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2016_11_17_ownership_borrowing.html
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<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">
<title>Rust Introduction</title>
<meta name="author" content="Sascha Grunert">
<link rel="stylesheet" href="css/reveal.css">
<link rel="stylesheet" href="css/theme/solarized.css">
<link rel="stylesheet" href="lib/css/zenburn.css">
<script>
var link = document.createElement( 'link' );
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = window.location.search.match( /print-pdf/gi ) ? 'css/print/pdf.css' : 'css/print/paper.css';
document.getElementsByTagName( 'head' )[0].appendChild( link );
</script>
</head>
<body>
<div class="reveal">
<div class="slides">
<section data-markdown>
<script type="text/template">
# Rust
## Ownership and Borrowing
<small>Second user group Meetup in Leipzig (2016-11-17)</small>
</script>
</section>
<section>
<section data-markdown>
<script type="text/template">
# Ownership
</script>
</section>
<section data-markdown>
<script type="text/template">
## Essentials for a safe language:
- You decide the lifetime of each value in your program
- Rust frees memory and other resources belonging to a value promptly
- Your program will never use a pointer to an object after it has been freed
- Rust restricts your program how to use pointers at compile time
</script>
</section>
<section data-markdown>
<script type="text/template">
## Ownership for variables
- Every value has a clear owner
- If the "owner" gets "dropped", the value is also gone
```rust
fn my_print() {
let mut vector = vec![1, 1, 1]; // allocated memory
for i in 1..10 {
let next = vector[i-1] + vector[i+1];
vector.push(next);
}
println!("{:?}", vector);
} // value dropped here
```
`Vector<T>` is mainly a fat pointer (pointer, capacity,
length) on the stack pointing to the data on the heap.
</script>
</section>
<section data-markdown>
<script type="text/template">
## Ownership for other types
- structs own their members
- tuples, arrays, vectors own their elements
- every value has a single owner
- single values may own other values
```rust
struct Person {
name: String,
age: u8,
}
let mut people = Vec::new();
people.push(
Person {
name: "Tom".to_string(),
age: 28,
}
);
people.push(
Person {
name: "Max".to_string(),
age: 33,
}
);
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Ownership restrictions
The ownership system will result in "trees", not flexible "graphs".
- borrowing possible (via referenes, non owning pointers)
- multiple owners possible within certain data types (Arc, Rc)
- move values from one owner to another (rearrange the tree)
</script>
</section>
</section>
<section>
<section data-markdown>
<script type="text/template">
# Moves
</script>
</section>
<section data-markdown>
<script type="text/template">
## Basics
- most operations move a value instead of copying
- results in low memory and processing time
- old owners after move uninitialized
```rust
let v = vec!["a".to_string(), "b".to_string(), "c".to_string()];
let a = v;
let b = v; // error: use of moved value
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Drop
```rust
let mut s = "Test".to_string();
s = "Example".to_string(); // value "Test" dropped here
```
```rust
let mut s = "Test".to_string();
let t = s;
s = "Example".to_string(); // nothing dropped here
```
<br>
Drop trait gives flexibility:
```rust
struct HasDrop;
impl Drop for HasDrop {
fn drop(&mut self) {
println!("Dropping!");
}
}
fn main() {
let _x = HasDrop;
} // "Dropping!"
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Move example
```rust
struct Person {
name: String,
age: u8,
}
let mut people = Vec::new(); // moves ownership to "people"
people.push(
Person {
name: "Tom".to_string(), // moves ownership, returns a String
age: 28,
} // Structure takes ownership of the String and u8
);
people.push(
Person {
name: "Max".to_string(),
age: 33,
}
); // Vector takes ownership of the person and its owners
```
- machine code optimized to be stored "where it belongs"
- moves just copy three words (a fat pointer)
</script>
</section>
<section data-markdown>
<script type="text/template">
## Control flow
```rust
let x = vec![1, 2, 3];
if condition {
my_func(x); // okay, to move from x here
} else {
another_func(x); // okay to also move from x here
}
test_func(x); // bad: x is uninitialized here
```
```rust
let x = vec![1, 2, 3];
loop {
my_func(x); // error: x is uninitialized after the first iteration
// x = get_a_vector(); // Solution: get a fresh value
}
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Indexed content
```rust
// Build a vector of the strings '1' to '5'
let mut v = Vec::new();
for i in 1..6 {
v.push(i.to_string());
}
let third = v[2]; // error: cannot move out of indexed content
let fifth = v.pop().unwrap(); // Works if unwrap(); succeeds
// Removes the 2nd element and return it,
// replacing it with the last element.
