半小時學Rust
文章翻譯自英文博客,內容有刪減。在此感謝原作者分享的精神!
原文地址:A half-hour to learn Rust
原文作者:amos loves to tinker
說明:文章將根據個人理解深度,將不定期修改編輯,也歡迎同學提出寶貴建議!
有時爲了加強編程語言的熟練程度,我們需要閱讀很多相關文檔。但如果不知道什麼意思你如何去閱讀呢?
在本文中,我將介紹儘可能多的Rust Snippets,而不去過多關注某幾個概念,而且會解釋他們所包含關鍵字和符號的意義。
準備好了嗎?那開始吧!
變量綁定
let
介紹了一種變量綁定的方式:
let x; // declare "x"
x = 42; // assign 42 to "x"
同樣可以將生命變量和指定值寫成一行:
let x = 42;
你也可以用類型註解符:
顯示地指定變量類型
let x: i32; // `i32` is a signed 32-bit integer
x = 42;
// there's i8, i16, i32, i64, i128
// also u8, u16, u32, u64, u128 for unsigned
當然,也可以寫成一行:
let x: i32 = 42;
如果你聲明瞭一個變量,但並沒有馬上對其進行初始化,那麼編譯器將會阻止你在對其進行初始化前使用它:
let x;
foobar(x); // error: borrow of possibly-uninitialized variable: `x`
x = 42;
然而,這樣做卻是完全可以的:
let x;
x = 42;
foobar(x); // the type of `x` will be inferred from here
下劃線是一個特殊的變量名稱,或者缺少名稱,基本意味着扔掉一些內容:
// this does *nothing* because 42 is a constant
let _ = 42;
// this calls `get_thing` but throws away its result
let _ = get_thing();
以下劃線開始的變量名稱是regular名稱,編譯器在該名稱未使用時不會給出警告信息:
// we may use `_x` eventually, but our code is a work-in-progress
// and we just wanted to get rid of a compiler warning for now.
let _x = 42;
允許分別綁定同一名稱的變量,這樣會隱藏一個變量綁定:
let x = 13;
let x = x + 3;
// using `x` after that line only refers to the second `x`,
// the first `x` no longer exists.
Rust擁有元組類型,可把它看成固定長度的不同類型值的集合
:
let pair = ('a', 17);
pair.0; // this is 'a'
pair.1; // this is 17
假如我們特別想註解pair
的類型,我們可以這麼寫:
let pair: (char, i32) = ('a', 17);
元組可以在賦值時被解構,即元組中的元素被分解到各自獨立的域中:
let (some_char, some_int) = ('a', 17);
// now, `some_char` is 'a', and `some_int` is 17
這在一個函數返回一個元組時相當有用:
let (left, right) = slice.split_at(middle);
當然,當解構一個元組時, 下劃線_
可以用來充當不需要顯示的值:
let (_, right) = slice.split_at(middle);
語句與表達式
分號標誌着語句的結束:
let x = 3;
let y = 5;
let z = y + x;
也就是說一個語句可以衍生(span)多行:
let x = vec![1,2,3,4,5,6,7,8]
.iter()
.map(|x| x + 3)
.fold(0, |x, y| x + y);
(待會解釋該代碼段什麼意思!)
函數
fn 聲明一個函數。
(1) 無返回值的函數:
fn greet() {
println!("Hi, there!");
}
(2) 返回一個i32整數值的函數,箭頭->
表示其返回值類型:
fn fair_dice_roll() -> i32 {
4
}
一對大括號代表一個代碼塊, 擁有自己的域:
// This prints "in", then "out"
fn main() {
let x = "out";
{
// this is a different `x`
let x = "in";
println!(x);
}
println!(x);
}
代碼塊也是表達式,也就是說代碼塊計算出的值給一個值
// this:
let x = 42;
// is equivalent to this:
let x = { 42 };
在一個代碼塊中,一般會有多個語句:
let x = {
let y = 1; // first statement
let z = 2; // second statement
y + z // this is the *tail* - what the whole block will evaluate to
};
也就是說爲什麼省略函數最後的大括號跟返回值具有一樣的效果,即等價關係如下:
fn fair_dice_roll() -> i32 {
return 4;
}
fn fair_dice_roll() -> i32 {
4
}
if
條件表達式:
fn fair_dice_roll() -> i32 {
if feeling_lucky {
6
} else {
4
}
}
match
表達式:
fn fair_dice_roll() -> i32 {
match feeling_lucky {
true => 6,
false => 4,
}
}
.
