Characteristics of Object-Oriented Languages
There is no consensus in the programming community about what features a language must have to be considered object-oriented. Rust is influenced by many programming paradigms, including OOP; for example, we explored the features that came from functional programming in Chapter 13. Arguably, OOP languages share certain common characteristics, namely objects, encapsulation, and inheritance. Let’s look at what each of those characteristics means and whether Rust supports it.
Objects Contain Data and Behavior
The book Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison-Wesley Professional, 1994), colloquially referred to as The Gang of Four book, is a catalog of object-oriented design patterns. It defines OOP 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.
Using this definition, 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, according to the Gang of Four’s definition of objects.
Encapsulation that Hides Implementation Details
Another aspect commonly associated with OOP is the idea of encapsulation, which means that the implementation details of an object aren’t accessible to code using that object. Therefore, the only way to interact with an object is through its public API; code using the object shouldn’t 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 how to control encapsulation in Chapter 7: we can use the pub
keyword to decide which 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
pub struct AveragedCollection {
list: Vec<i32>,
average: f64,
}
The struct is marked pub
so that other code can 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
pub struct AveragedCollection {
list: Vec<i32>,
average: f64,
}
impl AveragedCollection {
pub fn add(&mut self, value: i32) {
self.list.push(value);
self.update_average();
}
pub fn remove(&mut self) -> Option<i32> {
let result = self.list.pop();
match result {
Some(value) => {
self.update_average();
Some(value)
}
None => None,
}
}
pub fn average(&self) -> f64 {
self.average
}
fn update_average(&mut self) {
let total: i32 = self.list.iter().sum();
self.average = total as f64 / self.list.len() as f64;
}
}
The public methods add
, remove
, and average
are the only ways to access
or modify data in 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 handles
updating the average
field as well.
We leave the list
and average
fields private so there is no way for
external code to add or remove items to or from 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 the struct
AveragedCollection
, we can easily change aspects, such as the data structure,
in the future. For instance, we could use a HashSet<i32>
instead of a
Vec<i32>
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<i32>
and Vec<i32>
have
different methods for adding and removing items, so the external code would
likely have to change if it were 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 elements 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 one. There is no way to define a struct that inherits the parent struct’s fields and method implementations without using a macro.
However, if you’re used to having inheritance in your programming toolbox, you can use other solutions in Rust, depending on your reason for reaching for inheritance in the first place.
You would choose inheritance for two main reasons. One is for reuse of code:
you can implement particular behavior for one type, and inheritance enables you
to reuse that implementation for a different type. You can do this in a limited
way in Rust code using default trait method implementations, which you saw in
Listing 10-14 when we added a default implementation of the summarize
method
on the Summary
trait. Any type implementing the Summary
trait would have
the summarize
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 also having the implementation of the method. We can
also override the default implementation of the summarize
method when we
implement the Summary
trait, which is similar to a child class overriding the
implementation of a method inherited from a parent class.
The other 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 you can substitute multiple objects for each other at runtime if they share certain characteristics.
Polymorphism
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 necessary. 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. It also introduces the possibility of calling methods on subclasses that don’t make sense or that cause errors because the methods don’t apply to the subclass. In addition, some languages will only allow single inheritance (meaning a subclass can only inherit from one class), further restricting the flexibility of a program’s design.
For these reasons, Rust takes the different approach of using trait objects instead of inheritance. Let’s look at how trait objects enable polymorphism in Rust.