let third = v.swap_remove(2);
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Copy Types
- certain types implement the Copy (marker) trait
- integer and floating-point numeric types
- char and bool types
- tuple or fixed-size array of Copy types is itself a Copy type
- self defined types can also be copy:
```rust
#[derive(Copy, Clone)]
struct Label { number: u32 }
// Works
```
```rust
#[derive(Copy, Clone)]
struct Label { name: String }
// Does not work since String does not implement Copy
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Shared Ownership
- most values have unique owners in Rust
- Arc/Rc allows to live values as long as everybody is done using it (smart pointer)
- Arc: Atomic Refernce Count (can be shared safely between threads)
- Rc: Reference Count (faster, not thread safe)
```rust
use std::rc::Rc;
let s = Rc::new("Test".to_string());
// Just creates another pointer to the data in the heap:
let t = s.clone(); // increment reference count
let u = s.clone(); // increment reference count
```
Rust drops the String if the last instance is dropped
</script>
</section>
</section>
<section>
<section data-markdown>
<script type="text/template">
# Borrowing
</script>
</section>
<section data-markdown>
<script type="text/template">
## Basics
- "References" are non owning pointer types
- must never outlive their referents
- are never `NULL`
- Creating new references is called "Borrowing"
- two reference types available:
- _Shared Reference_ (Copy): multiple reader
```rust
&T // shared, like a pointer to const data
```
- _Mutable Reference_ (not Copy): single writer
```rust
&mut T // mutable, like a pointer
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Consuming data
```rust
use std::collections::HashMap;
// Create a simple HashMap
let mut map :HashMap<String, u8> = HashMap::new();
map.insert("Foo".to_owned(), 5);
map.insert("Bar".to_owned(), 15);
// Iterate over the map
for (name, number) in map {
println!("{} {}", name, number);
}
// does not work any more
// the hashmap is already consumed
let foo = map.get("Foo");
// ^^^ Value used after move
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Borrowing data
```rust
use std::collections::HashMap;
// Create a simple HashMap
let mut map :HashMap<String, u8> = HashMap::new();
map.insert("Foo".to_owned(), 5);
map.insert("Bar".to_owned(), 15);
// Iterate over the map
for (name, number) in &map {
println!("{} {}", name, number);
}
// Works now
let foo = map.get("Foo");
println!("{:?}", foo); // prints "Some(5)"
```
```rust
// Standard vector operations:
fn sort(&mut self); // Takes a mutable reference, call vector.sort();
fn get(&self, index: usize) -> Option<T>; // Takes just a reference
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Implicit dereferencing
References are close to `C/C++` pointers:
```rust
let x = 10;
let y = &x;
assert!(*y == 10);
```
```rust
let mut y = 10;
let x = &mut y;
*x *= 10;
assert!(*x == 100);
```
Dot `.` operator follows references automatically:
```rust
let point = (100, 200);
let r = &point;
assert_eq!(r.0, 100); // no (*r).0 needed
```
```rust
struct Point { x: i32, y: i32 }
let p = Point { x: 100, y: 200 };
let r1 :&Point = &p;
let r2 :&&Point = &r1;
let r3 :&&&Point = &r2;
assert_eq!(r3.y, 200);
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Fat references
Some references are fat pointers:
- references to Vec or slice of an array: `&[T]`
- references to String: `&str`
References to expressions are possible as well:
```rust
fn some_function() -> usize { 10usize }
let r = &some_function();
assert_eq!(r + &10, 20);
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Scope of borrowing
```rust
let y;
{
let x = 1;
y = &x;
}
assert_eq!(*y, 1); // error: x does not live long enough
```
- Rust assigns a lifetime to every reference
- Lifetime will be checked at compilation
Rust tries to infer the correct lifetime for you.
