How to convert a concrete boxed object to a boxed trait object

Question

There are definitively other ways to solve this problem but I was just curious whether there was any way to make the following code work in Rust in some way I do not know yet.

The following code sample ( playground )

use std::sync::{Arc, Mutex};

trait ProvidesFoo {
    fn magic(&mut self);
}

struct Foo {
    magic_value: u32
}

impl Default for Foo {
    fn default() -> Self {
        Self {magic_value: 42}
    }
}

impl ProvidesFoo for Foo {
    fn magic(&mut self) {
        println!("ProvidesFoo magic {}", self.magic_value);
    }
}

pub type SharedFooConcrete = Arc<Mutex<Box<Foo>>>;
pub type SharedFooTraitObj = Arc<Mutex<Box<dyn ProvidesFoo + Send + Sync>>>;

struct FooProvider {
    foo_as_trait_obj: SharedFooTraitObj

}

impl FooProvider {
    fn magic_and_then_some(&mut self) {
        let mut fooguard = self.foo_as_trait_obj.lock().unwrap();
        fooguard.magic();
        println!("Additional magic");
    }
}

fn uses_shared_foo_boxed_trait_obj(foo: SharedFooTraitObj) {
    let mut foo_provider = FooProvider {
        foo_as_trait_obj: foo
    };
    foo_provider.magic_and_then_some();
}
fn uses_shared_foo_concrete(foo: SharedFooConcrete) {
    let mut fooguard = foo.lock().unwrap();
    fooguard.magic();
}

fn main() {
    let shared_foo = Arc::new(Mutex::new(Box::new(Foo::default())));
    uses_shared_foo_concrete(shared_foo.clone());
    uses_shared_foo_boxed_trait_obj(shared_foo);
}

will fail to compile with the following error:

error[E0308]: mismatched types
  --> fsrc-example/src/bin/test2.rs:52:37
   |
52 |     uses_shared_foo_boxed_trait_obj(shared_foo);
   |     ------------------------------- ^^^^^^^^^^ expected trait object `dyn ProvidesFoo`, found struct `Foo`
   |     |
   |     arguments to this function are incorrect
   |
   = note: expected struct `Arc<Mutex<Box<(dyn ProvidesFoo + Send + Sync + 'static)>>>`
              found struct `Arc<Mutex<Box<Foo>>>`
note: function defined here

There is definitely a way to "cast" a boxed trait object back to its concrete type like shown here , but this is basically the other way around. I come from a C++ background, so I am familiar with this type of API where a derived object can be passed as a base class.

The other possible solution I mentioned and used was to have a wrapper struct which has the SharedFooTraitObject as a field and implements the magic_and_then_some() operation on top. One can then pass that wrapper struct around and clone the Arc<Mutex> ed field for library code which expects only the trait object.

I was just curious whether this type of type coercion / casting is possible in Rust.

Kind Regards, RM

Answer 1

I'm not certain I understand correctly what you want; sorry if I'm wrong.

I removed all the wrapper types in order to focus on reference to concrete type vs reference to dyn trait.

Providing a reference to a concrete type where a reference to a dyn trait is expected is implicitly handled by the language because all the information is provided at compile time. A fat pointer pointing to the appropriate virtual table can be provided since the concrete type is well known.

On the other hand, when using a dyn trait, the compiler does not know anymore the original type, then providing a reference to a dyn trait where a reference to a concrete type is expected is impossible.

If we really want to achieve this, we need some runtime check. The Any trait can help for this purpose. When we downcast , we are certain of the concrete type, then we can obtain a reference to this concrete type.

use std::any::Any;

trait MyTrait: Any {
    fn show(&self);
    fn action(&mut self);
    fn as_any(&self) -> &dyn Any;
    fn as_any_mut(&mut self) -> &mut dyn Any;
}

struct MyStructA {
    value: i32,
}

impl MyStructA {
    fn new(value: i32) -> Self {
        Self { value }
    }
}

impl MyTrait for MyStructA {
    fn show(&self) {
        println!("value is {}", self.value);
    }
    fn action(&mut self) {
        self.value += 1;
        self.show();
    }
    fn as_any(&self) -> &dyn Any {
        self
    }
    fn as_any_mut(&mut self) -> &mut dyn Any {
        self
    }
}

struct MyStructB {
    txt: String,
}

impl MyStructB {
    fn new(txt: String) -> Self {
        Self { txt }
    }
}

impl MyTrait for MyStructB {
    fn show(&self) {
        println!("txt is {}", self.txt);
    }
    fn action(&mut self) {
        self.txt.push_str(" •");
        self.show();
    }
    fn as_any(&self) -> &dyn Any {
        self
    }
    fn as_any_mut(&mut self) -> &mut dyn Any {
        self
    }
}

fn use_impl_trait(t: &mut impl MyTrait) {
    print!("impl: ");
    t.action();
}

fn use_dyn_trait(t: &mut dyn MyTrait) {
    print!("dyn: ");
    t.action();
}

fn main() {
    {
        println!("~~~~ using concrete types ~~~~");
        let mut msa = MyStructA::new(100);
        let msa_ref = &mut msa;
        use_dyn_trait(msa_ref);
        use_impl_trait(msa_ref);
        let mut msb = MyStructB::new("xxx".to_owned());
        let msb_ref = &mut msb;
        use_dyn_trait(msb_ref);
        use_impl_trait(msb_ref);
    }
    {
        println!("~~~~ using dynamic types ~~~~");
        let mut msa = MyStructA::new(200);
        let msa_dyn_ref: &mut dyn MyTrait = &mut msa;
        use_dyn_trait(msa_dyn_ref);
        if let Some(msa_ref) =
            msa_dyn_ref.as_any_mut().downcast_mut::<MyStructA>()
        {
            use_impl_trait(msa_ref);
        }
        let mut msb = MyStructB::new("yyy".to_owned());
        let msb_dyn_ref: &mut dyn MyTrait = &mut msb;
        use_dyn_trait(msb_dyn_ref);
        if let Some(msb_ref) =
            msb_dyn_ref.as_any_mut().downcast_mut::<MyStructB>()
        {
            use_impl_trait(msb_ref);
        }
    }
}
/*
~~~~ using concrete types ~~~~
dyn: value is 101
impl: value is 102
dyn: txt is xxx •
impl: txt is xxx • •
~~~~ using dynamic types ~~~~
dyn: value is 201
impl: value is 202
dyn: txt is yyy •
impl: txt is yyy • •
*/