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
|
% Operators and Overloading
Rust allows for a limited form of operator overloading. There are certain
operators that are able to be overloaded. To support a particular operator
between types, there’s a specific trait that you can implement, which then
overloads the operator.
For example, the `+` operator can be overloaded with the `Add` trait:
```rust
use std::ops::Add;
#[derive(Debug)]
struct Point {
x: i32,
y: i32,
}
impl Add for Point {
type Output = Point;
fn add(self, other: Point) -> Point {
Point { x: self.x + other.x, y: self.y + other.y }
}
}
fn main() {
let p1 = Point { x: 1, y: 0 };
let p2 = Point { x: 2, y: 3 };
let p3 = p1 + p2;
println!("{:?}", p3);
}
```
In `main`, we can use `+` on our two `Point`s, since we’ve implemented
`Add<Output=Point>` for `Point`.
There are a number of operators that can be overloaded this way, and all of
their associated traits live in the [`std::ops`][stdops] module. Check out its
documentation for the full list.
[stdops]: ../std/ops/index.html
Implementing these traits follows a pattern. Let’s look at [`Add`][add] in more
detail:
```rust
# mod foo {
pub trait Add<RHS = Self> {
type Output;
fn add(self, rhs: RHS) -> Self::Output;
}
# }
```
[add]: ../std/ops/trait.Add.html
There’s three types in total involved here: the type you `impl Add` for, `RHS`,
which defaults to `Self`, and `Output`. For an expression `let z = x + y`, `x`
is the `Self` type, `y` is the RHS, and `z` is the `Self::Output` type.
```rust
# struct Point;
# use std::ops::Add;
impl Add<i32> for Point {
type Output = f64;
fn add(self, rhs: i32) -> f64 {
// add an i32 to a Point and get an f64
# 1.0
}
}
```
will let you do this:
```rust,ignore
let p: Point = // ...
let x: f64 = p + 2i32;
```
# Using operator traits in generic structs
Now that we know how operator traits are defined, we can define our `HasArea`
trait and `Square` struct from the [traits chapter][traits] more generically:
[traits]: traits.html
```rust
use std::ops::Mul;
trait HasArea<T> {
fn area(&self) -> T;
}
struct Square<T> {
x: T,
y: T,
side: T,
}
impl<T> HasArea<T> for Square<T>
where T: Mul<Output=T> + Copy {
fn area(&self) -> T {
self.side * self.side
}
}
fn main() {
let s = Square {
x: 0.0f64,
y: 0.0f64,
side: 12.0f64,
};
println!("Area of s: {}", s.area());
}
```
For `HasArea` and `Square`, we just declare a type parameter `T` and replace
`f64` with it. The `impl` needs more involved modifications:
```ignore
impl<T> HasArea<T> for Square<T>
where T: Mul<Output=T> + Copy { ... }
```
The `area` method requires that we can multiply the sides, so we declare that
type `T` must implement `std::ops::Mul`. Like `Add`, mentioned above, `Mul`
itself takes an `Output` parameter: since we know that numbers don't change
type when multiplied, we also set it to `T`. `T` must also support copying, so
Rust doesn't try to move `self.side` into the return value.
|
