1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
//! `RefCell`s and code smells
//!
//!   - **Date:** July 23, 2016
//!   - **Subject:** Digging deeper on smart pointers and mutability with
//!     `Cell` and `RefCell`.
//!   - [**Audio**][mp3]
//!
//! [mp3]: https://www.podtrac.com/pts/redirect.mp3/cdn.newrustacean.com/file/newrustacean/e016.mp3
//!
//! <audio style="width: 100%" title="Borrow a Cell or Clone a Cow" controls preload=metadata><source src="https://www.podtrac.com/pts/redirect.mp3/cdn.newrustacean.com/file/newrustacean/e016.mp3"></audio>
//!
//!
//! Notes
//! -----
//!
//! What are the `Cell` and `RefCell` types, and when should we use them?
//!
//! Today, we follow up both the detailed discussion of smart pointers in
//! [e015] and the closely related discussion in [Interview 2] with Raph Levien,
//! and look at two types you need to have a good idea how to deal with if you
//! want to use these smart pointer types more *ergonomically*---that is, how to
//! use them without feeling like you're beating your head against a wall!
//!
//! [e015]: https://www.newrustacean.com/show_notes/e015/
//! [Interview 2]: https://www.newrustacean.com/show_notes/interview/_2/
//!
//! The descriptions of the code below are minimal; see the inline comments in
//! [the source][src] for the actual informative discussion.
//!
//! [src]: /src/show_notes/e016.rs.html
//!
//!
//! ### A comment on the code samples
//!
//! Note that in several cases below we use `&[]` to borrow a reference to a
//! slice, rather than requiring this to pass a reference to a `Vec`
//! specifically. Making the first argument be of type
//! `&Vec<RefCell<SimpleNonCopyable>>` would also work, but would be more
//! restrictive in what it could and couldn't accept. Since `Vec` implements
//! [`Deref`] to automatically convert to slices, this works just fine, *and* is
//! more general. This is how you should usually write function signatures which
//! operate on reference to vectors (and likewise for other types which can
//! dereference to slices). We'll talk about this more in a future episode!
//!
//! [`Deref`]: https://doc.rust-lang.org/1.10.0/std/ops/trait.Deref.html
//!
//!
//! Links
//! -----
//!
//!   - Rust 1.10
//!       + [blog post]
//!       + [full release notes]
//!       + cdylib
//!           * RFC: [text] | [discussion]
//!           * [implementation]
//!   - [rustup 0.3.0 release]
//!   - [Integer32] \(Carol Nichols' and Jake Goulding's new Rust consultancy)
//!       + [announcement blog post]
//!       + [reddit discussion][reddit i32]
//!   - [IntelliJ Rust]
//!       + [reddit discussion][reddit ijr]
//!   - Tango: [source][Tango] | [talk]
//!   - `Cell` and `RefCell`:
//!       + [Cell Types] in _The Rust Programming Language_
//!       + Standard library:
//!           * [`std::cell`] module docs (detailed explanation, complementary to the book)
//!           * [`Cell<T>`] docs (at `std::cell::Cell`)
//!           * [`RefCell<T>`] docs (at `stc::cell::RefCell`)
//!
//! [blog post]: https://blog.rust-lang.org/2016/07/07/Rust-1.10.html
//! [full release notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1100-2016-07-07
//! [text]: https://github.com/rust-lang/rfcs/blob/master/text/1510-rdylib.md
//! [discussion]: https://github.com/rust-lang/rfcs/pull/1510
//! [implementation]: https://github.com/rust-lang/rust/pull/33553
//! [rustup 0.3.0 release]: https://internals.rust-lang.org/t/beta-testing-rustup-rs/3316/144
//! [Integer32]: http://www.integer32.com
//! [announcement blog post]: http://www.integer32.com/2016/07/11/why-rust.html
//! [reddit i32]: https://www.reddit.com/r/rust/comments/4sdncw/why_were_starting_a_rust_consultancy/
//! [IntelliJ Rust]: https://intellij-rust.github.io
//! [reddit ijr]: https://www.reddit.com/r/rust/comments/4sbqaq/intellijrust_has_a_website_now/
//! [Tango]: https://github.com/pnkfelix/tango
//! [talk]: https://skillsmatter.com/skillscasts/8372-tango-literate-programming-in-rust
//! [Cell Types]: https://doc.rust-lang.org/book/choosing-your-guarantees.html#cell-types
//! [`std::cell`]: https://doc.rust-lang.org/std/cell/index.html
//! [`Cell<T>`]: https://doc.rust-lang.org/std/cell/struct.Cell.html
//! [`RefCell<T>`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html
//!
//!
//! Sponsors
//! --------
//!
//!   - Aleksey Pirogov
//!   - [Chris Palmer]
//!   - [Daniel Collin]
//!   - [Derek Morr]
//!   - Doug Reeves
//!   - Eric Fulmer
//!   - Hamza Sheikh
//!   - [Jakub "Limeth" Hlusička]
//!   - Keith Gray
//!   - Lachlan Collins
//!   - Leif Arne Storset
//!   - Luca Schmid
//!   - Micael Bergeron
//!   - Michael Clayton
//!   - [Pascal Hertleif]
//!   - Ralph Giles ("rillian")
//!   - Ralph "FriarTech" Loizzo
//!   - Raph Levien
//!   - reddraggone9
//!   - Ryan Ollos
//!   - Vesa Kaihlavirta
//!   - [William Roe]
//!
//! [Chris Palmer]: http://red-oxide.org/
//! [Daniel Collin]: twitter.com/daniel_collin
//! [Derek Morr]: https://twitter.com/derekmorr
//! [Jakub "Limeth" Hlusička]: https://github.com/Limeth
//! [Pascal Hertleif]: https://pascalhertleif.de/
//! [William Roe]: http://willroe.me
//!
//! (Thanks to the couple people donating who opted out of the reward tier, as
//! well. You know who you are!)
//!
//! ### Become a sponsor
//!
//!   - <a href="https://www.patreon.com/newrustacean" rel="payment">Patreon</a>
//!   - [Venmo](https://venmo.com/chriskrycho)
//!   - [Dwolla](https://www.dwolla.com/hub/chriskrycho)
//!   - [Cash.me](https://cash.me/$chriskrycho)
//!   - [Flattr](https://flattr.com/profile/chriskrycho)
//!   - [PayPal.me](https://paypal.me/chriskrycho)
//!
//!
//! Contact
//! -------
//!
//!   - New Rustacean:
//!     + Twitter: [@newrustacean](https://www.twitter.com/newrustacean)
//!     + Email: [hello@newrustacean.com](mailto:hello@newrustacean.com)
//!   - Chris Krycho
//!     + GitHub: [chriskrycho](https://github.com/chriskrycho)
//!     + Twitter: [@chriskrycho](https://www.twitter.com/chriskrycho)

