Previously, `RefCountedPtr<>` and `WeakRefCountedPtr<>` incorrectly allowed
implicit casting of any type to any other type. This hadn't caused a
problem until recently, but now that it has, we need to fix it. I have
fixed this by changing these smart pointer types to allow type
conversions only when the type used is convertible to the type of the
smart pointer. This means that if `Subclass` inherits from `Base`, then
we can set a `RefCountedPtr<BaseClass>` to a value of type
`RefCountedPtr<Subclass>`, but we cannot do the reverse.
We had been (ab)using this bug to make it more convenient to deal with
down-casting in subclasses of ref-counted types. For example, because
`Resolver` inherits from `InternallyRefCounted<Resolver>`, calling
`Ref()` on a subclass of `Resolver` will return `RefCountedPtr<Resolver>`
rather than returning the subclass's type. The ability to implicitly
convert to the subclass type made this a bit easier to deal with. Now
that that ability is gone, we need a different way of dealing with that
problem.
I considered several ways of dealing with this, but none of them are
quite as ergonomic as I would ideally like. For now, I've settled on
requiring callers to explicitly down-cast as needed, although I have
provided some utility functions to make this slightly easier:
- `RefCounted<>`, `InternallyRefCounted<>`, and `DualRefCounted<>` all
provide a templated `RefAsSubclass<>()` method that will return a new
ref as a subclass. The type used with `RefAsSubclass()` must be a
subclass of the type passed to `RefCounted<>`, `InternallyRefCounted<>`,
or `DualRefCounted<>`.
- In addition, `DualRefCounted<>` provides a templated `WeakRefAsSubclass<T>()`
method. This is the same as `RefAsSubclass()`, except that it returns
a weak ref instead of a strong ref.
- In `RefCountedPtr<>`, I have added a new `Ref()` method that takes
debug tracing parameters. This can be used instead of calling `Ref()`
on the underlying object in cases where the caller already has a
`RefCountedPtr<>` and is calling `Ref()` only to specify the debug
tracing parameters. Using this method on `RefCountedPtr<>` is more
ergonomic, because the smart pointer is already using the right
subclass, so no down-casting is needed.
- In `WeakRefCountedPtr<>`, I have added a new `WeakRef()` method that
takes debug tracing parameters. This is the same as the new `Ref()`
method on `RefCountedPtr<>`.
- In both `RefCountedPtr<>` and `WeakRefCountedPtr<>`, I have added a
templated `TakeAsSubclass<>()` method that takes the ref out of the
smart pointer and returns a new smart pointer of the down-casted type.
Just as with the `RefAsSubclass()` method above, the type used with
`TakeAsSubclass()` must be a subclass of the type passed to
`RefCountedPtr<>` or `WeakRefCountedPtr<>`.
Note that I have *not* provided an `AsSubclass<>()` variant of the
`RefIfNonZero()` methods. Those methods are used relatively rarely, so
it's not as important for them to be quite so ergonomic. Callers of
these methods that need to down-cast can use
`RefIfNonZero().TakeAsSubclass<>()`.
PiperOrigin-RevId: 592327447
Mark D. Roth