我想使用泛型计算阶乘，但在 main 中出现错误。
我的完整代码:
pub trait Body {
fn new() -> Self;
fn fact(&self, x: usize) -> usize {
match x {
1 => 1,
_ => x * self.fact(x - 1),
}
}
}
#[derive(Clone, Debug)]
pub struct RecursiveCall<T: Body> {
level: usize,
indicator: String,
n_repeat: usize,
body: T,
}
impl<T> RecursiveCall<T>
where T: Body
{
fn new(n_repeat: usize) -> RecursiveCall<T> {
RecursiveCall {
level: 0,
indicator: "- ".to_string(),
n_repeat: n_repeat,
body: <T as Body>::new(),
}
}
fn pre_trace(&self, fname: &str, arg: &usize) {
let args: String = arg.to_string();
println!("{}",
(vec![self.indicator.as_str(); self.level]).join("") +
self.level.to_string().as_str() + ":" + fname + "(" +
args.as_str() + ")");
}
fn post_trace(&self, fname: &str, arg: &usize, ret: &usize) {
println!("{}",
(vec![self.indicator.as_str(); self.level]).join("") +
self.level.to_string().as_str() + ":" + fname + "=" +
ret.to_string().as_str());
}
fn print_trace(&mut self) {
&self.pre_trace("fact", &self.n_repeat);
self.level += 1;
let ret = &self.body.fact(self.n_repeat);
self.level -= 1;
&self.post_trace("fact", &self.n_repeat, ret);
println!("Difference={}", &ret.to_string().as_str());
}
}
type B = Body;
fn main() {
let t = RecursiveCall::<B>::new();
}
此错误发生在main()中:
error: no associated item named `new` found for type `RecursiveCall<Body + 'static>` in the current scope
--> src/main.rs:61:13
|
61 | let t = RecursiveCall::<B>::new();
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= note: the method `new` exists but the following trait bounds were not satisfied: `Body : std::marker::Sized`, `Body : Body`
= help: items from traits can only be used if the trait is implemented and in scope; the following traits define an item `new`, perhaps you need to implement one of them:
= help: candidate #1: `Body`
= help: candidate #2: `std::sys_common::thread_info::NewThread`
= help: candidate #3: `std::iter::ZipImpl`
error[E0277]: the trait bound `Body + 'static: std::marker::Sized` is not satisfied
--> src/main.rs:61:13
|
61 | let t = RecursiveCall::<B>::new();
| ^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Sized` is not implemented for `Body + 'static`
|
= note: `Body + 'static` does not have a constant size known at compile-time
= note: required by `RecursiveCall`
error[E0277]: the trait bound `Body + 'static: Body` is not satisfied
--> src/main.rs:61:13
|
61 | let t = RecursiveCall::<B>::new();
| ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Body` is not implemented for `Body + 'static`
|
= note: required by `RecursiveCall`
error[E0038]: the trait `Body` cannot be made into an object
--> src/main.rs:61:13
|
61 | let t = RecursiveCall::<B>::new();
| ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Body` cannot be made into an object
|
= note: method `new` has no receiver
最佳答案
答案的关键在编译器的注释中:
note: the method
newexists but the following trait bounds were not satisfied:Body : std::marker::Sized,Body : Body
您已定义 B作为 Body 的类型别名，所以名称 B和Body两者在您的程序中含义相同(至少在本模块中)。
当你定义一个特征时，编译器也会定义一个同名的类型。但是，该类型无法直接实例化(与 C++/C#/Java/等中的类不同)。然而，这正是您想要做的!
特征界限Body : std::marker::Sized不满意，因为Body是编译器定义的与同名特征相对应的类型，是无大小类型。无大小类型只能在指针和引用中使用(例如 &Body 、 Box<Body> 等)。
特征界限Body : Body不满意，因为你的特质不是object-safe 。它不是对象安全的，因为方法 new没有self参数(这就是编译器在最后一个注释中的含义: method `new` has no receiver )。
通常，您会定义一个结构体或枚举并实现该类型的特征，然后在实例化 RecursiveCall 时使用该类型。尝试替换type B = Body;具有以下内容:
struct B;
impl Body for B {
fn new() -> B {
B
}
}
关于rust - 我该如何对待泛型？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42380121/