Factories, handles, and handle wrappers

Michael Mol <mikemol@gmail.com>
Wed, 16 Dec 2009 07:50:22 CST
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
    OBJHANDLE CreateObject();
    ERRCODE DestroyObject(OBJHANDLE);
    ERRCODE OperateOnObject(OBJHANDLE objHandle, int someArgument);
    OBJCOLLECTION objCollection;
} factoryObj;

// In some function, somewhere
OBJHANDLE objHandle = factoryObj.CreateObject();
factoryObj.OperateOnObject(objHandle, 42);

Providing objHandle as the first argument of every call to a distinct
marshalling class instance proves tedious, so a means of overloading
OBJHANDLE is desired such that explicit references to factoryObj are
unnecessary. The resulting OBJHANDLE (a wrapper for the type formally
known as OBJHANDLE) would not likely behave specially at construction
or destruction, nor likely have a custom copy constructor.

The two approaches under consideration are:

OBJHANDLE objHandle = factoryObj.CreateObject();
// or

and an approach which is used as follows:

OBJHANDLE objHandle = factoryObj.CreateObject();

Both approaches would have the exact same effects on the state of
factoryObj and the members of OBJCOLLECTION, which would also be the
same as the old code where the old OBJHANDLE type was passed as the
first argument.

The question is, which approach is more appropriate and intuitive for C
++, and why? I ask because this is a matter of matter of debate
between another coworker and I, and the third coworker consulted had
no opinion.

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