rust - 为什么对已删除对象的可变引用仍算作可变引用？

我会带着你远行 2023-01-20 13:59 21阅读 0赞

这是一个简化的示例:

struct Connection {} impl Connection { fn transaction(&mut self) -> Transaction { Transaction { conn: self } } } struct Transaction<'conn> { conn: &'conn Connection, } impl<'conn> Transaction<'conn> { fn commit(mut self) {} } fn main() { let mut db_conn = Connection {}; let mut trans = db_conn.transaction(); //1 let mut records_without_sync = 0_usize; const MAX_RECORDS_WITHOUT_SYNC: usize = 100; loop { //do something records_without_sync += 1; if records_without_sync >= MAX_RECORDS_WITHOUT_SYNC { trans.commit(); records_without_sync = 0; trans = db_conn.transaction(); //2 } } }

编译器在** 1 **和** 2 **上报告了两个可变借位，但事实并非如此。由于`trans.commit()`按值接受`self`，因此会删除`trans`，因此到** 2点**点，不应有任何可变引用。

*  为什么编译器看不到** 2 **没有可变引用？
 *  我该如何修复代码，而又保持相同的逻辑？

**最佳答案**

有一个可变的引用。  
  
如果将`transaction`更改为此:

fn transaction(&mut self) -> Transaction { let _: () = self; Transaction{conn: self} }

您会看到编译器出现以下错误:

= note: expected type `()` = note: found type `&mut Connection`

因此，`self`的类型为`&mut Connection` ...一个可变的引用。然后，将其传递给此函数返回的`Transaction`实例。  
  
这意味着您的可变借项在`trans`的有效期内存在(我添加了花括号以显示借项的范围):

let mut trans = db_conn.transaction(); { // <-------------------- Borrow starts here let mut records_without_sync = 0_usize; const MAX_RECORDS_WITHOUT_SYNC: usize = 100; loop { //do something records_without_sync += 1; if records_without_sync >= MAX_RECORDS_WITHOUT_SYNC { trans.commit(); records_without_sync = 0; trans = db_conn.transaction();// <--- ####### D'oh! Still mutably borrowed } } } // <-------------------- Borrow ends here

如果您正在寻找这种`parent-><-child`设置，我认为您必须接触`Rc<RefCell>`。  
  
具体来说，要引用的`Rc`会计算您传递连接的次数，而`RefCell`会在运行时(而不是在编译时)跟踪借入。是的，这的确意味着如果您尝试在运行时两次尝试并可变地借用它，就会感到 panic 。在不了解您的体系结构的情况下，很难说这是否合适。  
  
[Here is my solution anyway][]:

use std::cell::RefCell; use std::rc::Rc; struct Connection {} impl Connection { fn do_something_mutable(&mut self) { println!("Did something mutable"); } } type Conn = Rc<RefCell<Connection>>; struct Transaction { conn: Conn, } impl Transaction { fn new(connection: Conn) -> Transaction { Transaction { conn: connection } } fn commit(mut self) { self.conn.borrow_mut().do_something_mutable(); } } fn main() { let db_conn = Rc::new(RefCell::new(Connection {})); let mut trans = Transaction::new(db_conn.clone()); let mut records_without_sync = 0_usize; const MAX_RECORDS_WITHOUT_SYNC: usize = 100; loop { //do something records_without_sync += 1; if records_without_sync >= MAX_RECORDS_WITHOUT_SYNC { trans.commit(); records_without_sync = 0; trans = Transaction::new(db_conn.clone()); break; // Used to stop the loop crashing the playground } } }

[Here is my solution anyway]: http://play.integer32.com/?gist=7e227161a9f7d4c3f720bff01f816b40&version=stable