Rust - Smart Pointers

Rust allocates everything on the stack by default. You can store things on the heap by wrapping them in smart pointers like Box. Types like Vec and String implicitly help heap allocation. Smart pointers implement traits listed in the table below. These traits of the smart pointers differentiate them from an ordinary struct −

In this chapter, we will learn about the Box smart pointer. We will also learn how to create a custom smart pointer like Box.

Box

The Box smart pointer also called a box allows you to store data on the heap rather than the stack. The stack contains the pointer to the heap data. A Box does not have performance overhead, other than storing their data on the heap.

Let us see how to use a box to store an i32 value on the heap.

fn main() {
   let var_i32 = 5; 
   //stack
   let b = Box::new(var_i32); 
   //heap
   println!("b = {}", b);
}

Output

b = 5

In order to access a value pointed by a variable, use dereferencing. The * is used as a dereference operator. Let us see how to use dereference with Box.

fn main() {
   let x = 5; 
   //value type variable
   let y = Box::new(x); 
   //y points to a new value 5 in the heap

   println!("{}",5==x);
   println!("{}",5==*y); 
   //dereferencing y
}

The variable x is a value-type with the value 5. So, the expression 5==x will return true. Variable y points to the heap. To access the value in heap, we need to dereference using *y. *y returns value 5. So, the expression 5==*y returns true.

Output

true
true

Illustration - Deref Trait

The Deref trait, provided by the standard library, requires us to implement one method named deref, that borrows self and returns a reference to the inner data. The following example creates a structure MyBox, which is a generic type. It implements the trait Deref. This trait helps us access heap values wrapped by y using *y.

use std::ops::Deref;
struct MyBox<T>(T);
impl<T> MyBox<T> { 
   // Generic structure with static method new
   fn new(x:T)-> MyBox<T> {
      MyBox(x)
   }
}
impl<T> Deref for MyBox<T> {
   type Target = T;
   fn deref(&self) -> &T {
      &self.0 //returns data
   }
}
fn main() {
   let x = 5;
   let y = MyBox::new(x); 
   // calling static method
   
   println!("5==x is {}",5==x);
   println!("5==*y is {}",5==*y); 
   // dereferencing y
   println!("x==*y is {}",x==*y);
   //dereferencing y
}

Output

5==x is true
5==*y is true
x==*y is true

Illustration - Drop Trait

The Drop trait contains the drop() method. This method is called when a structure that implemented this trait goes out of scope. In some languages, the programmer must call code to free memory or resources every time they finish using an instance of a smart pointer. In Rust, you can achieve automatic memory deallocation using Drop trait.

use std::ops::Deref;

struct MyBox<T>(T);
impl<T> MyBox<T> {
   fn new(x:T)->MyBox<T>{
      MyBox(x)
   }
}
impl<T> Deref for MyBox<T> {
   type Target = T;
      fn deref(&self) -< &T {
      &self.0
   }
}
impl<T> Drop for MyBox<T>{
   fn drop(&mut self){
      println!("dropping MyBox object from memory ");
   }
}
fn main() {
   let x = 50;
   MyBox::new(x);
   MyBox::new("Hello");
}

In the above example, the drop method will be called twice as we are creating two objects in the heap.

dropping MyBox object from memory
dropping MyBox object from memory