rust - 尝试使用 Box<_> 应用多态性时出现错误 "cannot move a value ... the size cannot be statically determined"

Question

我正在应用多态性 solution在 Rust 中解决我的问题。我想将此解决方案与 Box<_> 一起使用因为它看起来最直接和简单，但它不起作用。

#[derive(Clone, Copy)]
pub struct NewPost;

#[derive(Clone, Copy)]
pub struct Post;

#[derive(Clone, Copy)]
pub struct PgConnection;

#[derive(Clone, Copy)]
pub struct DBPost;

pub trait DBAdapter {
    fn create(self, post: NewPost) -> Post;
    fn read(self) -> Vec<Post>;
}

impl DBPost {
    // DATABASE classes
    pub fn establish_connection(self) -> PgConnection {
        unimplemented!()
    }
}

impl DBAdapter for DBPost {
    fn create(self, _post: NewPost) -> Post {
        unimplemented!()
    }

    fn read(self) -> Vec<Post> {
        unimplemented!()
    }
}

struct GetPostsCase {
    db: Box<dyn DBAdapter>,
}

impl GetPostsCase {
    pub fn new(db: Box<dyn DBAdapter>) -> GetPostsCase {
        GetPostsCase { db: db }
    }

    pub fn run(&self) -> Vec<Post> {
        let result = self.db.read();
        result
    }
}

错误是:

error[E0161]: cannot move a value of type dyn DBAdapter: the size of dyn DBAdapter cannot be statically determined
  --> src/lib.rs:45:22
   |
45 |         let result = self.db.read();
   |                      ^^^^^^^

error[E0507]: cannot move out of `*self.db` which is behind a shared reference
  --> src/lib.rs:45:22
   |
45 |         let result = self.db.read();
   |                      ^^^^^^^ move occurs because `*self.db` has type `dyn DBAdapter`, which does not implement the `Copy` trait

Answer 1

您的 read 方法采用(未调整大小的)值而不是采用引用(其大小始终相同)。

可以通过更改DBAdapter的契约来解决问题

来自

fn read(self) -> Vec<Post> {

到

fn read(&self) -> Vec<Post> {
//      ^--- added the ampersand

(根据您的实现，您可能需要 &mut)