When do I need to use a dereference in rust?

Question

In the following example we are using a reference to self in the println! and show_type functions yet we aren't dereferencing self with * to get the value.

Why aren't we using dereference in the example?

When do we need to use a dereference?

How can we print to screen if a variable is holding a reference or a value?

struct Animal<T> { name: T, } impl<T: Display> Animal<T> { fn show_type(&self) { println!("{}", self.name); println!("{}", type_name::<T>()); } } fn main() { let dog = Animal { name: String::from("Rex") }; dog.show_type(); // Result: Rex, alloc::string::String }

Masklinn · Accepted Answer · 2022-02-22 13:42:35Z

Why aren't we using dereference in the example?

Because it's not necessary.

When do we need to use a dereference?

When the rust compiler does not auto-deref. Generally the rust compiler will auto-deref on attribute access and method calls (but only the "subject" of the call, parameters are not auto-deref'd). Though sometimes it's necessary to disambiguate.

How can we print to screen if a valubale is holding a reference or a value?

Because it's set up that way

impl<'_, T> Display for &'_ T where T: Display + ?Sized

So there is an implementation of Display for any reference to something which implements Display, and it simply delegates through the reference.

freakish · Accepted Answer · 2022-02-22 13:16:45Z

Field access expressions do automatic dereferncing in Rust: https://doc.rust-lang.org/reference/expressions/field-expr.html#automatic-dereferencing

And so I guess you would dereference manually when you need a whole value, not a specific field.

Note also that println! implicitely takes its arguments by reference.

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