这是来自尝试 Rustlings 运动特征 2。该练习要求我在
Vec
上实现该特征。测试已经存在,但它们失败了,这是一个很好的起点。我已经完成了 String
的特征实现,这很容易,但是 Vec
是另一个故事了。我不确定该方法需要返回什么,它会因各种返回而失败。我提供原始代码、我的尝试以及我的尝试中遇到的错误。希望这就足够了。
来自 Rustlings 存储库的原始代码:
// traits2.rs
//
// Your task is to implement the trait
// `AppendBar' for a vector of strings.
//
// To implement this trait, consider for
// a moment what it means to 'append "Bar"'
// to a vector of strings.
//
// No boiler plate code this time,
// you can do this!
// I AM NOT DONE
trait AppendBar {
fn append_bar(self) -> Self;
}
//TODO: Add your code here
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn is_vec_pop_eq_bar() {
let mut foo = vec![String::from("Foo")].append_bar();
assert_eq!(foo.pop().unwrap(), String::from("Bar"));
assert_eq!(foo.pop().unwrap(), String::from("Foo"));
}
}
以及我尝试解决它的方法:
// traits2.rs
//
// Your task is to implement the trait
// `AppendBar' for a vector of strings.
//
// To implement this trait, consider for
// a moment what it means to 'append "Bar"'
// to a vector of strings.
//
// No boiler plate code this time,
// you can do this!
// I AM NOT DONE
use std::clone::Clone;
trait AppendBar {
fn append_bar(&mut self) -> Self;
}
//TODO: Add your code here
impl<T: Clone> AppendBar for Vec<T> {
fn append_bar(&mut self) -> Self {
let bar: T = String::from("Bar");
self.to_vec().push(bar)
// self.to_vec()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn is_vec_pop_eq_bar() {
let mut foo = vec![String::from("Foo")].append_bar();
assert_eq!(foo, vec![String::from("Foo"), String::from("Bar")]);
assert_eq!(foo.pop().unwrap(), String::from("Bar"));
assert_eq!(foo.pop().unwrap(), String::from("Foo"));
}
}
编译会出现错误:
! Compiling of exercises/traits/traits2.rs failed! Please try again. Here's the output:
error[E0308]: mismatched types
--> exercises/traits/traits2.rs:22:22
|
20 | impl<T: Clone> AppendBar for Vec<T> {
| - this type parameter
21 | fn append_bar(&mut self) -> Self {
22 | let bar: T = String::from("Bar");
| - ^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found struct `std::string::String`
| |
| expected due to this
|
= note: expected type parameter `T`
found struct `std::string::String`
= help: type parameters must be constrained to match other types
= note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
error[E0308]: mismatched types
--> exercises/traits/traits2.rs:23:9
|
21 | fn append_bar(&mut self) -> Self {
| ---- expected `std::vec::Vec<T>` because of return type
22 | let bar: T = String::from("Bar");
23 | self.to_vec().push(bar)
| ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::vec::Vec`, found `()`
|
= note: expected struct `std::vec::Vec<T>`
found unit type `()`
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0308`.
我已经阅读并重新阅读了书中的建议部分和特征,但它超出了我的范围。我确信这是一个简单的解决方案,但我看不到它。
有几个问题:
String
推入通用 Vec<T>
,其中 T
可以是任何类型!fn append_bar(&mut self) -> Self
但应该是
fn append_bar(self) -> Self
Vec::push
的结果,但此方法不返回任何内容。要解决第一个问题,请实现
Vec<String>
而不是 Vec<T>
的特征。这就是作业的要求:
// Your task is to implement the trait // `AppendBar' for a vector of strings.
要解决第二个问题,您必须删除
&
,因此该方法接受 owned 值。
要解决最后一个问题,请在单独的语句中调用
self
后返回 Vec::push
:
self.push(bar);
self
Aloso 的答案存在差异。安德烈也给了。
当你接受
self
时:
fn append_bar(self) -> Self {
self.push("Bar".to_owned());
self
}
你正在接受一个可变的
Vec
:
let mut foo = vec![String::from("Foo")].append_bar();
assert_eq!(foo.pop().unwrap(), String::from("Bar"));
assert_eq!(foo.pop().unwrap(), String::from("Foo"));
即使变量
foo
被声明为可变的,方法append_bar()
也接受一个不可变的变量。您不需要借用 self
,因为您并不试图获得完全所有权,而是试图修改该变量中驻留的现有数据。正确答案是
fn append_bar(mut self) -> Self {
self.push("Bar".to_owned()); // || .to_string() || String::from("Bar")
// Whatever gets the point across. As the String literal is essentially a "Borrowed" string.
self
}
在
append_bar()
的范围内,您尝试改变 String
的集合并使用附加字符串返回它。
我相信这个问题的正确答案应该是这样的:
trait AppendBar {
fn append_bar(self) -> Self;
}
impl AppendBar for Vec<String> {
fn append_bar(mut self) -> Self {
self.push("Bar".to_string());
self
}
}
感谢@Aloso 的帮助和Jussi,我成功地让这个例子工作了。
为了编译需要进行突变,所以我最终得到了编译如下的代码:
// traits2.rs
//
// Your task is to implement the trait
// `AppendBar' for a vector of strings.
//
// To implement this trait, consider for
// a moment what it means to 'append "Bar"'
// to a vector of strings.
//
// No boiler plate code this time,
// you can do this!
// I AM NOT DONE
trait AppendBar {
fn append_bar(&mut self) -> Self;
}
//TODO: Add your code here
impl AppendBar for Vec<String> {
fn append_bar(&mut self) -> Self {
self.push(String::from("Bar"));
self.to_vec()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn is_vec_pop_eq_bar() {
let mut foo = vec![String::from("Foo")].append_bar();
assert_eq!(foo, vec![String::from("Foo"), String::from("Bar")]);
assert_eq!(foo.pop().unwrap(), String::from("Bar"));
assert_eq!(foo.pop().unwrap(), String::from("Foo"));
}
}