此问题在此处已有答案:
How to type None if it expects a function type as type argument?(3个答案)
20小时前关闭。
我有Option<impl Fn(i64) -> i64>
作为函数apply()
的参数,我想传递None
的值。apply(None)
.如何正确编写apply(None)
函数调用,以便它编译?
fn main(){
let a = apply(Some(|i| i + 1));
let b = apply(None);
let b2 = apply(None::<dyn Fn(i64) -> i64>);
println!("a: {0}; b: {1}", a, b)
}
fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
let i = 0_i64;
match f {
Some(f) => f(i),
None => i,
}
}
字符串
编译错误:
Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
--> src/main.rs:4:16
|
4 | let b = apply(None);
| ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
|
help: consider specifying the generic argument
|
4 | let b = apply(None::<T>);
| +++++
error[E0283]: type annotations needed
--> src/main.rs:4:16
|
4 | let b = apply(None);
| ----- ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
| |
| required by a bound introduced by this call
|
= note: multiple `impl`s satisfying `_: Fn<(i64,)>` found in the following crates: `alloc`, `core`:
- impl<A, F> Fn<A> for &F
where A: Tuple, F: Fn<A>, F: ?Sized;
- impl<Args, F, A> Fn<Args> for Box<F, A>
where Args: Tuple, F: Fn<Args>, A: Allocator, F: ?Sized;
note: required by a bound in `apply`
--> src/main.rs:10:25
|
10 | fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
| ^^^^^^^^^^^^^^ required by this bound in `apply`
help: consider specifying the generic argument
|
4 | let b = apply(None::<T>);
| +++++
Some errors have detailed explanations: E0282, E0283.
For more information about an error, try `rustc --explain E0282`.
型
我试过let b2 = apply(None::<dyn Fn(i64) -> i64>);
但它仍然不编译错误是
Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `dyn Fn(i64) -> i64` cannot be known at compilation time
--> src/main.rs:5:20
|
5 | let b2 = apply(None::<dyn Fn(i64) -> i64>);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `Sized` is not implemented for `dyn Fn(i64) -> i64`
note: required by a bound in `None`
--> /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/option.rs:567:5
For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to previous error
型
1条答案
按热度按时间yqkkidmi1#
类型
dyn Fn(i64) -> i64
是unsized,这意味着你不能将它存储在像Option
这样的枚举中。但是trait对象引用是有大小的,所以你可以使用
&dyn
来代替:字符串
但是,使用函数指针类型可能更好:
型
playground