Usage of dyn in rust

Asked Apr 25 '20 at 12:27

Active Apr 25 '20 at 12:36

Viewed 41 times

Boxes allow you to store data on the heap rather than the stack. What remains on the stack is the pointer to the heap data.

// Returns some struct that implements Animal, but we don't know which one at compile time.
fn random_animal(random_number: f64) -> Box<dyn Animal> {
    if random_number < 0.5 {
        Box::new(Sheep {})
    } else {
        Box::new(Cow {})
    }
}

If box is anyways providing a pointer to the Heap data, and pointer size will be the same for both the struct on stack, why we can just return Box<Animal> from this function? I wish to know why I cannot write Box<Animal> instead of Box<dyn Animal>?

edited Apr 25 '20 at 12:36

Simson

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asked Apr 25 '20 at 12:27

GraphicalDot

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Can you be more specific on your concern? Do you wish to know why you can write `Box` instead of `Box`? – E_net4 Apr 25 '20 at 12:32
Made changes and yes, this is exactly my question. – GraphicalDot Apr 25 '20 at 12:33
They are equivalent. The `dyn` keyword was made to avoid confusion between a trait (`Foo`) and its trait type (`dyn Foo`). – E_net4 Apr 25 '20 at 12:35

Usage of dyn in rust

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