[TOC] # Structs A `struct`, short for *structure*, is a custom data type that lets us name and package together multiple related values that make up a meaningful group. If you come from an object-oriented language, a `struct` is like an object’s data attributes. In the next section of this chapter, we’ll talk about how to define methods on our structs; methods are how you specify the *behavior* that goes along with a struct’s data. The `struct` and `enum` (that we will talk about in Chapter 6) concepts are the building blocks for creating new types in your program’s domain in order to take full advantage of Rust’s compile-time type checking. One way of thinking about structs is that they are similar to tuples, which we talked about in Chapter 3. Like tuples, the pieces of a struct can be different types. Unlike tuples, we name each piece of data so that it’s clearer what the values mean. Structs are more flexible as a result of these names: we don’t have to rely on the order of the data to specify or access the values of an instance. To define a struct, we enter the keyword `struct` and give the whole struct a name. A struct’s name should describe what the significance is of these pieces of data being grouped together. Then, inside curly braces, we define the names of the pieces of data, which we call *fields*, and specify each field’s type. For example, Listing 5-1 shows a struct to store information about a user account: ```rust struct User { username: String, email: String, sign_in_count: u64, active: bool, } ``` Listing 5-1: A `User` struct definition To use a struct once we've defined it, we create an *instance* of that struct by specifying concrete values for each of the fields. Creating an instance is done by stating the name of the struct, then curly braces with `key: value` pairs inside it where the keys are the names of the fields and the values are the data we want to store in those fields. The fields don’t have to be specified in the same order in which the struct declared them. In other words, the struct definition is like a general template for the type, and instances fill in that template with particular data to create values of the type. For example, we can declare a particular user like this: ```rust let user1 = User { email: String::from("someone@example.com"), username: String::from("someusername123"), active: true, sign_in_count: 1, }; ``` To get a particular value out of a struct, we can use dot notation. If we wanted just this user’s email address, we can say `user1.email`. ## Ownership of Struct Data In the `User` struct definition in Listing 5-1, we used the owned `String` type rather than the `&str` string slice type. This is a deliberate choice because we want instances of this struct to own all of its data, and for that data to be valid for as long as the entire struct is valid. It is possible for structs to store references to data owned by something else, but to do so requires the use of *lifetimes*, a feature of Rust that we'll discuss in Chapter 10. Lifetimes ensure that the data a struct references is valid for as long as the struct is. If you try to store a reference in a struct without specifying lifetimes, like this: Filename: src/main.rs ```rust,ignore struct User { username: &str, email: &str, sign_in_count: u64, active: bool, } fn main() { let user1 = User { email: "someone@example.com", username: "someusername123", active: true, sign_in_count: 1, }; } ``` The compiler will complain that it needs lifetime specifiers: ```text error[E0106]: missing lifetime specifier --> | 2 | username: &str, | ^ expected lifetime parameter error[E0106]: missing lifetime specifier --> | 3 | email: &str, | ^ expected lifetime parameter ``` We will talk about how to fix these errors in order to store references in structs in Chapter 10, but for now, fix errors like these by switching to owned types like `String` instead of references like `&str`. ## An Example Program To understand when we might want to use structs, let’s write a program that calculates the area of a rectangle. We’ll start off with single variables, then refactor our program until we’re using structs instead. Let’s make a new binary project with Cargo called *rectangles* that will take the length and width of a rectangle specified in pixels and will calculate the area of the rectangle. Listing 5-2 has a short program with one way of doing just that in our project’s *src/main.rs*: Filename: src/main.rs ```rust fn main() { let length1 = 50; let width1 = 30; println!( "The area of the rectangle is {} square pixels.", area(length1, width1) ); } fn area(length: u32, width: u32) -> u32 { length * width } ``` Listing 5-2: Calculating the area of a rectangle specified by its length and width in separate variables Let’s try running this program with `cargo run`: ```text The area of the rectangle is 1500 square pixels. ``` ### Refactoring with Tuples Our little program works okay; it figures out the area of the rectangle by calling the `area` function with each dimension. But we can do better. The length and the width are related to each other since together they describe one rectangle. The issue with this method is evident in the signature of `area`: ```rust,ignore fn area(length: u32, width: u32) -> u32 { ``` The `area` function is supposed to calculate the area of one rectangle, but our function takes two arguments. The arguments are related, but that’s not expressed anywhere in our program itself. It would be more readable and more manageable to group length and width together. We’ve already discussed one way we might do that in Chapter 3: tuples. Listing 5-3 has a version of our program which uses tuples: Filename: src/main.rs ```rust fn main() { let rect1 = (50, 30); println!