What Does Object-Oriented Mean?

There’s no consensus in the programming community about what features a language needs in order to be called object-oriented. Rust is influenced by many different programming paradigms including OOP; we explored, for example, the features that came from functional programming in Chapter 13. Arguably, object-oriented programming languages do tend to share certain common characteristics, namely objects, encapsulation, and inheritance. Let’s take a look at what each of those mean and whether Rust supports them.

Objects Contain Data and Behavior

The book “Design Patterns: Elements of Reusable Object-Oriented Software,” colloquially referred to as “The Gang of Four book,” is a catalog of object-oriented design patterns. It defines object-oriented programming in this way:

Object-oriented programs are made up of objects. An object packages both data and the procedures that operate on that data. The procedures are typically called methods or operations.

Under this definition, then, Rust is object-oriented: structs and enums have data and impl blocks provide methods on structs and enums. Even though structs and enums with methods aren’t called objects, they provide the same functionality, under the Gang of Four’s definition of objects.

Encapsulation that Hides Implementation Details

Another aspect commonly associated with object-oriented programming is the idea of encapsulation: that the implementation details of an object aren’t accessible to code using that object. The only way to interact with an object therefore is through its public API; code using the object should not be able to reach into the object’s internals and change data or behavior directly. This enables the programmer to change and refactor an object’s internals without needing to change the code that uses the object.

We discussed an example of this in Chapter 7: We can use the pub keyword to decide what modules, types, functions, and methods in our code should be public, and by default everything else is private. For example, we can define a struct AveragedCollection that has a field containing a vector of i32 values. The struct can also have a field that contains the average of the values in the vector, meaning the average doesn’t have to be computed on-demand whenever anyone needs it. In other words, AveragedCollection will cache the calculated average for us. Listing 17-1 has the definition of the AveragedCollection struct:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
pub struct AveragedCollection {
    list: Vec<i32>,
    average: f64,

Listing 17-1: An AveragedCollection struct that maintains a list of integers and the average of the items in the collection.

The struct itself is marked pub so that other code may use it, but the fields within the struct remain private. This is important in this case because we want to ensure that whenever a value is added or removed from the list, the average is also updated. We do this by implementing add, remove, and average methods on the struct as shown in Listing 17-2:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# pub struct AveragedCollection {
#     list: Vec<i32>,
#     average: f64,
# }
impl AveragedCollection {
    pub fn add(&mut self, value: i32) {

    pub fn remove(&mut self) -> Option<i32> {
        let result = self.list.pop();
        match result {
            Some(value) => {
            None => None,

    pub fn average(&self) -> f64 {

    fn update_average(&mut self) {
        let total: i32 = self.list.iter().sum();
        self.average = total as f64 / self.list.len() as f64;

Listing 17-2: Implementations of the public methods add, remove, and average on AveragedCollection

The public methods add, remove, and average are the only way to modify an instance of AveragedCollection. When an item is added to list using the add method or removed using the remove method, the implementations of each call the private update_average method that takes care of updating the average field as well.

We leave the list and average fields private so that there’s no way for external code to add or remove items to the list field directly, otherwise the average field might become out of sync when the list changes. The average method returns the value in the average field, allowing external code to read the average but not modify it.

Because we’ve encapsulated the implementation details of AveragedCollection, we can easily change aspects like the data structure in the future. For instance, we could use a HashSet instead of a Vec for the list field. As long as the signatures of the add, remove, and average public methods stay the same, code using AveragedCollection wouldn’t need to change. If we made list public instead, this wouldn’t necessarily be the case: HashSet and Vec have different methods for adding and removing items, so the external code would likely have to change if it was modifying list directly.

If encapsulation is a required aspect for a language to be considered object-oriented, then Rust meets that requirement. The option to use pub or not for different parts of code enables encapsulation of implementation details.

Inheritance as a Type System and as Code Sharing

Inheritance is a mechanism whereby an object can inherit from another object’s definition, thus gaining the parent object’s data and behavior without you having to define them again.

If a language must have inheritance to be an object-oriented language, then Rust is not. There is no way to define a struct that inherits the parent struct’s fields and method implementations. However, if you’re used to having inheritance in your programming toolbox, there are other solutions in Rust depending on your reason for reaching for inheritance in the first place.

There are two main reasons to choose inheritance. The first is for re-use of code: you can implement particular behavior for one type, and inheritance enables you to re-use that implementation for a different type. Rust code can be shared using default trait method implementations instead, which we saw in Listing 10-15 when we added a default implementation of the summary method on the Summarizable trait. Any type implementing the Summarizable trait would have the summary method available on it without any further code. This is similar to a parent class having an implementation of a method, and an inheriting child class then also having the implementation of the method. We can also choose to override the default implementation of the summary method when we implement the Summarizable trait, similar to a child class overriding the implementation of a method inherited from a parent class.

The second reason to use inheritance relates to the type system: to enable a child type to be used in the same places as the parent type. This is also called polymorphism, which means that multiple objects can be substituted for each other at runtime if they share certain characteristics.


To many people, polymorphism is synonymous with inheritance. But it’s actually a more general concept that refers to code that can work with data of multiple types. For inheritance, those types are generally subclasses. Rust instead uses generics to abstract over different possible types, and trait bounds to impose constraints on what those types must provide. This is sometimes called bounded parametric polymorphism.

Inheritance has recently fallen out of favor as a programming design solution in many programming languages because it’s often at risk of sharing more code than needs be. Subclasses shouldn’t always share all characteristics of their parent class, but will do so with inheritance. This can make a program’s design less flexible, and introduces the possibility of calling methods on subclasses that don’t make sense or that cause errors because the methods don’t actually apply to the subclass. Some languages will also only allow a subclass to inherit from one class, further restricting the flexibility of a program’s design.

For these reasons, Rust chose to take a different approach, using trait objects instead of inheritance. Let’s take a look at how trait objects enable polymorphism in Rust.