3 用户自定义类型
Rust自定义类型主要通过下面两个关键进行定义：
- struct：定义一个结构
- enum：定义一个枚举
常量能以通过const和static关键字创建。
3.1 结构
有三种类型的结构（“structs”），可以使用struct关键字来创建：
- 元组结构体，又名元组
- 传统C结构体
- 元结构体，无field，为了做成通用类型
// A unit struct
struct Nil;
// A tuple struct
struct Pair(i32, f64);
// A struct with two fields
struct Point {
x: f64,
y: f64,
}
// Structs can be reused as fields of another struct
#[allow(dead_code)]
struct Rectangle {
p1: Point,
p2: Point,
}
fn main() {
// Instantiate a `Point`
let point: Point = Point { x: 0.3, y: 0.4 };
// Access the fields of the point
println!("point coordinates: ({}, {})", point.x, point.y);
// Destructure the point using a `let` binding
let Point { x: my_x, y: my_y } = point;
let _rectangle = Rectangle {
// struct instantiation is an expression too
p1: Point { x: my_y, y: my_x },
p2: point,
};
// Instantiate a unit struct
let _nil = Nil;
// Instantiate a tuple struct
let pair = Pair(1, 0.1);
// Destructure a tuple struct
let Pair(integer, decimal) = pair;
println!("pair contains {:?} and {:?}", integer, decimal);
}
程序执行结果：
point coordinates: (0.3, 0.4) pair contains 1 and 0.1
3.2 枚举
enum关键字允许创建一个可能有许多变体的变量。每一个对结构体来说是合法的变体，对枚举同样是合法的。
// An attribute to hide warnings for unused code.
#![allow(dead_code)]
// Create an `enum` to classify someone. Note how both names
// and type information together specify the variant:
// `Skinny != Fat` and `Height(i32) != Weight(i32)`. Each
// is different and independent.
enum Person {
// An `enum` may either be `unit-like`,
Skinny,
Fat,
// like tuple structs,
Height(i32),
Weight(i32),
// or like structures.
Info { name: String, height: i32 }
}
// A function which takes a `Person` enum as an argument and
// returns nothing.
fn inspect(p: Person) {
// Usage of an `enum` must cover all cases (irrefutable)
// so a `match` is used to branch over it.
match p {
Person::Skinny => println!("Is skinny!"),
Person::Fat => println!("Is fat!"),
// Destructure `i` from inside the `enum`.
Person::Height(i) => println!("Has a height of {}.", i),
Person::Weight(i) => println!("Has a weight of {}.", i),
// Destructure `Info` into `name` and `height`.
Person::Info { name, height } => {
println!("{} is {} tall!", name, height);
},
}
}
fn main() {
let person = Person::Height(18);
let danny = Person::Weight(10);
// `to_owned()` creates an owned `String` from a string slice.
let dave = Person::Info { name: "Dave".to_owned(), height: 72 };
let john = Person::Fat;
let larry = Person::Skinny;
inspect(person);
inspect(danny);
inspect(dave);
inspect(john);
inspect(larry);
}
程序运行结果：
Has a height of 18. Has a weight of 10. Dave is 72 tall! Is fat! Is skinny!
3.2.1 use
可以使用use声明，所以不用手动指定作用域：
// An attribute to hide warnings for unused code.
#![allow(dead_code)]
enum Status {
Rich,
Poor,
}
enum Work {
Civilian,
Soldier,
}
fn main() {
// Explicitly `use` each name so they are available without
// manual scoping.
use Status::{Poor, Rich};
// Automatically `use` each name inside `Work`.
use Work::*;
// Equivalent to `Status::Poor`.
let status = Poor;
// Equivalent to `Work::Civilian`.
let work = Civilian;
match status {
// Note the lack of scoping because of the explicit `use` above.
Rich => println!("The rich have lots of money!"),
Poor => println!("The poor have no money..."),
}
match work {
// Note again the lack of scoping.
Civilian => println!("Civilians work!"),
Soldier => println!("Soldiers fight!"),
}
}
程序执行结果：
The poor have no money... Civilians work!
3.2.2 C-like
enum也能像C预言的枚举那样使用。
// An attribute to hide warnings for unused code.
#![allow(dead_code)]
// enum with implicit discriminator (starts at 0)
enum Number {
Zero,
One,
Two,
}
// enum with explicit discriminator
enum Color {
Red = 0xff0000,
Green = 0x00ff00,
Blue = 0x0000ff,
}
fn main() {
// `enums` can be cast as integers.
println!("zero is {}", Number::Zero as i32);
println!("one is {}", Number::One as i32);
println!("roses are #{:06x}", Color::Red as i32);
println!("violets are #{:06x}", Color::Blue as i32);
}
程序运行结果为：
zero is 0 one is 1 roses are #ff0000 violets are #0000ff
3.2.3 测试用例：链表
enums一个普通的使用时创建链表：
use List::*;
enum List {
// Cons: Tuple struct that wraps an element and a pointer to the next node
Cons(u32, Box<List>),
// Nil: A node that signifies the end of the linked list
Nil,
}
// Methods can be attached to an enum
impl List {
// Create an empty list
fn new() -> List {
// `Nil` has type `List`
Nil
}
// Consume a list, and return the same list with a new element at its front
fn prepend(self, elem: u32) -> List {
// `Cons` also has type List
Cons(elem, Box::new(self))
}
// Return the length of the list
fn len(&self) -> u32 {
// `self` has to be matched, because the behavior of this method
// depends on the variant of `self`
// `self` has type `&List`, and `*self` has type `List`, matching on a
// concrete type `T` is preferred over a match on a reference `&T`
match *self {
// Can't take ownership of the tail, because `self` is borrowed;
// instead take a reference to the tail
Cons(_, ref tail) => 1 + tail.len(),
// Base Case: An empty list has zero length
Nil => 0
}
}
// Return representation of the list as a (heap allocated) string
fn stringify(&self) -> String {
match *self {
Cons(head, ref tail) => {
// `format!` is similar to `print!`, but returns a heap
// allocated string instead of printing to the console
format!("{}, {}", head, tail.stringify())
},
Nil => {
format!("Nil")
},
}
}
}
fn main() {
// Create an empty linked list
let mut list = List::new();
// Append some elements
list = list.prepend(1);
list = list.prepend(2);
list = list.prepend(3);
// Show the final state of the list
println!("linked list has length: {}", list.len());
println!("{}", list.stringify());
}
程序运行结果：
linked list has length: 3 3, 2, 1, Nil
3.3 常量
Rust有两种不同类型的常量，这些常量可以在任何作用于定义，包括全局。两种方法都需要显示声明：
- const：一个不可改变的值（通常用法）
- static：一个可能在staic声明周期内可变的变量
“string”是一个特例。它可以直接赋值给一个static变量，而不需要修改，因为它的类型签名：&‘static str已经使用了‘static生命周期。所有其他的引用类型必须使用‘static声明走起做特殊注解。
// Globals are declared outside all other scopes.
static LANGUAGE: &'static str = "Rust";
const THRESHOLD: i32 = 10;
fn is_big(n: i32) -> bool {
// Access constant in some function
n > THRESHOLD
}
fn main() {
let n = 16;
// Access constant in the main thread
println!("This is {}", LANGUAGE);
println!("The threshold is {}", THRESHOLD);
println!("{} is {}", n, if is_big(n) { "big" } else { "small" });
// Error! Cannot modify a `const`.
//THRESHOLD = 5;
// FIXME ^ Comment out this line
}
程序运行结果：
This is Rust The threshold is 10 16 is big
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时间： 2024-10-13 10:33:27