( "The area of the rectangle is {} square pixels.", area(rect1) ); } fn area(dimensions: (u32, u32)) -> u32 { dimensions.0 * dimensions.1 } ``` Listing 5-3: Specifying the length and width of the rectangle with a tuple In one way, this is a little better. Tuples let us add a bit of structure, and we’re now passing just one argument. But in another way this method less clear: tuples don’t give names to their elements, so our calculation has gotten more confusing because we have to index into the parts of the tuple: ```rust,ignore dimensions.0 * dimensions.1 ``` It doesn’t matter if we mix up length and width for the area calculation, but if we were to draw the rectangle on the screen it would matter! We would have to remember that `length` was the tuple index `0` and `width` was the tuple index `1`. If someone else was to work on this code, they would have to figure this out and remember it as well. It would be easy to forget or mix these values up and cause errors, since we haven’t conveyed the meaning of our data in our code. ### Refactoring with Structs: Adding More Meaning Here is where we bring in structs. We can transform our tuple into a data type with a name for the whole as well as names for the parts, as shown in Listing 5-4: Filename: src/main.rs ```rust struct Rectangle { length: u32, width: u32, } fn main() { let rect1 = Rectangle { length: 50, width: 30 }; println!( "The area of the rectangle is {} square pixels.", area(&rect1) ); } fn area(rectangle: &Rectangle) -> u32 { rectangle.length * rectangle.width } ``` Listing 5-4: Defining a `Rectangle` struct Here we’ve defined a struct and given it the name `Rectangle`. Inside the `{}` we defined the fields to be `length` and `width`, both of which have type `u32`. Then in `main`, we create a particular instance of a `Rectangle` that has a length of 50 and a width of 30. Our `area` function now takes one argument that we’ve named `rectangle` whose type is an immutable borrow of a struct `Rectangle` instance. As we covered in Chapter 4, we want to borrow the struct rather than take ownership of it so that `main` keeps its ownership and can continue using `rect1`, so that’s why we have the `&` in the function signature and at the call site. The `area` function accesses the `length` and `width` fields of the `Rectangle` instance it got as an argument. Our function signature for `area` now says exactly what we mean: calculate the area of a `Rectangle`, using its `length` and `width` fields. This conveys that the length and width are related to each other, and gives descriptive names to the values rather than using the tuple index values of `0` and `1`. This is a win for clarity. ### Adding Useful Functionality with Derived Traits It’d be nice to be able to print out an instance of our `Rectangle` while we’re debugging our program and see the values for all its fields. Listing 5-5 tries using the `println!` macro as we have been: Filename: src/main.rs ```rust,ignore struct Rectangle { length: u32, width: u32, } fn main() { let rect1 = Rectangle { length: 50, width: 30 }; println!("rect1 is {}", rect1); } ``` Listing 5-5: Attempting to print a `Rectangle` instance If we run this, we get an error with this core message: ```text error[E0277]: the trait bound `Rectangle: std::fmt::Display` is not satisfied ``` The `println!` macro can do many kinds of formatting, and by default, `{}` tells `println!` to use formatting known as `Display`: output intended for direct end-user consumption. The primitive types we’ve seen so far implement `Display` by default, as there’s only one way you’d want to show a `1` or any other primitive type to a user. But with structs, the way `println!` should format the output is less clear as there are more display possibilities: Do you want commas or not? Do you want to print the struct `{}`s? Should all the fields be shown? Because of this ambiguity, Rust doesn’t try to guess what we want and structs do not have a provided implementation of `Display`. If we keep reading the errors, though, we’ll find this helpful note: ```text note: `Rectangle` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string ``` Let’s try it! The `println!` will now look like `println!("rect1 is {:?}", rect1);`. Putting the specifier `:?` inside the `{}` tells `println!` we want to use an output format called `Debug`. `Debug` is a trait that enables us to print out our struct in a way that is useful for developers so that we can see its value while we are debugging our code. Let’s try running with this change and… drat. We still get an error: ```text error: the trait bound `Rectangle: std::fmt::Debug` is not satisfied ``` Again, though, the compiler has given us a helpful note! ```text note: `Rectangle` cannot be formatted using `:?