## Union 结构

```c++
union quantity {
  short count;
  float weight;
  float volume;
};

// 写法一
union quantity q;
q.count = 4;

// 写法二
union quantity q = {.count=4};

// 写法三
// 不指定属性名，就会赋值给第一个属性
union quantity q = {4};

printf("count is %i\n", q.count); // count is 4
printf("weight is %f\n", q.weight); // 未定义行为

q.weight = 0.5;
printf("weight is %f\n", q.weight); // weight is 0.5
```


c.md

标量（scalar）类型表示单个值。
Rust 有 4 个基本的标量类型：整型、浮点型、布尔型和字符。

```rust
let guess: u32 = "42".parse().expect("Not a number!");

let x = 2.0; // f64
let y: f32 = 3.0; // f32

// addition
let sum = 5 + 10;

// subtraction
let difference = 95.5 - 4.3;

// multiplication
let product = 4 * 30;

// division
let quotient = 56.7 / 32.2;
let floored = 2 / 3; // Results in 0

// remainder
let remainder = 43 % 5;

let t = true;
let f: bool = false; // with explicit type annotation

let c = 'z';
let z = 'ℤ';
let heart_eyed_cat = '😻';
```

复合类型（compound type）可以将多个值组合成一个类型。
Rust 有两种基本的复合类型：元组（tuple）和数组（array）。

## 类型别名用来创建类型同义词

类型别名的主要用途是减少重复。

```rust
// Kilometers 是 i32 的 同义词（synonym）
let x: i32 = 5;
let y: Kilometers = 5;
println!("x + y = {}", x + y);

// 引入类型别名 Thunk 来减少重复
type Thunk = Box<dyn Fn() + Send + 'static>;
let f: Thunk = Box::new(|| println!("hi"));
fn takes_long_type(f: Thunk) {
    // --snip--
}
fn returns_long_type() -> Thunk {
    // --snip--
}
```

## 从不返回的 never type

- `!` 在函数从不返回的时候充当返回值。
- 从不返回的函数被称为 发散函数（diverging functions）

```rust
fn bar() -> ! {
    // --snip--
}

let guess: u32 = match guess.trim().parse() {
    Ok(num) => num,
    Err(_) => continue, // continue 的值是 !
};

impl<T> Option<T> {
    pub fn unwrap(self) -> T {
        match self {
            Some(val) => val,
            // panic! 是 ! 类型
            None => panic!("called `Option::unwrap()` on a `None` value"),
        }
    }
}

// 此表达式的值是 !
loop {
    print!("and ever ");
}
```

也就是说，对于如下泛型函数定义：

```rust
fn generic<T>(t: T) {
    // --snip--
}
// 实际上被当作如下处理：
fn generic<T: Sized>(t: T) {
    // --snip--
}
```


rust.md


## 内置类型列表

| name         | dart             | go         | js                   | python           |
| ------------ | ---------------- | ---------- | -------------------- | ---------------- |
| numbers      | num              |            | Number               | Numbers          |
| numbers      | int              | int        |                      | -int             |
| numbers      |                  | int8       |                      |                  |
| numbers      |                  | int16      |                      |                  |
| numbers      |                  | int32/rune |                      |                  |
| numbers      |                  | int64      |                      |                  |
| numbers      |                  | uint       |                      |                  |
| numbers      |                  | uint8/byte |                      |                  |
| numbers      |                  | uint16     |                      |                  |
| numbers      |                  | uint32     |                      |                  |
| numbers      |                  | uint64     |                      |                  |
| numbers      |                  | uintptr    |                      |                  |
| numbers      | double           | float32    |                      | -float           |
| numbers      |                  | float64    |                      |                  |
| numbers      |                  | complex64  |                      |                  |
| numbers      |                  | complex128 |                      |                  |
| complex      |                  |            |                      | complex          |
| strings      | String           | string     | String               | str              |
| booleans     | bool(true/false) | bool       | Boolean(true/false)  | bool(True/False) |
| lists/arrays | List             |            | -Array               | list             |
| maps         | map              |            | -Map                 | dict             |
| runes        | runes            |            |                      |                  |
| symbols      | symbol           |            | Symbol               |                  |
| Null         | -null            |            | Null(null)           |                  |
| Undefined    |                  |            | Undefined(undefined) |                  |
| Object       | Object           |            | Object               |                  |
| Set          | Set              |            | -Set                 | set              |
| ArrayBuffer  |                  |            | -ArrayBuffer         | bytearray        |
| Tuples       |                  |            |                      | tuple            |
| range        |                  |            |                      | range            |
| Function     | -Function        |            |                      |                  |
| dynamic      | dynamic          |            |                      |                  |
| error        |                  | error      |                      |                  |


数据类型

内置类型列表

了解更多

name	dart	go	js	python
numbers	num		Number	Numbers
numbers	int	int		-int
numbers		int8
numbers		int16
numbers		int32/rune
numbers		int64
numbers		uint
numbers		uint8/byte
numbers		uint16
numbers		uint32
numbers		uint64
numbers		uintptr
numbers	double	float32		-float
numbers		float64
numbers		complex64
numbers		complex128
complex				complex
strings	String	string	String	str
booleans	bool(true/false)	bool	Boolean(true/false)	bool(True/False)
lists/arrays	List		-Array	list
maps	map		-Map	dict
runes	runes
symbols	symbol		Symbol
Null	-null		Null(null)
Undefined			Undefined(undefined)
Object	Object		Object
Set	Set		-Set	set
ArrayBuffer			-ArrayBuffer	bytearray
Tuples				tuple
range				range
Function	-Function
dynamic	dynamic
error		error