use std::cell::{Cell, RefCell};

/// A container showing a type where `Cell<T>` works with `Vec<T>`.
#[derive(Copy, Clone)]
pub struct SimpleContainer {
    contents: i32,
}

/// Demonstrate how you need `Cell<T>` even just with a `Vec<T>`;
pub fn demonstrate_need_for_cell() -> Vec<Cell<SimpleContainer>> {
    let a_simple_container = SimpleContainer { contents: 42 };
    let another_one = SimpleContainer { contents: 84 };

    // The point is to illustrate vector behavior!
    #[allow(clippy::useless_vec)]
    let _a_vec = vec![a_simple_container, another_one];

    // So far so good, but say we want to change the contents of one of the
    // struct types here. Uncomment the following line; it will fail to compile.
    //
    // let retrieved = _a_vec.get_mut(1);
    //
    // If, however, we use a `Cell`, we can get at the mutable data. See the
    // test below!
    vec![Cell::new(a_simple_container), Cell::new(another_one)]
}

/// Operate mutably on the contenets of an immutable reference to a `Vec`.
pub fn double_cell(containers: &[Cell<SimpleContainer>]) {
    for container_cell in containers {
        let old = container_cell.get().contents;
        let new = old * 2;
        container_cell.set(SimpleContainer { contents: new });
    }
}

/// A container showing where `Cell<T>` doesn't work and `RefCell<T>` does.
pub struct SimpleNonCopyable {
    contents: String,
}

/// Demonstrate interior mutability with `Rc` and `RefCell`.
pub fn add_to_each_string(list_contained_strings: &[RefCell<SimpleNonCopyable>], to_push: &str) {
    for contained in list_contained_strings {
        let mut original = contained.borrow_mut();
        original.contents.push_str(to_push);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn need_for_cell() {
        // Here we have an immutable `Vec<Cell<SimpleContainer>>`
        let all_wrapped_up = demonstrate_need_for_cell();
        assert_eq!(all_wrapped_up.first().unwrap().get().contents, 42);
        // And yet its contents can be changed! The `Cell::set()` method lets us
        // update the *interior* of the item.
        all_wrapped_up
            .first()
            .unwrap()
            .set(SimpleContainer { contents: 12 });
        assert_eq!(all_wrapped_up.first().unwrap().get().contents, 12);
    }

    #[test]
    fn double_cell_works() {
        use std::cell::Cell;

        let some_containers = vec![
            Cell::new(SimpleContainer { contents: 3 }),
            Cell::new(SimpleContainer { contents: 5 }),
        ];

        double_cell(&some_containers);
        assert_eq!(some_containers.first().unwrap().get().contents, 6);
        assert_eq!(some_containers.get(1).unwrap().get().contents, 10);
    }

    #[test]
    fn add_to_each_string_works() {
        use std::cell::RefCell;

        let contained_strings = vec![
            RefCell::new(SimpleNonCopyable {
                contents: "Boom".to_string(),
            }),
            RefCell::new(SimpleNonCopyable {
                contents: "Bang".to_string(),
            }),
        ];

        add_to_each_string(&contained_strings, " razzle");

        assert_eq!(
            contained_strings.first().unwrap().borrow().contents,
            String::from("Boom razzle")
        );
        assert_eq!(
            contained_strings.get(1).unwrap().borrow().contents,
            String::from("Bang razzle")
        );
    }
}