The Rust Programming Language

The second trait important to the smart pointer pattern is Drop, which lets us customize what happens when a value is about to go out of scope. We can provide an implementation for the Drop trait on any type, and the code we specify can be used to release resources like files or network connections. We’re introducing Drop in the context of smart pointers because the functionality of the Drop trait is almost always used when implementing a smart pointer. For example, Box<T> customizes Drop to deallocate the space on the heap that the box points to.

In some languages, the programmer must call code to free memory or resources every time they finish using an instance of a smart pointer. If they forget, the system might become overloaded and crash. In Rust, we can specify that a particular bit of code should be run whenever a value goes out of scope, and the compiler will insert this code automatically. As a result, we don’t need to be careful about placing cleanup code everywhere in a program that an instance of a particular type is finished with, but we still won’t leak resources!

We specify the code to run when a value goes out of scope by implementing the Drop trait. The Drop trait requires us to implement one method named drop that takes a mutable reference to self. To see when Rust calls drop, let’s implement drop with println! statements for now.

Listing 15-14 shows a CustomSmartPointer struct whose only custom functionality is that it will print Dropping CustomSmartPointer! when the instance goes out of scope. This example demonstrates when Rust runs the drop function:

Filename: src/main.rs

struct CustomSmartPointer {
    data: String,
}

impl Drop for CustomSmartPointer {
    fn drop(&mut self) {
        println!("Dropping CustomSmartPointer with data `{}`!", self.data);
    }
}

fn main() {
    let c = CustomSmartPointer { data: String::from("my stuff") };
    let d = CustomSmartPointer { data: String::from("other stuff") };
    println!("CustomSmartPointers created.");
}

Listing 15-14: A CustomSmartPointer struct that implements the Drop trait where we would put our cleanup code

The Drop trait is included in the prelude, so we don’t need to import it. We implement the Drop trait on CustomSmartPointer and provide an implementation for the drop method that calls println!. The body of the drop function is where you would place any logic that you wanted to run when an instance of your type goes out of scope. We’re printing some text here to demonstrate when Rust will call drop.

In main, we create two instances of CustomSmartPointer and then print CustomSmartPointers created.. At the end of main, our instance of CustomSmartPointer will go out of scope, and Rust will call the code we put in the drop method, printing our final message. Note that we didn’t need to call the drop method explicitly.

When we run this program, we’ll see the following output:

CustomSmartPointers created.
Dropping CustomSmartPointer with data `other stuff`!
Dropping CustomSmartPointer with data `my stuff`!

Rust automatically called drop for us when our instance went out of scope, calling the code we specified. Variables are dropped in the reverse order of the order in which they were created, so d was dropped before c. This example just gives you a visual guide to how the drop method works, but usually you would specify the cleanup code that your type needs to run rather than a print message.

Unfortunately, it’s not straightforward to disable the automatic drop functionality. Disabling drop isn’t usually necessary; the whole point of the Drop trait is that it’s taken care of automatically. Occasionally, you might want to clean up a value early. One example is when using smart pointers that manage locks: you might want to force the drop method that releases the lock to run so other code in the same scope can acquire the lock. Rust doesn’t let us call the Drop trait’s drop method manually; instead we have to call the std::mem::drop function provided by the standard library if we want to force a value to be dropped before the end of its scope.

Let’s see what happens when we try to call the Drop trait’s drop method manually by modifying the main function in Listing 15-14, as shown in Listing 15-15:

Filename: src/main.rs

fn main() {
    let c = CustomSmartPointer { data: String::from("some data") };
    println!("CustomSmartPointer created.");
    c.drop();
    println!("CustomSmartPointer dropped before the end of main.");
}

Listing 15-15: Attempting to call the drop method from the Drop trait manually to clean up early

When we try to compile this code, we’ll get this error:

error[E0040]: explicit use of destructor method
  --> src/main.rs:14:7
   |
14 |     c.drop();
   |       ^^^^ explicit destructor calls not allowed

This error message states that we’re not allowed to explicitly call drop. The error message uses the term destructor, which is the general programming term for a function that cleans up an instance. A destructor is analogous to a constructor that creates an instance. The drop function in Rust is one particular destructor.

Rust doesn’t let us call drop explicitly because Rust would still automatically call drop on the value at the end of main. This would be a double free error because Rust would be trying to clean up the same value twice.

We can’t disable the automatic insertion of drop when a value goes out of scope, and we can’t call the drop method explicitly. So, if we need to force a value to be cleaned up early, we can use the std::mem::drop function.

The std::mem::drop function is different than the drop method in the Drop trait. We call it by passing the value we want to force to be dropped early as an argument. The function is in the prelude, so we can modify main in Listing 15-14 to call the drop function, as shown in Listing 15-16:

Filename: src/main.rs

# struct CustomSmartPointer {
#     data: String,
# }
#
# impl Drop for CustomSmartPointer {
#     fn drop(&mut self) {
#         println!("Dropping CustomSmartPointer!");
#     }
# }
#
fn main() {
    let c = CustomSmartPointer { data: String::from("some data") };
    println!("CustomSmartPointer created.");
    drop(c);
    println!("CustomSmartPointer dropped before the end of main.");
}

Listing 15-16: Calling std::mem::drop to explicitly drop a value before it goes out of scope

Running this code will print the following:

CustomSmartPointer created.
Dropping CustomSmartPointer with data `some data`!
CustomSmartPointer dropped before the end of main.

The text Dropping CustomSmartPointer with data `some data`! is printed between the CustomSmartPointer created. and CustomSmartPointer dropped before the end of main. text, showing that the drop method code is called to drop c at that point.

We can use code specified in a Drop trait implementation in many ways to make cleanup convenient and safe: for instance, we could use it to create our own memory allocator! With the Drop trait and Rust’s ownership system, we don’t have to remember to clean up because Rust does it automatically.

We also don’t have to worry about accidentally cleaning up values still in use because that would cause a compiler error: the ownership system that makes sure references are always valid also ensures that drop gets called only once when the value is no longer being used.

Now that we’ve examined Box<T> and some of the characteristics of smart pointers, let’s look at a few other smart pointers defined in the standard library.