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Deref<T>

  • Deref<T> trait is used to customize the behavior of dereference operator (*).
  • If we implement the Deref<T> trait, then the smart pointer can be treated as a reference. Therefore, the code that works on the references can also be used on the smart pointers too.

Regular References

Regular reference is a kind of pointer that points to some value which is stored somewhere else. Let's see a simple example to create the reference of i32 type value and then we use the dereference operator with this reference.

Output:

a and *b are equal

In the above example, a holds the i32 type value, 20 while b contains the reference of 'a' variable. If we use *b, then it represents the value, 20. Therefore, we can compare the variable a and *b, and it will return the true value. If we use &b instead of *b, then the compiler throws an error "cannot compare {integer} with {&integer}".

Box<T> as a Reference

The Box<T> pointer can be used as a reference.

Let's see a simple example:

Output:

Value of *b is 11

In the above example, Box<T> behaves similarly as the regular references. The only difference between them is that b contains the box pointing to the data rather than referring to the value by using the '&' operator.

Smart Pointer as References

Now, we create the smart pointer similar to the Box<T> type, and we will see how they behave differently from the regular references.

  • The Box<T> can be defined as the tuple struct with one element for example, MyBox<T>.
  • After creating the tuple struct, we define the function on type MyBox<T>.

Let's see a simple example:

Output:

Rust Deref

In the above example, we create the smart pointer, b, but it cannot be dereferenced. Therefore, we conclude that the customized pointers which are similar to the Box<T> type cannot be dereferenced.

Implementing a Deref Trait

  • The Deref trait is defined in the standard library which is used to implement the method named deref.
  • The deref method borrows the self and returns a reference to the inner data.

Let's see a simple example:

Output:

10

Program Explanation

  • The Deref trait is implemented on the MyBox type.
  • The Deref trait implements the deref() method, and the deref() method returns the reference of 'a' variable.
  • The type Target = T; is an associated type for a Deref trait. Associated type is used to declare the generic type parameter.
  • We create the instance of MyBox type, b.
  • The deref() method is called by using the instance of MyBox type, b.deref() and then the reference which is returned from the deref() method is dereferenced.

Deref Coercion

  • Deref Coercion is a process of converting the reference that implements the Deref trait into the reference that Deref can convert the original type into.
  • Deref Coercion is performed on the arguments of the functions and methods.
  • Deref Coercion happens automatically when we pass the reference of a particular type to a function that does not match with the type of an argument in the function definition.

Let's see a simple example:

Output:

5

In the above example, we are calling the print(&b) function with an argument &b, which is the reference of &Box<i32>. In this case, we implement the Deref trait that converts the &Box<i32> into &i32 through the process of Deref Coercion.

Interaction of Derif Coercion with mutability

Till now, we use the Deref Trait to override the * operator on immutable references, and we can use the DerefMut trait to override the * operator on mutable references.

Rust performs Deref coercion in the following three cases:

  • When T: Deref<Target = U> where T and U are the immutable references , then &T is converted into &U type.
  • When T: DerefMut<Target = U> where T and U are the mutable references, then &mut T is converted into &mut U.
  • When T: Deref<Target = U> where T is a mutable reference and U is an immutable reference, then &mut T is converted into &U.

Note: Rust can coerce the mutable reference into the immutable reference, but it cannot coerce the immutable reference into the mutable reference because of the borrowing rules.


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