常被用於訪問值的域:
let a = (10, 20);
a.0; // this is 10
let amos = get_some_struct();
amos.nickname; // this is "fasterthanlime"
或者,在一個值上調用方法:
let nick = "fasterthanlime";
nick.len(); // this is 14
雙冒::
類似於作用在命名空間上,在該例子中,std
是一個crate
(庫),cmp
是一個模塊(代碼文件),min
是一個函數:
let least = std::cmp::min(34, 90); // this is 34
use指令可用於將其他名稱空間中的名稱“引入範圍”:
use std::cmp::min;
let least = min(7, 1); // this is 1
在use指令中,花括號的另一含義是:“globs”。假如我們要同時引用min
和max
,我們可以做如下操作:
// this works:
use std::cmp::min;
use std::cmp::max;
// this also works:
use std::cmp::{min, max};
// this also works!
use std::{cmp::min, cmp::max};
通配符*
允許你從一個命名空間中引入每個symbol:
// this brings `min` and `max` in scope, and many other things
use std::cmp::*;
類型也是命名空間,方法可以像常規函數一樣被調用:
let x = "amos".len(); // this is 4
let x = str::len("amos"); // this is also 4
str
是一個原始類型,但是很多非原始類型也在默認的範圍內。
// `Vec` is a regular struct, not a primitive type
let v = Vec::new();
// this is exactly the same code, but with the *full* path to `Vec`
let v = std::vec::Vec::new();
如下引用也是起作用的,因爲Rust將在每個模塊的開始插入如下引用:
use std::prelude::v1::*;
(反過來又重新導出了很多符號,例如Vec,String,Option和Result)
關鍵字struct
用於結構體聲明:
struct Vec2 {
x: f64, // 64-bit floating point, aka "double precision"
y: f64,
}
可以使用結構文字初始化它們:
let v1 = Vec2 { x: 1.0, y: 3.0 };
let v2 = Vec2 { y: 2.0, x: 4.0 };
// the order does not matter, only the names do
可以使用縮略形式初始化剩餘的來自於其他結構體的域:
let v3 = Vec2 {
x: 14.0,
..v2
};
這種形式被稱作“結構體更新語法”,只能發生在結構體的最後一個位置,且不能跟逗號.
請注意,其餘字段可以表示所有字段:
let v4 = Vec2 { ..v3 };
結構體,像元組,是可以被解構的。像let
模式的例子是有效的:
let (left, right) = slice.split_at(middle);
這樣的形式也可以:
let v = Vec2 { x: 3.0, y: 6.0 };
let Vec2 { x, y} = v;
//`x` is now 3.0, `y` is now `6.0`
還有這個:
let Vec2 { x, .. } = v;
// this throws away `v.y`
let
模式可以被用於if
的條件:
struct Number {
odd: bool,
value: i32,
}
fn main() {
let one = Number { odd: true, value: 1 };
let two = Number { odd: false, value: 2 };
print_number(one);
print_number(two);
}
fn print_number(n: Number) {
if let Number { odd: true, value } = n {
println!("Odd number: {}", value);
} else if let Number { odd: false, value } = n {
println!("Even number: {}", value);
}
}
// this prints:
// Odd number: 1
// Even number: 2
match
操作同樣也是模式,像if let
:
fn print_number(n: Number) {
match n {
Number { odd: true, value } => println!("Odd number: {}", value),
Number { odd: false, value } => println!("Even number: {}", value),
}
}
// this prints the same as before
match
必須是詳盡的:至少有一個能匹配的分支。
fn print_number(n: Number) {
match n {
Number { value: 1, .. } => println!("One"),
Number { value: 2, .. } => println!("Two"),
Number { value, .. } => println!("{}", value),
// if that last arm didn't exist, we would get a compile-time error
}
}
若列出所有匹配分支是比較困難的,那麼下劃線_
可以用作“catch-all”模式:
fn print_number(n: Number) {
match n.value {
1 => println!("One"),
2 => println!("Two"),
_ => println!("{}", n.value),
}
}
你也可以聲明方法在自定義類型上:
struct Number {
odd: bool,
value: i32,
}
impl Number {
fn is_strictly_positive(self) -> bool {
self.value > 0
}
}
並且可以像通常的做法使用它們:
fn main() {
let minus_two = Number {
odd: false,
value: -2,
};
println!("positive? {}", minus_two.is_strictly_positive());
// this prints "positive? false"
}
變量綁定默認是不可變的:
fn main() {
let n = Number {
odd: true,
value: 17,
};
n.odd = false; // error: cannot assign to `n.odd`,
// as `n` is not declared to be mutable
}
不可變變量綁定不能對其內部進行更改(就像我們剛纔嘗試的那樣),但是也不能將其分配
fn main() {
let n = Number {
odd: true,
value: 17,
};
n = Number {
odd: false,
value: 22,
}; // error: cannot assign twice to immutable variable `n`
}
mut
關鍵字使得變量爲可變綁定:
fn main() {
let mut n = Number {
odd: true,
value: 17,
}
n.value = 19; // all good
}
Traits
Trait 是多種類型共同擁有的一種性質:
trait Signed {
fn is_strictly_negative(self) -> bool;
}
你可以實現爲:
(1)如果要實現外部定義的 trait 需要先將其導入作用域。
(2)不允許對外部類型實現外部 trait;
(3)可以對外部類型實現自定義的 trait;
(4)可以對自定義類型上實現外部 trait。
這些規則稱爲“孤兒規則”。
(1) 爲自定義類型實現Trait的例子:
impl Signed for Number {
fn is_strictly_negative(self) -> bool {
self.value < 0
}
}
fn main() {
let n = Number { odd: false, value: -44 };
println!("{}", n.is_strictly_negative()); // prints "true"
}
(2) 爲外部類型(原始類型,比如i32等)實現自定義Trait:
impl Signed for i32 {
fn is_strictly_negative(self) -> bool {
self < 0
}
}
fn main() {
let n: i32 = -44;
println!("{}", n.is_strictly_negative()); // prints "true"
}
(3)爲自定義類型實現外部Trait:
// the `Neg` trait is used to overload `-`, the
// unary minus operator.