`; if it is defined in your crate, add `#[derive(Debug)]` or manually implement it ``` Rust *does* include functionality to print out debugging information, but we have to explicitly opt-in to having that functionality be available for our struct. To do that, we add the annotation `#[derive(Debug)]` just before our struct definition, as shown in Listing 5-6: ```rust #[derive(Debug)] struct Rectangle { length: u32, width: u32, } fn main() { let rect1 = Rectangle { length: 50, width: 30 }; println!("rect1 is {:?}", rect1); } ``` Listing 5-6: Adding the annotation to derive the `Debug` trait and printing the `Rectangle` instance using debug formatting At this point, if we run this program, we won’t get any errors and we’ll see the following output: ```text rect1 is Rectangle { length: 50, width: 30 } ``` Nice! It’s not the prettiest output, but it shows the values of all the fields for this instance, which would definitely help during debugging. If we want output that is a bit prettier and easier to read, which can be helpful with larger structs, we can use `{:#?}` in place of `{:?}` in the `println!` string. If we use the pretty debug style in this example, the output will look like: ``` rect1 is Rectangle { length: 50, width: 30 } ``` There are a number of traits Rust has provided for us to use with the `derive` annotation that can add useful behavior to our custom types. Those traits and their behaviors are listed in Appendix C. We’ll be covering how to implement these traits with custom behavior, as well as creating your own traits, in Chapter 10. Our `area` function is pretty specific—it only computes the area of rectangles. It would be nice to tie this behavior together more closely with our `Rectangle` struct, since it’s behavior that our `Rectangle` type has specifically. Let’s now look at how we can continue to refactor this code by turning the `area` function into an `area` *method* defined on our `Rectangle` type. ## Method Syntax *Methods* are similar to functions: they’re declared with the `fn` keyword and their name, they can take arguments and return values, and they contain some code that gets run when they’re called from somewhere else. Methods are different from functions, however, because they’re defined within the context of a struct (or an enum or a trait object, which we will cover in Chapters 6 and 13, respectively), and their first argument is always `self`, which represents the instance of the struct that the method is being called on. ### Defining Methods Let’s change our `area` function that takes a `Rectangle` instance as an argument and instead make an `area` method defined on the `Rectangle` struct, as shown in Listing 5-7: Filename: src/main.rs ```rust #[derive(Debug)] struct Rectangle { length: u32, width: u32, } impl Rectangle { fn area(&self) -> u32 { self.length * self.width } } fn main() { let rect1 = Rectangle { length: 50, width: 30 }; println!( "The area of the rectangle is {} square pixels.", rect1.area() ); } ``` Listing 5-7: Defining an `area` method on the `Rectangle` struct In order to make the function be defined within the context of `Rectangle`, we start an `impl` block (`impl` is short for *implementation*). Then we move the function within the `impl` curly braces, and change the first (and in this case, only) argument to be `self` in the signature and everywhere within the body. Then in `main` where we called the `area` function and passed `rect1` as an argument, we can instead use *method syntax* to call the `area` method on our `Rectangle` instance. Method syntax is taking an instance and adding a dot followed by the method name, parentheses, and any arguments. In the signature for `area`, we get to use `&self` instead of `rectangle: &Rectangle` because Rust knows the type of `self` is `Rectangle` due to this method being inside the `impl Rectangle` context. Note we still need to have the `&` before `self`, just like we had `&Rectangle`. Methods can choose to take ownership of `self`, borrow `self` immutably as we’ve done here, or borrow `self` mutably, just like any other argument. We’ve chosen `&self` here for the same reason we used `&Rectangle` in the function version: we don’t want to take ownership, and we just want to be able to read the data in the struct, not write to it. If we wanted to be able to change the instance that we’ve called the method on as part of what the method does, we’d put `&mut self` as the first argument instead. Having a method that takes ownership of the instance by having just `self` as the first argument is rarer; this is usually used when the method transforms `self` into something else and we want to prevent the caller from using the original instance after the transformation. The main benefit of using methods over functions, in addition to getting to use method syntax and not having to repeat the type of `self` in every method’s signature, is for organization. We’ve put all the things we can do with an instance of a type together in one `impl` block, rather than make future users of our code search for capabilities of `Rectangle` all over the place. PROD: START BOX ### Where’s the `->` Operator? In languages like C++, there are two different operators for calling methods: `.