[英]Writing generic implementation of sum on an iterator in Rust
I have been learning Rust, coming from a Swift, C and C++ background.我一直在学习 Rust,来自 Swift、C 和 C++ 背景。 I feel like I have a basic understanding of ownership, borrowing and traits.
我觉得我对所有权、借用和特征有了基本的了解。 To exercise a bit, I decided to implement a
sum
function on a generic slice [T]
where T
has a default value and can be added to itself.为了练习一下,我决定在通用切片
[T]
上实现sum
function,其中T
具有默认值并且可以添加到自身。
This is how far I got:这是我走了多远:
trait Summable {
type Result;
fn sum(&self) -> Self::Result;
}
impl<T> Summable for [T]
where
T: Add<Output = T> + Default,
{
type Result = T;
fn sum(&self) -> T {
let x = T::default();
self.iter().fold(x, |a, b| a + b)
}
}
Compiler complains with expected type parameter T, found &T
for a + b
.编译器抱怨
expected type parameter T, found &T
for a + b
。
I understand why the error happens, but not exactly how to fix it.我明白为什么会发生错误,但不知道如何解决它。 Yes, the type of
x
is T
.是的,
x
的类型是T
。 It cannot be &T
because, if nothing else, if the slice is empty, that's the value that is returned and I cannot return a reference to something created inside the function. Plus, the default function returns a new value that the code inside the function owns.它不能是
&T
因为,如果没有别的,如果切片是空的,那就是返回的值,我不能返回对 function 内部创建的东西的引用。另外,默认 function 返回一个新值,function 内部的代码拥有。 Makes sense.说得通。 And yes,
b
should be a shared reference to the values in the slice since I don't want to consume them (not T
) and I don't want to mutate them (not &mut T
).是的,
b
应该是对切片中值的共享引用,因为我不想使用它们(不是T
)并且我不想改变它们(不是&mut T
)。
But that means I need to add T
to &T
, and return a T
because I am returning a new value (the sum) which will be owned by the caller.但这意味着我需要将
T
添加到&T
,并返回一个T
因为我要返回一个将由调用者拥有的新值(总和)。 How?如何?
PS: Yes, I know this function exists already. PS:是的,我知道这个 function 已经存在了。 This is a learning exercise.
这是一个学习练习。
The std::ops::Add
trait has an optional Rhs
type parameter that defaults to Self
: std::ops::Add
特性有一个可选的Rhs
类型参数,默认为Self
:
pub trait Add<Rhs = Self> {
type Output;
fn add(self, rhs: Rhs) -> Self::Output;
}
Because you've omitted the Rhs
type parameter from the T: Add<Output = T>
bound, it defaults to T
: hence to your a
you can add a T
, but not an &T
.因为您从
T: Add<Output = T>
绑定中省略了Rhs
类型参数,所以它默认为T
: 因此您可以在a
中添加T
,但不能添加&T
。
Either specify that T: for<'a> Add<&'a T, Output = T>
;要么指定
T: for<'a> Add<&'a T, Output = T>
; or else somehow obtain an owned T
from b
, eg via T: Copy
or T: Clone
.或者以某种方式从
b
获取拥有的T
,例如通过T: Copy
或T: Clone
。
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