impl std::ops::Neg for Number {
type Output = Number;
fn neg(self) -> Number {
Number {
value: -self.value,
odd: self.odd,
}
}
}
fn main() {
let n = Number { odd: true, value: 987 };
let m = -n; // this is only possible because we implemented `Neg`
println!("{}", m.value); // prints "-987"
}
一個impl
塊總是對應着一個類型,因此在該塊中,Self
對應這個類型:
impl std::ops::Neg for Number {
type Output = Self;
fn neg(self) -> Self {
Self {
value: -self.value,
odd: self.odd,
}
}
}
一些Trait是標籤-它們不是說某一類型實現某些方法,而是說某些事情可以藉助於某一類型來完成。比如說,i32
實現了Copy
Trait.(簡言之,i32
是 Copy
),因此以下例子是可以正常工作的。
fn main() {
let a: i32 = 15;
let b = a; // `a` is copied
let c = a; // `a` is copied again
}
也可以這樣:
fn print_i32(x: i32) {
println!("x = {}", x);
}
fn main() {
let a: i32 = 15;
print_i32(a); // `a` is copied
print_i32(a); // `a` is copied again
}
但是Number
結構體是不能如此操作的,因爲其並沒有實現Copy
Trait,所以這樣做會有問題:
fn main() {
let n = Number { odd: true, value: 51 };
let m = n; // `n` is moved into `m`
let o = n; // error: use of moved value: `n`
}
也不能這樣做:
fn print_number(n: Number) {
println!("{} number {}", if n.odd { "odd" } else { "even" }, n.value);
}
fn main() {
let n = Number { odd: true, value: 51 };
print_number(n); // `n` is moved
print_number(n); // error: use of moved value: `n`
}
但是,如果將print_number
的參數換成不可變引用形式,那麼函數將可以正常工作:
fn print_number(n: &Number) {
println!("{} number {}", if n.odd { "odd" } else { "even" }, n.value);
}
fn main() {
let n = Number { odd: true, value: 51 };
print_number(&n); // `n` is borrowed for the time of the call
print_number(&n); // `n` is borrowed again
}
如果將函數參數換成可變引用也是能工作的,但只要變量綁定是mut
.
fn invert(n: &mut Number) {
n.value = -n.value;
}
fn print_number(n: &Number) {
println!("{} number {}", if n.odd { "odd" } else { "even" }, n.value);
}
fn main() {
// this time, `n` is mutable
let mut n = Number { odd: true, value: 51 };
print_number(&n);
invert(&mut n); // `n is borrowed mutably - everything is explicit
print_number(&n);
}
Trait方法也可以引用或者可變引用作爲自身:
impl std::clone::Clone for Number {
fn clone(&self) -> Self {
Self { ..self }
}
}
當調用Trait方法時,隱式借用了接收者:
fn main() {
let n = Number { odd: true, value: 51 };
let mut m = n.clone();
m.value += 100;
print_number(&n);
print_number(&m);
}
特別強調下:這些是等價的:
let m = n.clone();
let m = std::clone::Clone::clone(&n);
標籤Trait像Copy
沒有方法:
// note: `Copy` requires that `Clone` is implemented too
impl std::clone::Clone for Number {
fn clone(&self) -> Self {
Self { ..*self }
}
}
impl std::marker::Copy for Number {}
現在,Clone
仍然可以使用:
fn main() {
let n = Number { odd: true, value: 51 };
let m = n.clone();
let o = n.clone();
}
但Number
值將永遠不被移動:
fn main() {
let n = Number { odd: true, value: 51 };
let m = n; //`m` is a copy of `n`
let o = n; // same. `n` is neither moved nor borrowed.
