Creating Weak<T> from &T?

1

I presume I am missing something obvious. In some methods on a struct, I want to create a weak reference to the structure. As far as I can tell, for good reasons, I need to get to an Rc, and then I can call Rc::downgrade(...) to get to a Weak. But when I try to figure out how to create an Rc, I end up seeing various Clone methods. I don't want a reference to a clone of the object. And &Self is not an Rc. What am I missing?
Thanks,
Joel

2

For an Rc type the data is on the heap, so you can get a weak reference from it. But if you have a struct value on the stack, that should not work. Well, at least that is what I would assume -- I have never used weak references in Rust yet.

3

In Rust objects are not reference counted, and there's no garbage collection. If something isn't in Rc already, then it won't support Rc's Weak. There's no magic to uncover a hidden Weak. There's no special way of upgrading temporary references. Rc is always in-your-face explicit and if it's not obviously there, then it's not.

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4

Thank you both. Sounds like I can't do what I want wiht a method on &self, but have to define methods that take Rc as a parameter, and probably a build method to build the Rc. I guess I am biased in my patterns because so much of what I work with assumes graphs / bi-directional trees, ...
Yours,
Joel

5

A reference is internally implemented in Rust as a simple pointer. It is basically only an address. A reference points directly to the referenced value.

But a Rc<T> and Weak<T is not simply a pointer or a reference, but a custom type (struct) containing a raw pointer to some other type containing additional management data, something similarly like this:

struct Rc<T> {
    pointer: *mut InternalRcNode<T>,
}

struct Weak<T> {
    pointer: *mut InternalRcNode<T>,
}

struct InternalRcNode<T> {
    strong_count: usize,
    weak_count: usize,
    value: MaybeUninit<T>,
}

(The real implementation differs)

So, obviously you can't create a Rc<T> or Weak<T> from a reference.

6

You can make fn method(self: Rc<Self>) when it requires use of reference counting.

&self is not merely a default, but also a type system promise that the method won't be able to keep self by any means.

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7

If I am reading that right, it looks like what I am going to need. If I define a method as you give above, fn method (self: Rc, ...).... as part of the implementation of a struct (say DataStore), then can I in other places where I have an instance I of Rc, can I call I.method(....)? Or do I need to call DataStore::method(I, ...)?
Thank you,
Joel

8

Thanks for the further elaboration. I should have realized it from what I had read, but simply didn't put it together.
Yours,
Joel

9

Yes, you can do that whenever there is a self parameter. That is what makes a function callable using method call (dot) syntax.

If a function does not have a self parameter but is in an impl, then it is not a method, but only an associated function, and you can only call it using Type::function() syntax. (All methods are associated functions, but not all associated functions are methods.)

10

If your object is already in an Rc, then Rc::clone will not create a clone of the internal object; only new refcounted handle.

1 Like
11

Thanks. Still working through getting all this consistent, just wanted to acknoweldge that this is helping a great deal.
Yours,
Joel