Sunday, May 13, 2012

smartPtr - remember the 3 types involved

I feel it's easy to confuse the 3 data types involved in even the simplest smart pointer. It's less confusing when you instantiate the class template and slightly more confusing when you have to program in templates.

Say you use smartPtr<T>, there are really 3 data types involved here
1) T
2) pointer to T
3) smartPtr<T>

There are also at least 3 objects involved
1) a single pointee object, of type T
2) 1 or more 32-bit raw-pointer objects of type T*
3) Multiple instances of smartPtr<T>

At clean-up time, we need to
1) deallocate the pointee object, assuming it's on heap
2) don't worry about the raw pointer objects. Someone would take care of it. However, we assume no one calls delete() on one of them!
3) the smartPtr instances are typically low-maintenance. They are often stack-allocated or are (non-ref) fields of bigger objects -- non-ref meaning a field of type smartPtr not smartPtr* or smartPtr&. When such a smartPtr instance is cleaned up, we don't need to worry about anything.

Now we are ready to understand circular reference.
- A smartPtr<Car> instance myCar holding a raw pointer to a Car instance, which has a field which is a smartPtr<Driver> instance;
- this smartPtr instance holds raw pointer to a Driver instance.
- the driver instance has a field that is myCar.

All 6 objects are unreachable and unusable, but look at the 2 reference counts.

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