}
有些Trait
可以通過derive
屬性自動實現標籤Trait:
#[derive(Clone, Copy)]
struct Number {
odd: bool,
value: i32,
}
// this expands to `impl Clone for Number` and `impl Copy for Number` blocks.
函數泛型:
fn foobar<T>(arg: T) {
// do something with `arg`
}
函數可以有多個類型參數,並可以在函數聲明和函數提中使用,而不是具體類型:
fn foobar<L, R>(left: L, right: R) {
// do something with `left` and `right`.
}
類型參數通常具有約束,因此您實際上可以對它們做一些事情。最簡單的限制就是Trait名稱:
fn print<T: Display>(value: T) {
println!("value = {}", value);
}
fn print<T: Debug>(value: T) {
println!("value = {:?}", value);
}
類型參數約束的語法更長:
fn print<T>(value: T)
where
T: Display {
println!("value = {}");
}
約束可能更復雜:它們可能需要一個類型參數來實現多個Traits:
use std::fmt::Debug;
fn compare<T>(left: T, right: T)
where
T: Debug + PartialEq,
{
println!("{:?} {} {:?}", left, if left == right {"=="})
}
fn main() {
compare("tea", "coffee");
// prints: "tea" != "coffee"
}
泛型函數可以被認爲是名稱空間,其中包含無限個具有不同具體類型的函數。
和使用crate
, module
,type
和泛型函數一樣,可以使用::
來“導航”;
fn main() {
use std::any::type_name;
println!("{}", type_name::<i32>()); // prints "i32"
println!("{}", type_name::<(f64, char)>()); // prints "(f64, char)"
}
這被稱爲渦輪魚語法,因爲::<>看起來像一條魚。結構體也可以是泛型的:
struct Pair<T> {
a: T,
b: T,
}
fn print_type_name<T>(_val: &T) {
println!("{}", std::any::type_name::<T>());
}
fn main() {
let p1 = Pair{ a: 3, b: 9 };
let p2 = Pair { a: true, b: false };
print_type_name(&p1); // prints "Pair<i32>"
print_type_name(&p2); // prints "Pair<bool>"
}
標準庫類型Vec(堆分配的數組),是泛型的:
fn main() {
let mut v1 = Vec::new();
v1.push(1);
let mut v2 = Vec::new();
v2.push(false);
print_type_name(&v1); // prints "Vec<i32>"
print_type_name(&v2); // prints "Vec<bool>"
}
說起Vec
, 一般由vec!
宏來定義:
fn main() {
let v1 = vec![1, 2, 3];
let v2 = vec![true, false, true];
print_type_name(&v1); // prints "Vec<i32>"
print_type_name(&v2); // prints "Vec<bool>"
}
所有類似於name!(),name![]或name!{}調用的是一個宏。宏展開成一般的代碼。
事實上,println!
是一個宏:
fn main() {
println!("{}", "Hello there!");
}
這段代碼展開成普通代碼具有相同效果:
fn main() {
use std::io::{self, Write};
io::stdout().lock().write_all(b"Hello there!\n").unwrap();
}
panic!
也是一個宏,如果啓用,它將猛烈停止執行並顯示錯誤消息和錯誤的文件名/行號:
fn main() {
panic!("This panics");
}
// output: thread 'main' panicked at 'This panics', src/main.rs:3:5
一些方法也是Panic,比如Option
類型能爲Some(x),也可能爲None。如果Option
調用.unwrap(),如果爲None,那麼將會Panic。
fn main() {
let o1: Option<i32> = Some(128);
o1.unwrap(); //this is fine
let o2: Option<i32> = None;
o2.unwrap(); // this is panics!
}
// output: thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21
Option
不是一個結構體,而是一個枚舉類型,且只有兩個變量;
enum Option<T> {
None,
Some(T),
}
impl<T> Option<T> {
fn unwrap(self) -> T {
match self {
Self::Some(t) => t,
Self::None => panic!(".unwrap() called on a None option")
}
}
}
use self::Option::{None, Some};
fn main() {
let o1: Option<i32> = Some(128);
o1.unwrap(); // this is fine
let o2: Option<i32> = None;
o2.unwrap(); // this panics!
