仍用 在线环境 运行。看 dyn 的介绍，似乎是在指定函数返回类型时，额外说明“类型不一定”，估计是 dynamic 的缩写。强烈建议关键词尽量避免缩写！至少在官方文档里说清楚。
上来运行了七八遍，寻思怎么老是baaaah？不是随机后看 <0.5 么？再看一眼随机数如何生成。。。命名能不能不那么误导。直接把 random_number 改名为某数，感觉好点。
struct 羊 {}
struct 牛 {}
trait 动物 {
// Instance method signature
fn 嘶鸣(&self) -> &'static str;
}
// Implement the `动物` trait for `羊`.
impl 动物 for 羊 {
fn 嘶鸣(&self) -> &'static str {
"baaaaah!"
}
}
// Implement the `动物` trait for `牛`.
impl 动物 for 牛 {
fn 嘶鸣(&self) -> &'static str {
"moooooo!"
}
}
// Returns some struct that implements 动物, but we don't know which one at compile time.
fn 抽个动物(某数: f64) -> Box<dyn 动物> {
if 某数 < 0.5 {
Box::new(羊 {})
} else {
Box::new(牛 {})
}
}
fn main() {
let 某数 = 0.234;
let 动物 = 抽个动物(某数);
println!("You've randomly chosen an animal, and it says {}", 动物.嘶鸣());
}
如果去掉 Box<dyn>，报错如下，果然 type 和 trait 含义不同：
error[E0782]: expected a type, found a trait
--> src/main.rs:24:21
|
24 | fn 抽个动物(某数: f64) -> 动物 {
| ^^^^
|
help: use `impl 动物` to return an opaque type, as long as you return a single underlying type
|
24 | fn 抽个动物(某数: f64) -> impl 动物 {
| ++++
help: alternatively, you can return an owned trait object
|
24 | fn 抽个动物(某数: f64) -> Box<dyn 动物> {
| +++++++ +
For more information about this error, try `rustc --explain E0782`.
如果用第一个建议 -> impl 动物，则提示函数体不同分支的返回类型（type）不同：
error[E0308]: `if` and `else` have incompatible types
--> src/main.rs:28:9
|
25 | / if 某数 < 0.5 {
26 | | Box::new(羊 {})
| | --------------- expected because of this
27 | | } else {
28 | | Box::new(牛 {})
| | ^^^^^^^^^^^^^^^ expected `Box<羊>`, found `Box<牛>`
29 | | }
| |_____- `if` and `else` have incompatible types
|
= note: expected struct `Box<羊>`
found struct `Box<牛>`
For more information about this error, try `rustc --explain E0308`.
有点意外的是，此次报错的建议里没有提到用 dyn。照理说可以分析出 羊 和 牛 实现了同样的 trait，就可以像上面那样提示用 dyn，不过“owned trait object”不知啥意思.
另外，impl xx for xxx 挺接近自然语法，不知自然语言里这么常用的 for 还用于其他语法不。
再说缩写，看 impl、fn 似乎是想将关键词长度控制在六个字母（如struct）以内，但 str 有必要么？随便点点其他章节，结果发现 这里:
fn university(&self) -> String;
难道 str 不是 String 吗？!
回头再看一下代码，-> &'static str 重复了三次，挺扎眼。
于是把这里的一处 ->... 去掉试试：
impl 动物 for 羊 {
fn 嘶鸣(&self) -> &'static str {
"baaaaah!"
}
}
报错如下：
error[E0053]: method `嘶鸣` has an incompatible type for trait
--> src/main.rs:11:17
|
11 | fn 嘶鸣(&self) {
| ^ expected `&'static str`, found `()`
|
note: type in trait
--> src/main.rs:6:21
|
6 | fn 嘶鸣(&self) -> &'static str;
| ^^^^^^^^^^^^
= note: expected signature `fn(&羊) -> &'static str`
found signature `fn(&羊) -> ()`
help: change the output type to match the trait
|
11 | fn 嘶鸣(&self) -> &'static str {
| +++++++++++++++
error[E0308]: mismatched types
--> src/main.rs:12:9
|
11 | fn 嘶鸣(&self) {
| - expected `()` because of default return type
12 | "baaaaah!"
| ^^^^^^^^^^ expected `()`, found `&str`
Some errors have detailed explanations: E0053, E0308.
For more information about an error, try `rustc --explain E0053`.
如果 impl 动物 for 羊 时，里面实现的 嘶鸣 必须和 动物 的同名方法有相同签名的话，为啥不能省略掉呢？
从 “expected (), found &str” 看起来 "baaaaah!" 的返回类型是 &str，那么如果把 -> &'static str 都改成 &str，应该没问题吧。
运行的确如初。看来 'static 就是强调返回值为静态。
如果把 -> 全都去掉的话，除了上面的第二个，还有显示方法必须要匹配返回类型：
error[E0277]: `()` doesn't implement `std::fmt::Display`
--> src/main.rs:35:66
|
35 | println!("You've randomly chosen an animal, and it says {}", 动物.嘶鸣());
| ^^^^^^^^^^^ `()` cannot be formatted with the default formatter
|
= help: the trait `std::fmt::Display` is not implemented for `()`
= note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
= 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)
Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
话说这个 () 算是个啥类型？术语体系里有个命名吗？
最后试了把 -> Box<dyn 动物> 去掉，报错如下：
error[E0308]: mismatched types
--> src/main.rs:26:9
|
26 | Box::new(羊 {})
| ^^^^^^^^^^^^^^^ expected `()`, found `Box<羊>`
|
= note: expected unit type `()`
found struct `Box<羊>`
help: consider using a semicolon here
|
26 | Box::new(羊 {});
| +
help: try adding a return type
|
24 | fn 抽个动物(某数: f64) -> Box<羊> {
| ++++++++++
error[E0308]: mismatched types
--> src/main.rs:28:9
|
28 | Box::new(牛 {})
| ^^^^^^^^^^^^^^^ expected `()`, found `Box<牛>`
|
= note: expected unit type `()`
found struct `Box<牛>`
help: consider using a semicolon here
|
28 | Box::new(牛 {});
| +
help: try adding a return type
|
24 | fn 抽个动物(某数: f64) -> Box<牛> {
| ++++++++++
error[E0599]: no method named `嘶鸣` found for unit type `()` in the current scope
--> src/main.rs:35:69
|
35 | println!("You've randomly chosen an animal, and it says {}", 动物.嘶鸣());
| ^^^^ method not found in `()`
|
= help: items from traits can only be used if the trait is implemented and in scope
note: `动物` defines an item `嘶鸣`, perhaps you need to implement it
--> src/main.rs:4:1
|
4 | trait 动物 {
| ^^^^^^^^^^
Some errors have detailed explanations: E0308, E0599.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `playground` (bin "playground") due to 3 previous errors
整整五十行！！感觉这种报错反馈风格对于新手来说视觉和心理上冲击会比较大，尤其是非英文母语开发者。
不过发现了 () 的术语：unit type。
得记着 str 和 String 的疑问，先到这里。