Rust - How to check for NULL values

Last Updated on January 30, 2020

Contents

1 NULL not supported
- 1.1 Rust Enforces Variable Initialization
  - 1.1.1 Compile Error 1
  - 1.1.2 Compile Error 2
2 Using Enum Option
3 Summary

NULL not supported

Rust does not support NULL unlike some other modern programming languages like Java. Actually, it does not have NULL or null keyword. However, it provides an enum Option used in similar way to Java 8 Optional.

Rust Enforces Variable Initialization

Rust enforces variable initialization both for “global” and “local” variables.

// Global variable
static MAX_THREADS: i32;

fn main() {

    // Local variable
    let message:&str;

    print!("{}", message);
    print!("{}", MAX_THREADS);

}

// Global variable

static MAX_THREADS: i32;

fn main() {

// Local variable

let message:&str;

print!("{}", message);

print!("{}", MAX_THREADS);

}

These codes result to the following two separate error messages.

Compile Error 1

error: expected one of `!`, `(`, `+`, `::`, `<`, or `=`, found `;`
 --> src\main.rs:1:24
  |
1 | static MAX_THREADS: i32;
  |                        ^ expected one of `!`, `(`, `+`, `::`, `<`, or `=` here

error: expected one of `!`, `(`, `+`, `::`, `<`, or `=`, found `;`

--> src\main.rs:1:24

1 | static MAX_THREADS: i32;

| ^ expected one of `!`, `(`, `+`, `::`, `<`, or `=` here

Compile Error 2

error[E0381]: borrow of possibly uninitialized variable: `message`
 --> src\main.rs:9:18
  |
9 |     print!("{}", message);
  |                  ^^^^^^^ use of possibly uninitialized `message`

error[E0381]: borrow of possibly uninitialized variable: `message`

--> src\main.rs:9:18

9 | print!("{}", message);

| ^^^^^^^ use of possibly uninitialized `message`

Using Enum Option

Below is a short code snippet demonstrating how to use the enum Option.

fn main() {

    let recipient:Option<&str> = None;
    let message:Option<&str> = Some("Rust is an awesome programming language");

    print!("Has recipient? {}\n", recipient.is_some());
    print!("Has no recipient? {}\n", recipient.is_none());
    print!("Has message? {}\n", message.is_some());
    print!("Has no message? {}\n", message.is_none());

    // This causes runtime error
    // print!("Recipient: {}\n", recipient.unwrap());
    print!("Message: {}\n", message.unwrap());
}

fn main() {

let recipient:Option<&str> = None;

let message:Option<&str> = Some("Rust is an awesome programming language");

print!("Has recipient? {}\n", recipient.is_some());

print!("Has no recipient? {}\n", recipient.is_none());

print!("Has message? {}\n", message.is_some());

print!("Has no message? {}\n", message.is_none());

// This causes runtime error

// print!("Recipient: {}\n", recipient.unwrap());

print!("Message: {}\n", message.unwrap());

}

Output:

C:/Users/karldev/.cargo/bin/cargo.exe run --color=always --package rust-project1 --bin rust-project1
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target\debug\rust-project1.exe`
Has recipient? false
Has no recipient? true
Has message? true
Has no message? false
Message: Rust is an awesome programming language

Process finished with exit code 0

C:/Users/karldev/.cargo/bin/cargo.exe run --color=always --package rust-project1 --bin rust-project1

Finished dev [unoptimized + debuginfo] target(s) in 0.03s

Running `target\debug\rust-project1.exe`

Has recipient? false

Has no recipient? true

Has message? true

Has no message? false

Message: Rust is an awesome programming language

Process finished with exit code 0

Tested with Rust 1.37.0.

Summary

Since NULL or null is not supported, there is no need to check for NULL values in Rust.

Rust – How to check for NULL values

NULL not supported

Rust Enforces Variable Initialization

Compile Error 1

Compile Error 2

Using Enum Option

Summary

Karl San Gabriel

NULL not supported

Rust Enforces Variable Initialization

Compile Error 1

Compile Error 2

Using Enum Option

Summary

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Karl San Gabriel