}
// output: thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21
Result
也是一個枚舉類型:
enum Result<T, E> {
Ok(T),
Err(E),
}
它也會在Result值是Err(E)時發生panic。
Lifetime
變量綁定有一個生命週期“lieftime”:
fn main() {
// `x` doesn't exist yet
{
let x = 42; // `x` starts existing
println!("x = {}", x);
// `x` stops existing
}
// `x` no longer exists
}
類似地,引用有生命週期:
fn main() {
// `x` doesn't exist yet
{
let x = 42; // `x` starts existing
let x_ref = &x; // `x_ref` starts existing - it borrows `x`
println!("x_ref = {}", x_ref);
// `x_ref` stops existing
// `x` stops existing
}
// `x` no longer exists
}
引用的生命週期長度不能超過它所借用的變量綁定的生命週期:
fn main() {
let x_ref = {
let x = 42;
&x
};
println!("x_ref = {}", x_ref);
// error: `x` does not live long enough
}
不可變變量綁定可被借用多次:
fn main() {
let x = 42;
let x_ref1 = &x;
let x_ref2 = &x;
let x_ref3 = &x;
println!("{} {} {}", x_ref1, x_ref2, x_ref3);
}
當一個變量被借用,那麼變量綁定將不可變:
fn main() {
let mut x = 42;
let x_ref = &x;
x = 13;
println!("x_ref = {}", x_ref);
// error: cannot assign to `x` because it is borrowed
}
當一個變量被不可變借用時,那麼該變量不允許可變借用:
fn main() {
let mut x = 42;
let x_ref1 = &x;
let x_ref2 = &mut x;
// error: cannot borrow `x` as mutable because it is also borrowed as immutable
println!("x_ref1 = {}", x_ref1);
}
函數的引用參數也有生命週期:
fn print(x:&i32) {
// `x` is borrowed (from the outside) for the
// entire time this function is called.
}
具有引用參數的函數可以使用具有不同生命週期的借用來調用:
(1)All functions that take references are generic
(2)生命週期是泛型參數;
生命週期的名稱起始字符帶有單引號'
:
// elided (non-named) lifetimes:
fn print(x: &i32) {}
// named lifetimes:
fn print<'a>(x: &'a i32) {}
返回引用的生命週期要依賴於某一個函數參數的生命週期:
// elided (non-named) lifetimes:
fn print(x: &i32) {}
// named lifetimes:
fn print<'a>(x: &'a i32) {}
當函數只有一個輸入生命週期(帶有生命週期限制的函數參數),沒有必要去標註生命週期,所有項都具有相同的生命週期,因此以下兩個函數是等價的:
fn number_value<'a>(num: &'a Number) -> &'a i32 {
&num.value
}
fn number_value(num: &Number) -> &i32 {
&num.value
}
結構體在生命週期中也可以是泛型的,這使得結構體持有引用:
struct NumRef<'a> {
x: &'a i32,
}
fn main() {
let x:i32 = 99;
let x_ref = NumRef {x: &x};
// `x_ref` cannot outlive `x`, etc.
}
同樣的代碼,但是增加了一個函數:
struct NumRef<'a> {
x: &'a i32,
}
fn as_num_ref(x: &'a i32) -> NumRef<'a> {
NumRef {x: &x}
}
fn main() {
let x: i32 = 99;
let x_ref = as_num_ref(&x);
// `x_ref` cannot outlive `x`, etc.
}
同樣代碼,但省略了生命週期:
struct NumRef<'a> {
x: &'a i32,
}
fn as_num_ref(x: &'a i32) -> NumRef<'_> {
NumRef {x: &x}
}
fn main() {
let x: i32 = 99;
let x_ref = as_num_ref(&x);
// `x_ref` cannot outlive `x`, etc.
}s_
impl
塊也可以泛型方式使用生命週期:
impl<'a> NumRef<'a> {
fn as_i32_ref(&'a self) -> &'a i32 {
self.x
}
}
fn main() {
let x: i32 = 99;
let x_num_ref = NumRef { x: &x};
let x_i32_ref = x_num_ref.as_i32_ref();
// neither ref cannot outlive `x`
}
當然,對於只有一個生命週期參數的情況,也可以省略:
impl<'a> NumRef<'a> {
fn as_i32_ref(&self) -> &i32 {
self.x
}
}
如果沒有顯示的生命週期符號,那省略的將更加堅決:
impl NumRef<'_> {
fn as_i32_ref(&self) -> &i32 {
self.x
}
}
特殊生命週期 static,其在程序的整個生命週期內都有效,以下爲String
語法的例子:
struct Person {
name: &'static str,
}
fn main() {
let p = Person {
name: "fasterthanlime",
};
}
但是已有所主的字符串是不能static
的,以下例子中的引用生命週期是不能長於所引用變量的生命週期的:
struct Person {
name: &'static str,
}
fn main() {
let name = format!("fasterthan{}", lime);
let p = Person { name: &name };
// error: `name` does not live long enough
}
In that last example, the local name is not a &static str, it’s a String. It’s been allocated dynamically, and it will be freed. Its lifetime is less than the whole program (even though it happens to be in main)."