</script>
</section>
</section>
<section>
<section data-markdown>
<script type="text/template">
# Lifetimes
</script>
</section>
<section data-markdown>
<script type="text/template">
## Basics
- start with a `'` (e.g. 'a or 'static)
- 'static is the largest possible lifetime (must be initialized)
<br>
Functions can take lifetime parameters to specify lifetimes:
```rust
fn f<'a>(p: &'a i32) { ... }
```
<br>
Example, does this work?
```rust
fn f(p: &'static i32) { ... }
let x = 10;
f(&x);
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## References in function arguments
```rust
// Usually Rust infers this
fn smallest<'a>(v: &'a [i32]) -> &'a i32 {
let mut s = &v[0];
for r in &v[1..] {
if *r < *s { s = r; }
}
s
}
```
```rust
let s;
{
let items = [5, 3, 2, 0, 1, 6, 9];
s = smallest(&items);
}
assert_eq!(*s, 0); // error: items does not live long enough
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## References in structures
- references in structures needs explicit lifetime parameters
```rust
struct Items<'a> {
count: &'a i32
}
```
```rust
let storage;
{
let x = 5;
storage = Items { count: &x };
}
assert_eq!(*storage.count, 5); // error: x does not live long enough
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Distinct lifetimes #1
```rust
struct Point<'a> {
x: &'a i32,
y: &'a i32,
}
```
```rust
let x = 5;
let r;
{
let y = 10;
{
let point = Point { x: &x, y: &y };
r = point.x;
}
}
```
Will it work?
</script>
</section>
<section data-markdown>
<script type="text/template">
## Distinct lifetimes #2
The constraints are impossible to satisfy: no lifetime
is shorter than y's scope, but longer than r's.
Solution:
```rust
struct Point<'a, 'b> {
x: &'a i32,
y: &'b i32
}
```
```rust
let x = 5;
let r;
{
let y = 10;
{
let point = Point { x: &x, y: &y };
r = point.x;
}
}
```
</script>
</section>
<section data-markdown>
<script type="text/template">
## Share or Mutate?
Shared references makes the variable read only:
```rust
let v = vec![1, 2, 3];
let r = &v;
let move = v; // moves
println!("{}", r[0]);
// error: cannot move out of `v` because it is borrowed
```
Solution:
```rust
let v = vec![1, 2, 3];
{
let r = &v;
println!("{}", r[0]); // Works now
}
let move = v;
```
This makes iterator invalidation impossible!
</script>
</section>
<section data-markdown>
<script type="text/template">
Remember the Ownership tree:
<br>
## Shared access is read only access
_Multiple readers are possible_
<br>
## Mutable access is exclusive
_No other usable path to this referent possible_
</script>
</section>
<section data-markdown>
<script type="text/template">
## Examples
```rust
let mut x = 10;
let r1 = &x;
let r2 = &x; // multiple shared borrows
x += 10; // error: x already borrowed
let m = &mut x; // error: cannot borrow `x` as mutable because it is
// also borrowed as immutable
```
```rust
let mut y = 20;
let m1 = &mut y;
let m2 = &mut y; // error: cannot borrow as mutable more than once
let z = y; // error: cannot use `y` because it was mutably borrowed
```
```rust
let mut w = (1, 2);
let r = &w;
let r0 = &r.0; // okay: re borrowing shared as shared
let m1 = &mut r.1; // error: can't re borrow shared as mutable
```
```rust
let mut v = (1, 2);
let m = &mut v;
let m0 = &mut m.0; // okay: re borrowing mutable from mutable
*m0 = 42;
let r1 = &m.1; // okay: re borrowing shared from mutable,
// and doesn't overlap with m0
v.1; // error: access through other paths still forbidden
```
</script>
</section>
</section>
<section data-markdown>
<script type="text/template">
# Thank you!
</script>
</section>
</div>
</div>
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