ch15-04-rc.md

## `Rc<T>`, the Reference Counted Smart Pointer

In the majority of cases, ownership is clear: you know exactly which variable
owns a given value. However, there are cases when a single value might have
multiple owners. For example, in graph data structures, multiple edges might
point to the same node, and that node is conceptually owned by all of the edges
that point to it. A node shouldn’t be cleaned up unless it doesn’t have any
edges pointing to it and so has no owners.

You have to enable multiple ownership explicitly by using the Rust type
`Rc<T>`, which is an abbreviation for *reference counting*. The `Rc<T>` type
keeps track of the number of references to a value to determine whether or not
the value is still in use. If there are zero references to a value, the value
can be cleaned up without any references becoming invalid.

Imagine `Rc<T>` as a TV in a family room. When one person enters to watch TV,
they turn it on. Others can come into the room and watch the TV. When the last
person leaves the room, they turn off the TV because it’s no longer being used.
If someone turns off the TV while others are still watching it, there would be
uproar from the remaining TV watchers!

We use the `Rc<T>` type when we want to allocate some data on the heap for
multiple parts of our program to read and we can’t determine at compile time
which part will finish using the data last. If we knew which part would finish
last, we could just make that part the data’s owner, and the normal ownership
rules enforced at compile time would take effect.

Note that `Rc<T>` is only for use in single-threaded scenarios. When we discuss
concurrency in Chapter 16, we’ll cover how to do reference counting in
multithreaded programs.

### Using `Rc<T>` to Share Data

Let’s return to our cons list example in Listing 15-5. Recall that we defined
it using `Box<T>`. This time, we’ll create two lists that both share ownership
of a third list. Conceptually, this looks similar to Figure 15-3:

<img alt="Two lists that share ownership of a third list" src="img/trpl15-03.svg" class="center" />

<span class="caption">Figure 15-3: Two lists, `b` and `c`, sharing ownership of
a third list, `a`</span>

We’ll create list `a` that contains 5 and then 10. Then we’ll make two more
lists: `b` that starts with 3 and `c` that starts with 4. Both `b` and `c`
lists will then continue on to the first `a` list containing 5 and 10. In other
words, both lists will share the first list containing 5 and 10.

Trying to implement this scenario using our definition of `List` with `Box<T>`
won’t work, as shown in Listing 15-17:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-17/src/main.rs}}
```

<span class="caption">Listing 15-17: Demonstrating we’re not allowed to have
two lists using `Box<T>` that try to share ownership of a third list</span>

When we compile this code, we get this error:

```console
{{#include ../listings/ch15-smart-pointers/listing-15-17/output.txt}}
```

The `Cons` variants own the data they hold, so when we create the `b` list, `a`
is moved into `b` and `b` owns `a`. Then, when we try to use `a` again when
creating `c`, we’re not allowed to because `a` has been moved.

We could change the definition of `Cons` to hold references instead, but then
we would have to specify lifetime parameters. By specifying lifetime
parameters, we would be specifying that every element in the list will live at
least as long as the entire list. This is the case for the elements and lists
in Listing 15-17, but not in every scenario.

Instead, we’ll change our definition of `List` to use `Rc<T>` in place of
`Box<T>`, as shown in Listing 15-18. Each `Cons` variant will now hold a value
and an `Rc<T>` pointing to a `List`. When we create `b`, instead of taking
ownership of `a`, we’ll clone the `Rc<List>` that `a` is holding, thereby
increasing the number of references from one to two and letting `a` and `b`
share ownership of the data in that `Rc<List>`. We’ll also clone `a` when
creating `c`, increasing the number of references from two to three. Every time
we call `Rc::clone`, the reference count to the data within the `Rc<List>` will
increase, and the data won’t be cleaned up unless there are zero references to
it.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-18/src/main.rs}}
```

<span class="caption">Listing 15-18: A definition of `List` that uses
`Rc<T>`</span>

We need to add a `use` statement to bring `Rc<T>` into scope because it’s not
in the prelude. In `main`, we create the list holding 5 and 10 and store it in
a new `Rc<List>` in `a`. Then when we create `b` and `c`, we call the
`Rc::clone` function and pass a reference to the `Rc<List>` in `a` as an
argument.

We could have called `a.clone()` rather than `Rc::clone(&a)`, but Rust’s
convention is to use `Rc::clone` in this case. The implementation of
`Rc::clone` doesn’t make a deep copy of all the data like most types’
implementations of `clone` do. The call to `Rc::clone` only increments the
reference count, which doesn’t take much time. Deep copies of data can take a
lot of time. By using `Rc::clone` for reference counting, we can visually
distinguish between the deep-copy kinds of clones and the kinds of clones that
increase the reference count. When looking for performance problems in the
code, we only need to consider the deep-copy clones and can disregard calls to
`Rc::clone`.

### Cloning an `Rc<T>` Increases the Reference Count

Let’s change our working example in Listing 15-18 so we can see the reference
counts changing as we create and drop references to the `Rc<List>` in `a`.

In Listing 15-19, we’ll change `main` so it has an inner scope around list `c`;
then we can see how the reference count changes when `c` goes out of scope.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch15-smart-pointers/listing-15-19/src/main.rs:here}}
```

<span class="caption">Listing 15-19: Printing the reference count</span>

At each point in the program where the reference count changes, we print the
reference count, which we get by calling the `Rc::strong_count` function. This
function is named `strong_count` rather than `count` because the `Rc<T>` type
also has a `weak_count`; we’ll see what `weak_count` is used for in the
[“Preventing Reference Cycles: Turning an `Rc<T>` into a
`Weak<T>`”][preventing-ref-cycles]<!-- ignore --> section.

This code prints the following:

```console
{{#include ../listings/ch15-smart-pointers/listing-15-19/output.txt}}
```

We can see that the `Rc<List>` in `a` has an initial reference count of 1; then
each time we call `clone`, the count goes up by 1. When `c` goes out of scope,
the count goes down by 1. We don’t have to call a function to decrease the
reference count like we have to call `Rc::clone` to increase the reference
count: the implementation of the `Drop` trait decreases the reference count
automatically when an `Rc<T>` value goes out of scope.

What we can’t see in this example is that when `b` and then `a` go out of scope
at the end of `main`, the count is then 0, and the `Rc<List>` is cleaned up
completely. Using `Rc<T>` allows a single value to have multiple owners, and
the count ensures that the value remains valid as long as any of the owners
still exist.

Via immutable references, `Rc<T>` allows you to share data between multiple
parts of your program for reading only. If `Rc<T>` allowed you to have multiple
mutable references too, you might violate one of the borrowing rules discussed
in Chapter 4: multiple mutable borrows to the same place can cause data races
and inconsistencies. But being able to mutate data is very useful! In the next
section, we’ll discuss the interior mutability pattern and the `RefCell<T>`
type that you can use in conjunction with an `Rc<T>` to work with this
immutability restriction.

[preventing-ref-cycles]: ch15-06-reference-cycles.html#preventing-reference-cycles-turning-an-rct-into-a-weakt