本文介绍了如何指示异步函数返回值的生命周期与参数相同?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我们可以像这样用非静态参数匹配一个普通函数:
We can match a normal function with non-static parameter like this:
fn processor(data: &i32) -> &i32 {
data
}
fn process<'b>(data: &'b i32, processor: impl 'static + for<'a> Fn(&'a i32) -> &'a i32) -> &'b i32 {
processor(data)
}
fn main() {
let data = 1;
println!("data: {}", process(&data, processor));
}
由于异步函数返回匿名future,我们不能表明匿名future的生命周期与参数相同:
Since async functions return anonymous futures, we cannot indicate that the lifetime of anonymous future is the same as the parameter:
use std::future::Future;
async fn processor(data: &i32) -> &i32 {
data
}
async fn process<'b, F>(data: &'b i32, processor: impl 'static + Fn(&i32) -> F) -> &'b i32
where
F: 'b + Future<Output = &'b i32>,
{
processor(data).await
}
async fn _main() {
let data = 1;
println!("data: {}", process(&data, processor).await);
}
编译器会抱怨:
error[E0271]: type mismatch resolving `for<'r> <for<'_> fn(&i32) -> impl std::future::Future {processor} as std::ops::FnOnce<(&'r i32,)>>::Output == _`
--> src/lib.rs:16:26
|
7 | async fn process<'b, F>(data: &'b i32, processor: impl 'static + Fn(&i32) -> F) -> &'b i32
| ------- - required by this bound in `process`
...
16 | println!("data: {}", process(&data, processor).await);
| ^^^^^^^ expected bound lifetime parameter, found concrete lifetime
我该如何匹配?
推荐答案
您需要声明:
- 闭包接受与参数具有相同生命周期的引用.
- 返回的 future 返回一个与参数具有相同生命周期的引用.
- 返回的 future 捕获与参数具有相同生命周期的引用.
async fn process<'b, F, Fut>(data: &'b i32, processor: F) -> &'b i32
where
F: Fn(&'b i32) -> Fut,
// ^^ [1]
F: 'static,
Fut: Future<Output = &'b i32> + 'b,
// ^^ [2] ^^ [3]
{
processor(data).await
}
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