铁锈中相互冲突的生命

Question

代码是让score_handler在方法运算符((中处理来自某个学校的分数，handle((只是做一些计算，而不是保留self.school的引用：

trait Class{
fn student_count(&self, )->usize;
fn student_score(&self, i: usize) ->u64;
}
trait School<'a>{
fn class_count(&self)->usize;
fn class(&'a self, i:usize)->&'a dyn Class;
}
trait ScoreHandler<'a> {
fn handle(&'a mut self, school: &'a dyn School<'a>);
}
struct Coordinator<'a>{
some_value: u64,
school: &'a dyn School<'a>,
score_handler: &'a mut dyn ScoreHandler<'a>
}
impl Coordinator<'_>{
pub fn main(&mut self){
self.operate();
if self.some_value == 0 {
println!("0");
}
}
fn operate(&mut self){
self.score_handler.handle(self.school);
}
}

我有错误：

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
-> mytest/main/src/main.rs:29:28
|
29 |         self.score_handler.handle(self.school);
|                            ^^^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 28:5...
-> mytest/main/src/main.rs:28:5
|
28 | /     fn operate(&mut self){
29 | |         self.score_handler.handle(self.school);
30 | |     }
| |_____^
note: ...so that reference does not outlive borrowed content
-> mytest/main/src/main.rs:29:9
|
29 |         self.score_handler.handle(self.school);
|         ^^^^^^^^^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'_` as defined on the impl at 21:18...
--> mytest/main/src/main.rs:21:18
|
21 | impl Coordinator<'_>{
|                  ^^
note: ...so that the types are compatible
--> mytest/main/src/main.rs:29:28
|
29 |         self.score_handler.handle(self.school);
|                            ^^^^^^
= note: expected `&mut dyn ScoreHandler<'_>`
found `&mut dyn ScoreHandler<'_>`
...

所以我更改了<'_>到<'a>，像这样：

trait Class{
fn student_count(&self, )->usize;
fn student_score(&self, i: usize) ->u64;
}
trait School<'a>{
fn class_count(&self)->usize;
fn class(&'a self, i:usize)->&'a dyn Class;
}
trait ScoreHandler<'a> {
fn handle(&'a mut self, school: &'a dyn School<'a>);
}
struct Coordinator<'a>{
some_value: u64,
school: &'a dyn School<'a>,
score_handler: &'a mut dyn ScoreHandler<'a>
}
impl<'a> Coordinator<'a>{
pub fn main(&'a mut self){
self.operate();
if self.some_value == 0 {
println!("0");
}
}
fn operate(&'a mut self){
self.score_handler.handle(self.school);
}
}

我有错误：

error[E0503]: cannot use `self.some_value` because it was mutably borrowed
--> mytest/main/src/main.rs:24:12
|
21 | impl<'a> Coordinator<'a>{
|      -- lifetime `'a` defined here
22 |     pub fn main(&'a mut self){
23 |         self.operate();
|         --------------
|         |
|         borrow of `*self` occurs here
|         argument requires that `*self` is borrowed for `'a`
24 |         if self.some_value == 0 {
|            ^^^^^^^^^^^^^^^ use of borrowed `*self`
error[E0503]: cannot use `self.some_value` because it was mutably borrowed
--> mytest/main/src/main.rs:24:12
|
21 | impl<'a> Coordinator<'a>{
|      -- lifetime `'a` defined here
22 |     pub fn main(&'a mut self){
23 |         self.operate();
|         --------------
|         |
|         borrow of `*self` occurs here
|         argument requires that `*self` is borrowed for `'a`
24 |         if self.some_value == 0 {
|            ^^^^^^^^^^^^^^^ use of borrowed `*self`
...

我想知道是否有人能帮我找到一个好的解决方案，谢谢！

Answer 1

首先：我强烈建议您重新考虑您的类型架构。为什么Class和School特征？您的不同类是否有不同的实现细节，例如，一个类在内部使用Vec，另一个类使用HashMap？

正如rodrigo在评论中已经指出的那样，您可以简单地删除几个'a，它就会编译。原因是，在你的代码中，你可以确切地说，Coordinator::operate()的可变自借用将与Coordinator::main()一样长，但随后你在那里重新借用它。

然后看起来是这样的：

trait Class {
fn student_count(&self) -> usize;
fn student_score(&self, i: usize) -> u64;
}
trait School<'a> {
fn class_count(&self) -> usize;
fn class(&'a self, i: usize) -> &'a dyn Class;
}
trait ScoreHandler<'a> {
fn handle(&mut self, school: &'a dyn School<'a>);
}
struct Coordinator<'a> {
some_value: u64,
school: &'a dyn School<'a>,
score_handler: &'a mut dyn ScoreHandler<'a>,
}
impl<'a> Coordinator<'a> {
pub fn main(&mut self) {
self.operate();
if self.some_value == 0 {
println!("0");
}
}
fn operate(&mut self) {
self.score_handler.handle(self.school);
}
}

当然，您还必须定义一些实际的结构来实现这些特性，我将举一个例子：

struct ClassA {
student_scores: Vec<u64>,
}
impl Class for ClassA {
fn student_count(&self) -> usize {
self.student_scores.len()
}
fn student_score(&self, i: usize) -> u64 {
self.student_scores[i]
}
}
struct SchoolA<'a> {
classes: Vec<&'a dyn Class>,
}
impl<'a> School<'a> for SchoolA<'a> {
fn class_count(&self) -> usize {
self.classes.len()
}
fn class(&'a self, i: usize) -> &'a dyn Class {
self.classes[i]
}
}
struct ScoreHandlerA {
some_internal_state: u64,
}
impl<'a> ScoreHandler<'a> for ScoreHandlerA {
fn handle(&mut self, school: &'a dyn School<'a>) {
for i in 0..school.class_count() {
println!("Handling a class in a school");
}
}
}

再说一遍，你的主要可能看起来像这样：

fn main() {
let classes = vec![ClassA {
student_scores: vec![13, 14, 15],
}];
let school = SchoolA {
classes: classes.iter().map(|c| c as &dyn Class).collect(),
};
let mut coordinator = Coordinator {
some_value: 13,
school: &school,
score_handler: &mut ScoreHandlerA {
some_internal_state: 0,
},
};
coordinator.main();
}