引用与借用
Estimated reading time: 2 minutes
引用
Rust语言中，由于所有权机制，在变量传入到函数中，会发生所有权的移动，我们就无法在函数后面继续使用该变量了。
示例代码如下：
fn takes_ownership(some_string: String) {
println!("{}", some_string);
}
#[test]
fn test_takes_ownership() {
let s = String::from("hello");
takes_ownership(s);
println!("s:{}", s); // 这里发生编译错误
}
执行结果：
error[E0382]: borrow of moved value: `s`
--> crates/ownership_demo/src/lib.rs:28:26
|
26 | let s = String::from("hello");
| - move occurs because `s` has type `String`, which does not implement the `Copy` trait
27 | takes_ownership(s);
| - value moved here
28 | println!("s:{}", s);
| ^ value borrowed here after move
|
note: consider changing this parameter type in function `takes_ownership` to borrow instead if owning the value isn't necessary
--> crates/ownership_demo/src/lib.rs:1:33
|
1 | fn takes_ownership(some_string: String) {
| --------------- ^^^^^^ this parameter takes ownership of the value
| |
| in this function
= note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider cloning the value if the performance cost is acceptable
|
27 | takes_ownership(s.clone());
| ++++++++
For more information about this error, try `rustc --explain E0382`.
error: could not compile `ownership_demo` (lib test) due to 1 previous error
错误信息显示值 s 被移动了，无法执行println!("s:{}", s);
这里我们可以使用引用，指向一个地址，通过引用可以直接使用该值，但不获取其所有权。
引用所存储的的值是一个地址，这个地址才是真正指向变量。
借用
我们将创建一个引用的行为称为 借用（borrowing）。正如现实生活中，如果一个人拥有某样东西，你可以从他那里借来。当你使用完毕，必须还回去。我们并不拥有它。
fn takes_ownership_references(some_string: &String) {
println!("fn s:{}", some_string);
}
#[test]
fn test_gives_ownership() {
let s = gives_ownership();
takes_ownership_references(&s);
println!("打印:{}", s);
}
执行结果：
running 1 test
test tests::test_gives_ownership ... ok
successes:
---- tests::test_gives_ownership stdout ----
fn s:hello
打印:hello
successes:
tests::test_gives_ownership
解引用
与引用相反的是解引用，解引用运算符是 *，解引用指针以访问数据。