爲了在Person
結構體中存儲一個非‘static字符串,需要:
A)使用生命週期泛型:
struct Person<'a> {
name: &'a str,
}
fn main() {
let name = format!("fasterthan{}", "lime");
let p = Person { name: &name };
// `p` cannot outlive `name`
}
B) 獲取字符串的所有權
struct Person {
name: String,
}
fn main() {
let name = format!("fasterthan{}", "lime");
let p = Person { name: name };
// `name` was moved into `p`, their lifetimes are no longer tied.
}
說起:在結構體中,當一個域被設置爲變量綁定爲相同的名稱,(即值與域名稱相同):
let p = Person { name: name};
可以簡寫爲:
let p = Person { name };
Rust中很多類型有owned
和non-owned
變量:
(1)Strings:String
是owned, &str是引用的;
(2)Paths: PathBuf
是owned, &Path是引用的;
(3)Collections: Vec<T>
是owned, &[T]是引用的;
Rust有slice(切片),他們是多連續元素的引用。可以通過以下例子中的方式借用動態數組(vector):
fn main() {
let v = vec![1,2,3,4,5];
let v2 = &v[2..4];
println!("v2 = {:?}", v2);
}
//output:
// v2 = [3, 4]
以上列子並非難以理解,查詢操作符(foo[index])重載了Index
和IndexMut
Trait。..
語義表示範圍,僅僅是一些在標準庫中定義的結構體。且索引範圍是半開半閉區間內的元素,如果最右端的前面加上=
:
fn main() {
// 0 or greater
println!("{:?}", (0..).contains(&100)); // true
// strictly less than 20
println!("{:?}", (..20).contains(&20)); // false
// 20 or less than 20
println!("{:?}", (..=20).contains(&20)); // true
// only 3, 4, 5
println!("{:?}", (3..6).contains(&4)); // true
}
借用規則同樣可應用於slices:
fn tail(s: &[u8]) -> &[u8] {
&s[1..]
}
fn main() {
let x = &[1, 2, 3, 4, 5];
let y = tail(x);
println!("y = {:?}", y);
}
相同效果的例子:
fn tail<'a>(s: &'a [u8]) -> &'a [u8] {
&s[1..]
}
這樣是合法的:
fn main() {
let y = {
let x = &[1, 2, 3, 4, 5];
tail(x)
};
println!("y = {:?}", y);
}
因爲[1,2,3,4,5]
是’static數組,因此,這是不合法的:
fn main() {
let y = {
let v = vec![1, 2, 3, 4, 5];
tail(&v)
// error: `v` does not live long enough
};
println!("y = {:?}", y);
}
因爲動態數組是基於堆分配的,並沒有‘static生命週期。
&str
值是切片
fn file_ext(name: &str) -> Option<&str> {
// this does not create a new string - it returns
// a slice of the argument.
name.split(".").last()
}
fn main() {
let name = "Read me. Or don't.txt";
if let Some(ext) = file_ext(name) {
println!("file extension: {}", ext);
} else {
println!("no file extension");
}
}
因此借用規則同樣適用於此:
fn main() {
let ext = {
let name = String::from("Read me. Or don't.txt");
file_ext(&name).unwrap_or("")
// error: `name` does not live long enough
};
println!("extension: {:?}", ext);
}
函數執行失敗時通常會返回一個Result
:
fn main() {
let s = std::str::from_utf8(&[240, 159, 141, 137]);
println!("{:?}", s);
// prints: Ok("🍉")
let s = std::str::from_utf8(&[195, 40]);
println!("{:?}", s);
// prints: Err(Utf8Error { valid_up_to: 0, error_len: Some(1) })
}
在執行失敗時,若你想Panic, 那麼調用.unwrap():
fn main() {
let s = std::str::from_utf8(&[240, 159, 141, 137]).unwrap();
println!("{:?}", s);
// prints: "🍉"
let s = std::str::from_utf8(&[195, 40]).unwrap();
// prints: thread 'main' panicked at 'called `Result::unwrap()`
// on an `Err` value: Utf8Error { valid_up_to: 0, error_len: Some(1) }',
// src/libcore/result.rs:1165:5
}
或者想獲取自定義的信息,可以調用.expect():
fn main() {
let s = std::str::from_utf8(&[195, 40]).expect("valid utf-8");
// prints: thread 'main' panicked at 'valid utf-8: Utf8Error
// { valid_up_to: 0, error_len: Some(1) }', src/libcore/result.rs:1165:5
}
或者使用match
fn main() {
match std::str::from_utf8(&[240, 159, 141, 137]) {
Ok(s) => println!("{}", s),
Err(e) => panic!(e),
}
// prints 🍉
}
或者使用if let
:
fn main() {
if let Ok(s) = std::str::from_utf8(&[240, 159, 141, 137]) {
println!("{}", s);
}
// prints 🍉
}
或者可以拋出錯誤:
fn main() -> Result<(), std::str::Utf8Error> {
match std::str::from_utf8(&[240, 159, 141, 137]) {
Ok(s) => println!("{}", s),
Err(e) => return Err(e),
}
Ok(())
}
或者可以使用操作符?
