rust - 切片中的不可变引用是如何更新的？为什么它不改变引用变量的值？

Question

这个问题在这里已经有了答案:





What's the difference between placing "mut" before a variable name and after the ":"?

(3 个回答)


1年前关闭。




我正在阅读 Rust 文档，在那里我读到了 slices .根据页面的文本，切片是不可变的引用:

This is also why string literals are immutable; &str is an immutable reference.

我希望下面的代码中有两件事:

yetnewstring = "we";给出编译时错误

即使我对上述内容有误，我仍然希望 newstring2 的最后一个打印语句的输出成为 westtheSlice .

下面的代码是如何工作的？

fn main() {
    let mut newstring2: String; //declare new mutable string
    newstring2 = String::from("Hello");    // add a value to it 
    println!("this is new newstring: {}", newstring2); // output = Hello

    let mut yetnewstring = test123(&mut newstring2); // pass a mutable reference of newstring2 , and get back a string literal
    println!("this is yetnewstring :{}", yetnewstring); // output = "te"
    yetnewstring = "we"; // how come this is mutable now ? arent string literals immutable?
    println!("this is the Changed yetnewstring  :{}", yetnewstring); // output = "we"
    println!("this is newstring2 after change of yetnewstring = 'we' : {}" , newstring2); // output  = "testtheSlice"
    // if string literal yetnewstring was reference to a slice of  newstring2 ,
    //then shouldnt above  output have to be :"westtheSlice"
}

fn test123(s: &mut String) -> &str {
     *s = String::from("testtheSlice");  
    &s[0..2]
} 

Answer 1

yetnewstring的类型是 &str ，只有绑定(bind)是可变的。分配是有效的，因为您正在分配另一个 &str值为 &str变量，这很好。