without garbage
collection, there's absolutely no way to ensure that the invalid
remains invalid.

3) Using a smart-pointer makes the possibility of using an already
destroyed object highly unlikely.

True. But it does so by not destroying the objects when they should
destroyed, and possibly never. The cure is as bad as the disease.

Feeling a bit thick, could you possibly elaborate the last point?


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