使得代碼更加簡潔:
fn main() -> Result<(), std::str::Utf8Error> {
let s = std::str::from_utf8(&[240, 159, 141, 137])?;
println!("{}", s);
Ok(())
}
解引用操作符*
,但你並不需要使用解引用來訪問域或者調用方法:
struct Point {
x: f64,
y: f64,
}
fn main() {
let p = Point { x: 1.0, y: 3.0 };
let p_ref = &p;
println!("({}, {})", p_ref.x, p_ref.y);
}
// prints `(1, 3)`
並且當類型是Copy
語義時,你可以簡單地這樣做。
首先看下沒有Copy
語義時,如下例子:
struct Point {
x: f64,
y: f64,
}
fn negate(p: Point) -> Point {
Point {
x: -p.x,
y: -p.y,
}
}
fn main() {
let p = Point { x: 1.0, y: 3.0 };
let p_ref = &p;
negate(*p_ref);
// error: cannot move out of `*p_ref` which is behind a shared reference
}
但當有Copy
語義時:
// now `Point` is `Copy`
#[derive(Clone, Copy)]
struct Point {
x: f64,
y: f64,
}
fn negate(p: Point) -> Point {
Point {
x: -p.x,
y: -p.y,
}
}
fn main() {
let p = Point { x: 1.0, y: 3.0 };
let p_ref = &p;
negate(*p_ref); // ...and now this works
}
閉包
閉包是具有某些捕獲上下文的Fn,FnMut或FnOnce類型的函數。它們的參數是一對管道內(’|’),逗號分隔名稱列表。它們不需要花括號,除非要使用多個語句。
fn for_each_planet<F>(f: F)
where F: Fn(&'static str)
{
f("Earth");
f("Mars");
f("Jupiter");
}
fn main() {
for_each_planet(|planet| println!("Hello, {}", planet));
}
// prints:
// Hello, Earth
// Hello, Mars
// Hello, Jupiter
借用規則同樣適用於這裏:
fn for_each_planet<F>(f: F)
where F: Fn(&'static str)
{
f("Earth");
f("Mars");
f("Jupiter");
}
fn main() {
let greeting = String::from("Good to see you");
for_each_planet(|planet| println!("{}, {}", greeting, planet));
// our closure borrows `greeting`, so it cannot outlive it
}
比如,以下例子就不能正常通過:
fn for_each_planet<F>(f: F)
where F: Fn(&'static str) + 'static // `F` must now have "'static" lifetime
{
f("Earth");
f("Mars");
f("Jupiter");
}
fn main() {
let greeting = String::from("Good to see you");
for_each_planet(|planet| println!("{}, {}", greeting, planet));
// error: closure may outlive the current function, but it borrows
// `greeting`, which is owned by the current function
}
但是這樣是可以的:
fn main() {
let greeting = String::from("You're doing great");
for_each_planet(move |planet| println!("{}, {}", greeting, planet));
// `greeting` is no longer borrowed, it is *moved* into
// the closure.
}
一個FnMut
需要可變借用才能被調用,因此它在某一時刻只能被調用一次,比如以下例子是合法的:
fn foobar<F>(f: F)
where F: Fn(i32) -> i32
{
println!("{}", f(f(2)));
}
fn main() {
foobar(|x| x * 2);
}
// output: 8
而這個例子是非法的:
fn foobar<F>(mut f: F)
where F: FnMut(i32) -> i32
{
println!("{}", f(f(2)));
// error: cannot borrow `f` as mutable more than once at a time
}
fn main() {
foobar(|x| x * 2);
}
這樣操作又變成合法的了:
fn foobar<F>(mut f: F)
where F: FnMut(i32) -> i32
{
let tmp = f(2);
println!("{}", f(tmp));
}
fn main() {
foobar(|x| x * 2);
}
// output: 8
FnMut
存在因爲一些閉包可變借用局部變量:
fn foobar<F>(mut f: F)
where F: FnMut(i32) -> i32
{
let tmp = f(2);
println!("{}", f(tmp));
}
fn main() {
let mut acc = 2;
foobar(|x| {
acc += 1;
x * acc
});
}
// output: 24
這些閉包不能被傳給需要Fn
的函數:
fn foobar<F>(f: F)
where F: Fn(i32) -> i32
{
println!("{}", f(f(2)));
}
fn main() {
let mut acc = 2;
foobar(|x| {
acc += 1;
// error: cannot assign to `acc`, as it is a
// captured variable in a `Fn` closure.
