Sharing a struct with trait objects as properties across threads

Question

I have the code below. With the commented out parts, it\'s working. When I uncomment the parts it does not compile anymore.

How can I adjust the commented parts to m

Answer 1

I find the error message pretty straightforward:

• the trait std::marker::Send is not implemented for Expr + 'static
• required because of the requirements on the impl of std::marker::Send for std::sync::Arc<Expr + 'static>
• required because it appears within the type Container
• required because of the requirements on the impl of std::marker::Send for std::sync::Arc<Container>
• required because it appears within the type [closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]
• required by std::thread::spawn

You are trying to move your Arc<Container> to another thread, but it contains an Arc<Expr + 'static>, which cannot be guaranteed to be safely sent (Send) or shared (Sync) across threads.

Either add Send and Sync as supertraits to Expr:

pub trait Expr: Send + Sync { /* ... */ }

Or add them as trait bounds to your trait objects:

pub struct AddExpr {
    expr1: Box<Expr + Send + Sync>,
    expr2: Box<Expr + Send + Sync>,
}

impl AddExpr {
    pub fn new(expr1: Box<Expr + Send + Sync>, expr2: Box<Expr + Send + Sync>) -> Self {
        Self { expr1, expr2 }
    }
}

struct Container {
    x: i32,
    cached_expr: Arc<Expr + Send + Sync>,
}

See also:

• How can I share references across threads?
• Multithreaded application fails to compile with error-chain
• Is there any way to implement the Send trait for ZipFile?
• How do I share a generic struct between threads using Arc<Mutex<MyStruct<T>>>?