Allowing both static variables and boxes as a function argument?

Question

I have a struct, which sometimes I instantiate statically, and sometimes I'd like users to allocate on the heap. Is it possible to allow both in as arguments to a function?

pub struct MyData {
    x: i32
}

static ALLOCATED_STATICALLY: MyData = MyData {x: 1};

// what should my signature be?
fn use_data(instance: Box<MyData>) {
    println!("{}", instance.x);
}

fn main () {
    use_data(Box::new(MyData{x: 2}));
    // this doesn't work currently
    use_data(ALLOCATED_STATICALLY);
}

Answer 1

In both instances, you can pass a pointer to the function.

pub struct MyData {
    x: i32
}

static ALLOCATED_STATICALLY: MyData = MyData { x: 1 };

// what should my signature be?
fn use_data(instance: &MyData) {
    println!("{}", instance.x);
}

fn main () {
    use_data(&Box::new(MyData{ x: 2 }));
    use_data(&ALLOCATED_STATICALLY);
}

Note that in both cases, the caller needs to use the & operator to take the address of the value. In the first call, the operator yields a &Box<MyData>, but the compiler automatically converts it to a &MyData because Box implements the Deref trait.