// the compiler suggests "changing foobar
// to accept closures that implement `FnMut`"
x * acc
});
}
FnOnce
閉包只能被調用一次。它們之所以存在,是因爲某些閉包將捕獲時移出的變量移出:
fn foobar<F>(f: F)
where F: FnOnce() -> String
{
println!("{}", f());
}
fn main() {
let s = String::from("alright");
foobar(move || s);
// `s` was moved into our closure, and our
// closures moves it to the caller by returning
// it. Remember that `String` is not `Copy`.
}
這自然是強制執行的,因爲需要移動FnOnce閉包才能調用它,因此,以下例子是非法的:
fn foobar<F>(f: F)
where F: FnOnce() -> String
{
println!("{}", f());
println!("{}", f());
// error: use of moved value: `f`
}
並且,如果你需要有說服力的證據表明閉包確實會移動s
,那麼下面這個例子也是非法的:
fn main() {
let s = String::from("alright");
foobar(move || s);
foobar(move || s);
// use of moved value: `s`
}
但是這樣卻是可以的:
fn main() {
let s = String::from("alright");
foobar(|| s.clone());
foobar(|| s.clone());
}
這裏有一個帶有兩個參數的閉包:
fn foobar<F>(x: i32, y: i32, is_greater: F)
where F: Fn(i32, i32) -> bool
{
let (greater, smaller) = if is_greater(x, y) {
(x, y)
} else {
(y, x)
};
println!("{} is greater than {}", greater, smaller);
}
fn main() {
foobar(32, 64, |x, y| x > y);
}
閉包允許忽略參數:
fn main() {
foobar(32, 64, |_, _| panic!("Comparing is futile!"));
}
稍微令人擔心的閉包:
fn countdown<F>(count: usize, tick: F)
where F: Fn(usize)
{
for i in (1..=count).rev() {
tick(i);
}
}
fn main() {
countdown(3, |i| println!("tick {}...", i));
}
// output:
// tick 3...
// tick 2...
// tick 1...
"馬桶式"閉包(之所以這麼叫是因爲|_| ()
看起來像個馬桶):
fn main() {
countdown(3, |_| ());
}
循環
任何可迭代的元素集合均可使用類似於for in
循環。我們知道在某一範圍內可以使用該形式的循環,其實在Vec
中也是可以使用的:
fn main() {
for i in vec![52, 49, 21] {
println!("I like the number {}", i);
}
}
或者是切片(slice)
fn main() {
for i in &[52, 49, 21] {
println!("I like the number {}", i);
}
}
// output:
// I like the number 52
// I like the number 49
// I like the number 21
或者是一個實際的迭代器:
fn main() {
// note: `&str` also has a `.bytes()` iterator.
// Rust's `char` type is a "Unicode scalar value"
for c in "rust".chars() {
println!("Give me a {}", c);
}
}
// output:
// Give me a r
// Give me a u
// Give me a s
// Give me a t
即使迭代器項已被過濾,映射或者展開:
fn main() {
for c in "sHE'S brOKen"
.chars()
.filter(|c| c.is_uppercase() || !c.is_ascii_alphabetic())
.flat_map(|c| c.to_lowercase())
{
print!("{}", c);
}
println!();
}
// output: he's ok
依舊可以從一個函數返回一個閉包:
fn make_tester(answer: String) -> impl Fn(&str) -> bool {
move |challenge| {
challenge == answer
}
}
fn main() {
// you can use `.into()` to perform conversions
// between various types, here `&'static str` and `String`
let test = make_tester("hunter2".into());
println!("{}", test("******"));
println!("{}", test("hunter2"));
}
你甚至能移動函數某一參數的引用到函數返回的閉包中:
fn make_tester<'a>(answer: &'a str) -> impl Fn(&str) -> bool + 'a {
move |challenge| {
challenge == answer
}
}
fn main() {
let test = make_tester("hunter2");
println!("{}", test("*******"));
println!("{}", test("hunter2"));
}
// output:
// false
// true
或者帶有省略生命週期的形式:
fn make_tester(answer: &str) -> impl Fn(&str) -> bool + '_ {
move |challenge| {
challenge == answer
}
}
這樣一來,我們達到了預計30分鐘閱讀時間的計劃,你應該能夠閱讀大部分在網上找到的Rust代碼。
編寫Rust代碼跟閱讀Rust是兩種截然不同的體驗,一方面是你並不是閱讀一個問題的解決方案,你要去解決該問題,另一方面,Rust編譯器可以提供很多幫助。
對於上述所有有意而爲之的錯誤代碼(“此代碼是非法的”等),rustc始終具有非常好的錯誤消息和有見地的建議。而且,當缺少提示時,編譯器團隊不會害怕添加它。
也許你需要查看更多的資料:
The Rust Book
Rust By Example
Read Rust
This Week In Rust
我也寫一些Rust相關的博客和發一些Rust相關的twitter,因此如果你比較喜歡這篇文章,你知道該怎麼做了!
Have fun!