【Rust】快速教程——冻结&表达式

前言

以前谁是魔我杀谁，现在我杀谁谁就是魔。——《拔魔》

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Rust基本知识

结构体

#[derive(Debug)]
struct Window_table{
    width : i32,
    height : i32,
    position : (i32,i32),
    id : i32,
    name :String,
}



fn main(){

    let W=1920;
    let H=1080;
    let a = Window_table{
        width : 960,
        height : 600,
        position : (W/2-480,H/2-300),
        id : 0x0000_0000_0000_00ff,  //255
        name : String::from("new win")
    };
    println!("{:#?}",a);
}

元组+结构体

struct Color(u8,u8,u8); //3个字节
struct Point(f64,f64);  //16个字节

fn main(){
    let a=Color(166,70,70);
    println!("a=({},{},{})",a.0,a.1,a.2);
    let a=Point(400.0,300.0);
    println!("a=({},{})",a.0,a.1);
}

局部作用域

目前这个例子还是和其他的语言一样，没什么特殊的。

fn main() {
    let a = 1;

    {
        let b = 2;
        println!("b={}", b);
        let a = 5_f32;
        println!("a={}", a);
    }

    println!("a={}", a);

    let a = 'a'; //shadowing
    println!("a={}", a);
}

冻结

fn main() {
    let mut a = 7i32;

    {
        let a = a;
        //a = 50; //报错，因为外部可变的a被内部不可变的a冻结了！
    }

    a = 3;
}

字面量

对于没有表明类型的整数，编译器默认是i32；对于没有表明类型的浮点数，编译器默认是f64

fn main() {
    let x = 117u8;
    let y = 20u32;
    let z = 3.1f32;

    let i = 5;
    let f = 3.1415;

    println!("size of `x` in bytes: {}", std::mem::size_of_val(&x));
    println!("size of `y` in bytes: {}", std::mem::size_of_val(&y));
    println!("size of `z` in bytes: {}", std::mem::size_of_val(&z));
    println!("size of `i` in bytes: {}", std::mem::size_of_val(&i));
    println!("size of `f` in bytes: {}", std::mem::size_of_val(&f));
}

Vec元素的类型由第一个push进的类型确定

fn main() {
    let a = 5u8;
    let b=String::from("Hello");
    let c:u8 = 1;
    let mut v = Vec::new();
   
    v.push(a);
    //v.push(b); //错误，因为类型被确定成u8了
    v.push(c);

    println!("{:?}", v);
}

type别名

//别名用type
type T1 = u64;
type T2 = u64;
type u8_ = u64;

fn main() {
    let a: T1 = 5 as u8_;
    let b: T2 = 2 as u8_;
    println!("{} a + {} b = {} unit?", a, b, a + b);
}

from和into

from函数就是把传入类型转换成自定义的类型，通过是把传入数据赋值给自定义类型的属性

use std::convert::From;

#[derive(Debug)]
struct Number {
    value: i32,
}

impl From<i32> for Number {
    fn from(item: i32) -> Self {
        Number { value: item }
    }
}

fn main() {
    let num = Number::from(30);
    println!("num={:?}", num);

    let a = 5;
    let b: Number = a.into();
    println!("b={:?}", b);
}

TryFrom和TryInto

和from、into一样，但是带错误处理

use std::convert::TryFrom;
use std::convert::TryInto;

#[derive(Debug, PartialEq)]
struct EvenNumber(i32);

impl TryFrom<i32> for EvenNumber {
    type Error = ();

    fn try_from(value: i32) -> Result<Self, Self::Error> {
        if value % 2 == 0 {
            Ok(EvenNumber(value))  //传入偶数就返回自定义类型
        } else {
            Err(())  //传入奇数就报错
        }
    }
}

fn main() {
    assert_eq!(EvenNumber::try_from(8), Ok(EvenNumber(8))); //ok=ok
    assert_eq!(EvenNumber::try_from(5), Err(()));           //err=err



    let a: Result<EvenNumber, ()> = 8i32.try_into();  //和a.into()的用法一样
    assert_eq!(a, Err(()));            //ok!=err
   
    let a: Result<EvenNumber, ()> = 5i32.try_into();
    assert_eq!(a, Err(()));            //err=err
}

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表达式

fn main() {
    let x = 5u32;

    let y = {
        let a = x * x;
        let b = a * x;

        //返回值不需要分号，意味着最后一句的结果默认是外面接收的
        b + a + x
    };

    let z = {   
        2 * x;
    };

    println!("x is {:?}", x);
    println!("y is {:?}", y);
    println!("z is {:?}", z);
}

表达式最后一句加了分号就表示没有返回值