Store any type in a struct field

Question

I'm working on code like the following:

use std::cell::RefCell;

struct CallbackWithArgs<T> {
    callback: Box<Fn(&mut T) -> ()>,
    arg: RefCell<T>,
}

struct S {
    args: CallbackWithArgs<_>,
}

The compiler has an error:

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/main.rs:9:28
  |
9 |     args: CallbackWithArgs<_>,
  |                            ^ not allowed in type signatures

What is the correct way to do this?

"So could anyone enlighten me with the correct way to do this?" What is "this"? What is it that you want to do? — mcarton, Jan 15 '18 at 08:22
The compiler error is something like `expect `std::any::Any + 'static`, found `std::any::Any``. Thus, after I set lifetime properly, this error has been fixed. I think the problem's gone. However, I'm still a little bewildered with the combination of lifetime and trait declaration in generic type declaration. — framlog, Jan 15 '18 at 12:16
[*The Rust Programming Language*](https://doc.rust-lang.org/book/second-edition/) has an [*entire chapter*](https://doc.rust-lang.org/book/second-edition/ch10-00-generics.html) on generics. — Shepmaster, Jan 15 '18 at 13:39

score 3 · Answer 1 · edited Jan 15 '18 at 13:36

3

You cannot use _ in a struct declaration; the compiler needs to know the struct's size at compile time.

If you want the type to be generic you can add a type parameter to S just as you did to CallbackWithArgs:

struct CallbackWithArgs<T> {
    callback: Box<Fn(&mut T) -> ()>,
    arg: RefCell<T>,
}

struct S<T> {
    args: CallbackWithArgs<T>,
}

Playground Link

For an explanation of _, see What is Vec<_>?

edited Jan 15 '18 at 13:36

Shepmaster

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answered Jan 15 '18 at 08:25

Neikos

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Yeap, I know I can't use `_` in that place. I just wanna express the idea of `Any`. Thanks anyway. – framlog Jan 15 '18 at 12:13

Store any type in a struct field

1 Answers1