Rust Functions and Methods

Are you a Rust programmer looking to take your skills to the next level? Do you want to learn more about Rust functions and methods? Look no further! In this article, we'll dive deep into the world of Rust functions and methods, exploring their differences, use cases, and best practices.

What are Rust Functions?

Functions are a fundamental building block of any programming language, and Rust is no exception. In Rust, functions are defined using the fn keyword, followed by the function name, and a set of parentheses that may or may not contain arguments. Here's an example:

fn add_numbers(a: i32, b: i32) -> i32 {
    a + b

In this example, we define a function called add_numbers that takes two arguments of type i32 and returns their sum as an i32. Notice that the last line of the function body is an expression that evaluates to the sum of the two arguments. This expression is also the return value of the function.

Functions in Rust can be called with or without arguments, and their return value can be assigned to a variable or used directly in an expression. Here's an example:

let result = add_numbers(2, 3);
println!("The result is {}", result);

In this example, we call the add_numbers function with the arguments 2 and 3, and assign the result to a variable called result. We then use the println! macro to print the result to the console.

What are Rust Methods?

Methods are similar to functions in that they are also a way to encapsulate behavior in Rust. However, methods are associated with a particular type, and are called on instances of that type using the dot notation. Here's an example:

struct Rectangle {
    width: u32,
    height: u32,

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height

let rect = Rectangle { width: 10, height: 20 };
let area = rect.area();
println!("The area is {}", area);

In this example, we define a struct called Rectangle that has two fields: width and height. We then define an implementation block for the Rectangle type, and define a method called area that calculates the area of the rectangle. Notice that the method takes a reference to self as its first argument, which allows it to access the fields of the Rectangle instance.

We then create an instance of Rectangle called rect, and call the area method on it using the dot notation. The result is assigned to a variable called area, which is then printed to the console.

Differences between Functions and Methods

While functions and methods share many similarities, there are some key differences between them that are worth exploring.


The syntax for defining functions and methods is slightly different. Functions are defined using the fn keyword, followed by the function name and a set of parentheses that may or may not contain arguments. Methods, on the other hand, are defined as part of a type's implementation block, and are associated with a particular instance of that type using the dot notation.


Functions and methods can both take arguments, but the way they are passed is slightly different. Functions take arguments as a comma-separated list inside the parentheses, while methods take a reference to self as their first argument, followed by any other arguments.

Return Values

Functions and methods can both return values, but the way they do so is slightly different. Functions use the return keyword to explicitly return a value, while methods use the last expression in their body as the return value.


Functions and methods can both be marked as pub to make them visible outside of their module. However, methods can also be marked as pub or pub(crate) to control their visibility within their module.

Best Practices for Rust Functions and Methods

Now that we've explored the differences between Rust functions and methods, let's take a look at some best practices for using them effectively.

Use Functions for General-Purpose Logic

Functions are great for encapsulating general-purpose logic that can be reused across multiple parts of your codebase. When defining a function, try to keep it as generic as possible, so that it can be used in a variety of contexts.

Use Methods for Type-Specific Behavior

Methods are great for encapsulating behavior that is specific to a particular type. When defining a method, try to keep it closely related to the fields and behavior of the type it is associated with.

Use References for Large Data Structures

When passing large data structures as arguments to functions or methods, it's often more efficient to use references instead of copying the data. This can help reduce memory usage and improve performance.

Use the &self and &mut self Conventions

When defining methods, it's a good idea to follow the convention of using &self for read-only methods and &mut self for methods that modify the instance. This makes it clear to users of your code which methods are safe to call in a concurrent environment.

Use Unit Tests to Verify Behavior

When defining functions and methods, it's important to write unit tests to verify their behavior. This can help catch bugs early and ensure that your code is working as expected.


Functions and methods are a fundamental part of Rust programming, and understanding their differences and best practices is key to writing effective and maintainable code. By following the guidelines outlined in this article, you can take your Rust programming skills to the next level and write code that is efficient, readable, and easy to maintain. Happy coding!

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