Subesh Yadav

Posted on Sep 17

Day 28 of #100DaysOfRust: Smart Pointers and Deref Coercion

#rust #webdev #learning #100daysofrust

Today’s learning was all about treating smart pointers like regular references by implementing the Deref trait, exploring deref coercion, and understanding how it interacts with mutability.

📌 Following References to Values

A regular reference (&T) points to a value stored elsewhere. To access the value, we use the dereference operator *:

fn main() {
    let x = 5;
    let y = &x;

    assert_eq!(5, x);
    assert_eq!(5, *y);
}

Here *y follows the reference to get the value 5. Without *, the comparison won’t compile because {integer} and &{integer} are different types.

📦 Using Box Like a Reference

Smart pointers like Box<T> behave similarly to references when dereferenced:

fn main() {
    let x = 5;
    let y = Box::new(x);

    assert_eq!(5, x);
    assert_eq!(5, *y);
}

🛠 Defining Our Own Smart Pointer

We can define our own smart pointer MyBox<T>:

struct MyBox<T>(T);

impl<T> MyBox<T> {
    fn new(x: T) -> MyBox<T> {
        MyBox(x)
    }
}

But trying to dereference *y will fail until we implement the Deref trait.

🔗 Implementing the Deref Trait

To enable dereferencing:

use std::ops::Deref;

impl<T> Deref for MyBox<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

Now *y works just like with Box<T>.

Behind the scenes:

*(y.deref())

This happens automatically whenever we use *.

🔄 Implicit Deref Coercions

Deref coercion allows automatic conversion between references:

From &T to &U when T: Deref<Target=U>
From &mut T to &mut U when T: DerefMut<Target=U>
From &mut T to &U when T: Deref<Target=U>

Example with MyBox<String>:

fn hello(name: &str) {
    println!("Hello, {name}!");
}

fn main() {
    let m = MyBox::new(String::from("Rust"));
    hello(&m); // Works because of deref coercion
}

Without coercion, we’d need:

hello(&(*m)[..]);

Much harder to read!

🔐 Deref and Mutability

&T → &U works with Deref.
&mut T → &mut U works with DerefMut.
&mut T → &U is also allowed.
❌ But &T → &mut U is not allowed, because it could break Rust’s borrowing rules.

This ensures safety and prevents undefined behavior.

🧠 Summary

Regular references require * to access the value.
Box<T> and custom smart pointers can implement Deref to behave like references.
Deref coercion simplifies function calls by automatically converting types.
Mutability rules ensure safety: mutable → immutable works, but immutable → mutable does not.

Rust’s deref system is a zero-cost abstraction: all conversions happen at compile time with no runtime cost.

🚀 That’s all for Day 28! Tomorrow, I’ll continue exploring more about **smart pointers and their advanced use

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