How to define the tokio spawn JoinHandle type ?

[fy2462]

Hi everybody:

I want to add the spawn handle in a HashMap, but I don't know how to define the type of map value.

I defined as follow:

let handle: JoinHandle<Send + 'static> = tokio::task::spawn(async move { 
     loop  {
             // do something
             // no return data
     }
});

The compiler remind me:

 handles_map: HashMap<String, Arc<JoinHandle<Send + 'static>>>,
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn std::marker::Send + 'static)`
note: required by a bound in `tokio::task::JoinHandle`
   --> /home/yanfeng/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.13.0/src/runtime/task/join.rs:144:27
    |
144 |     pub struct JoinHandle<T> {
    |                           ^ required by this bound in `tokio::task::JoinHandle`

error[E0277]: the size for values of type `(dyn std::marker::Send + 'static)` cannot be known at compilation time

How to define the JoinHandle<<???>> in HashMap?

[Darksonn]

The generic parameter in a JoinHandle is the return type of the task. It is () for tasks that don't return anything. If you don't know what the return type is, you can put any type in the spot and the compiler error will tell you what to put instead.

(It doesn't work when you put Send, but that's because Send is not a type — it's a trait.)

[fy2462]

Got it, thanks your help.