` if you’re calling a method on the object directly, and `->` if you’re calling the method on a pointer to the object and thus need to dereference the pointer first. In other words, if `object` is a pointer, `object->something()` is like `(*object).something()`. Rust doesn’t have an equivalent to the `->` operator; instead, Rust has a feature called *automatic referencing and dereferencing*. Calling methods is one of the few places in Rust that has behavior like this. Here’s how it works: when you call a method with `object.something()`, Rust will automatically add in `&`, `&mut`, or `*` so that `object` matches the signature of the method. In other words, these are the same: ```rust p1.distance(&p2); (&p1).distance(&p2); ``` The first one looks much, much cleaner. This automatic referencing behavior works because methods have a clear receiver — the type of `self`. Given the receiver and name of a method, Rust can figure out definitively whether the method is just reading (so needs `&self`), mutating (so `&mut self`), or consuming (so `self`). The fact that Rust makes borrowing implicit for method receivers is a big part of making ownership ergonomic in practice. PROD: END BOX ### Methods with More Arguments Let’s practice some more with methods by implementing a second method on our `Rectangle` struct. This time, we’d like for an instance of `Rectangle` to take another instance of `Rectangle` and return `true` if the second rectangle could fit completely within `self` and `false` if it would not. That is, if we run the code in Listing 5-8, once we've defined the `can_hold` method: Filename: src/main.rs ```rust,ignore fn main() { let rect1 = Rectangle { length: 50, width: 30 }; let rect2 = Rectangle { length: 40, width: 10 }; let rect3 = Rectangle { length: 45, width: 60 }; println!("Can rect1 hold rect2? {}", rect1.can_hold(&rect2)); println!("Can rect1 hold rect3? {}", rect1.can_hold(&rect3)); } ``` Listing 5-8: Demonstration of using the as-yet-unwritten `can_hold` method We want to see this output, since both of `rect2`’s dimensions are smaller than `rect1`’s, but `rect3` is wider than `rect1`: ```text Can rect1 hold rect2? true Can rect1 hold rect3? false ``` We know we want to define a method, so it will be within the `impl Rectangle` block. The method name will be `can_hold`, and it will take an immutable borrow of another `Rectangle` as an argument. We can tell what the type of the argument will be by looking at a call site: `rect1.can_hold(&rect2)` passes in `&rect2`, which is an immutable borrow to `rect2`, an instance of `Rectangle`. This makes sense, since we only need to read `rect2` (rather than write, which would mean we’d need a mutable borrow) and we want `main` to keep ownership of `rect2` so that we could use it again after calling this method. The return value of `can_hold` will be a boolean, and the implementation will check to see if `self`’s length and width are both greater than the length and width of the other `Rectangle`, respectively. Let’s add this new method to the `impl` block from Listing 5-7: Filename: src/main.rs ```rust impl Rectangle { fn area(&self) -> u32 { self.length * self.width } fn can_hold(&self, other: &Rectangle) -> bool { self.length > other.length && self.width > other.width } } ``` If we run this with the `main` from Listing 5-8, we will get our desired output! Methods can take multiple arguments that we add to the signature after the `self` parameter, and those arguments work just like arguments in functions do. ### Associated Functions One more useful feature of `impl` blocks: we’re allowed to define functions within `impl` blocks that *don’t* take `self` as a parameter. These are called *associated functions*, since they’re associated with the struct. They’re still functions though, not methods, since they don’t have an instance of the struct to work with. You’ve already used an associated function: `String::from`. Associated functions are often used for constructors that will return a new instance of the struct. For example, we could provide an associated function that would take one dimension argument and use that as both length and width, thus making it easier to create a square `Rectangle` rather than having to specify the same value twice: Filename: src/main.rs ```rust impl Rectangle { fn square(size: u32) -> Rectangle { Rectangle { length: size, width: size } } } ``` To call this associated function, we use the `::` syntax with the struct name: `let sq = Rectange::square(3);`, for example. This function is namespaced by the struct: the `::` syntax is used for both associated functions and namespaces created by modules, which we’ll learn about in Chapter 7. ## Summary Structs let us create custom types that are meaningful for our domain. By using structs, we can keep associated pieces of data connected to each other and name each piece to make our code clear. Methods let us specify the behavior that instances of our structs have, and associated functions let us namespace functionality that is particular to our struct without having an instance available. Structs aren’t the only way we can create custom types, though; let’s turn to the `enum` feature of Rust and add another tool to our toolbox.