ownership

chapter 4 ownership

4.1 what is ownership

ownership rule

• Each value in Rust has a variable that’s called its owner.
• There can only be one owner at a time.
• When the owner goes out of scope, the value will be dropped.

在rust中，当heap中的值发生，赋值、调用、返回时，都会发生所有权的“转移”

// assign 
let s1 = String::from("hello");
let s2 = s1;

// invoke
let s1 = String::from("hello");
takes_ownership(s1);

// invoke and return 
let s2 = String::from("hello");
let s3 = takes_and_gives_back(s2);

• but It’s quite annoying that anything we pass in also needs to be passed back if we want to use it again!

4.2 References and Borrowing

reference allow refer to some value without taking ownership.

fn main() {
    let s1 = String::from("hello");

    let len = calculate_length(&s1);

    println!("The length of '{}' is {}.", s1, len);
}

fn calculate_length(s: &String) -> usize {
    s.len()
}

mutable reference

fn main() {
    let mut s = String::from("hello");

    change(&mut s);
}

fn change(some_string: &mut String) {
    some_string.push_str(", world");
}

rule of reference

• 在任何时间，你只能拥有一个可变引用，或者，多个不可变引用。
• 引用必须是存在的，不允许悬挂的引用(dangling reference)。

4.3 Slice Type 切片

切片是另一种没有所有权的数据类型。切片允许你引用一个集合的连续元素。

let s = String::from("hello world");
let hello = &s[0..5]; //切片 imutable reference
let world = &s[6..11]; //切片

let s = "hello world";

• 变量s的类型是&str，等号右边称为”字符串字面量”；字符串是硬编码(hardcoding)在代码中，既不存在stack也不在heap，而是在程序的二进制代码中,即s是指向二进制代码中一段位置的一个切片。