Defining and Instantiating Structs
Structs are similar to tuples, which were discussed in Chapter 3. Like tuples, the pieces of a struct can be different types. Unlike tuples, we name each piece of data so it’s clear what the values mean. As a result of these names, structs are more flexible than tuples: 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 name the entire struct. A
struct’s name should describe the significance of the pieces of data being
grouped together. Then, inside curly braces, we define the names and types of
the pieces of data, which we call fields. For example, Listing 5-1 shows a
struct to store information about a user account:
struct User {
username: String,
email: String,
sign_in_count: u64,
active: bool,
}
Listing 5-1: A User struct definition
To use a struct after we’ve defined it, we create an instance of that struct
by specifying concrete values for each of the fields. We create an instance by
stating the name of the struct, and then add curly braces containing key:
value pairs where the keys are the names of the fields and the values are the
data we want to store in those fields. We don’t have to specify the fields in
the same order in which we declared them in the struct. 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 as shown in Listing 5-2:
# struct User {
# username: String,
# email: String,
# sign_in_count: u64,
# active: bool,
# }
#
let user1 = User {
email: String::from("[email protected]"),
username: String::from("someusername123"),
active: true,
sign_in_count: 1,
};
Listing 5-2: Creating an instance of the User
struct
To get a specific value from a struct, we can use dot notation. If we wanted
just this user’s email address, we can use user1.email wherever we want to
use this value. If the instance is mutable, we can change a value by using the
dot notation and assigning into a particular field. Listing 5-3 shows how to
change the value in the email field of a mutable User instance:
# struct User {
# username: String,
# email: String,
# sign_in_count: u64,
# active: bool,
# }
#
let mut user1 = User {
email: String::from("[email protected]"),
username: String::from("someusername123"),
active: true,
sign_in_count: 1,
};
user1.email = String::from("[email protected]");
Listing 5-3: Changing the value in the email field of a
User instance
Note that the entire instance must be mutable; Rust doesn’t allow us to mark only certain fields as mutable. Also note that as with any expression, we can construct a new instance of the struct as the last expression in the function body to implicitly return that new instance.
Listing 5-4 shows a build_user function that returns a User instance with
the given email and username. The active field gets the value of true, and
the sign_in_count gets a value of 1.
# struct User {
# username: String,
# email: String,
# sign_in_count: u64,
# active: bool,
# }
#
fn build_user(email: String, username: String) -> User {
User {
email: email,
username: username,
active: true,
sign_in_count: 1,
}
}
Listing 5-4: A build_user function that takes an email
and username and returns a User instance
It makes sense to name the function arguments with the same name as the struct
fields, but having to repeat the email and username field names and
variables is a bit tedious. If the struct had more fields, repeating each name
would get even more annoying. Luckily, there's a convenient shorthand!
Using the Field Init Shorthand when Variables and Fields Have the Same Name
Because the parameter names and the struct field names are exactly the same in
Listing 5-4, we can use the field init shorthand syntax to rewrite
build_user so that it behaves exactly the same but doesn’t have the
repetition of email and username in the way shown in Listing 5-5.
# struct User {
# username: String,
# email: String,
# sign_in_count: u64,
# active: bool,
# }
#
fn build_user(email: String, username: String) -> User {
User {
email,
username,
active: true,
sign_in_count: 1,
}
}
Listing 5-5: A build_user function that uses field init
shorthand since the email and username parameters have the same name as
struct fields
Here, we’re creating a new instance of the User struct, which has a field
named email. We want to set the email field’s value to the value in the
email parameter of the build_user function. Because the email field and
the email parameter have the same name, we only need to write email rather
than email: email.
Creating Instances From Other Instances With Struct Update Syntax
It’s often useful to create a new instance of a struct that uses most of an old instance’s values, but changes some. We do this using struct update syntax.
First, Listing 5-6 shows how we create a new User instance in user2 without
the update syntax. We set new values for email and username, but otherwise
use the same values from user1 that we created in Listing 5-2:
# struct User {
# username: String,
# email: String,
# sign_in_count: u64,
# active: bool,
# }
#
# let user1 = User {
# email: String::from("[email protected]"),
# username: String::from("someusername123"),
# active: true,
# sign_in_count: 1,
# };
#
let user2 = User {
email: String::from("[email protected]"),
username: String::from("anotherusername567"),
active: user1.active,
sign_in_count: user1.sign_in_count,
};
Listing 5-6: Creating a new User instance using some of
the values from user1
Using struct update syntax, we can achieve the same effect with less code,
shown in Listing 5-7. The syntax .. specifies that the remaining fields not
explicitly set should have the same value as the fields in the given instance.
# struct User {
# username: String,
# email: String,
# sign_in_count: u64,
# active: bool,
# }
#
# let user1 = User {
# email: String::from("[email protected]"),
# username: String::from("someusername123"),
# active: true,
# sign_in_count: 1,
# };
#
let user2 = User {
email: String::from("[email protected]"),
username: String::from("anotherusername567"),
..user1
};
Listing 5-7: Using struct update syntax to set a new
email and username values for a User instance but use the rest of the
values from the fields of the instance in the user1 variable
The code in Listing 5-7 also creates an instance in user2 that has a
different value for email and username but has the same values for the
active and sign_in_count fields from user1.
Tuple Structs without Named Fields to Create Different Types
We can also define structs that look similar to tuples, called tuple structs, that have the added meaning the struct name provides, but don’t have names associated with their fields, just the types of the fields. Tuple structs are useful when you want to give the whole tuple a name and make the tuple be a different type than other tuples, but naming each field as in a regular struct would be verbose or redundant.
To define a tuple struct you start with the struct keyword and the struct
name followed by the types in the tuple. For example, here are definitions and
usages of two tuple structs named Color and Point:
struct Color(i32, i32, i32);
struct Point(i32, i32, i32);
let black = Color(0, 0, 0);
let origin = Point(0, 0, 0);
Note that the black and origin values are different types, since they’re
instances of different tuple structs. Each struct we define is its own type,
even though the fields within the struct have the same types. For example, a
function that takes a parameter of type Color cannot take a Point as an
argument, even though both types are made up of three i32 values. Otherwise,
tuple struct instances behave like tuples, which we covered in Chapter 3: you
can destructure them into their individual pieces, you can use a . followed
by the index to access an individual value, and so on.
Unit-Like Structs without Any Fields
We can also define structs that don’t have any fields! These are called
unit-like structs since they behave similarly to (), the unit type.
Unit-like structs can be useful in situations such as when you need to
implement a trait on some type, but you don’t have any data that you want to
store in the type itself. We’ll be discussing traits in Chapter 10.
Ownership of Struct Data
In the
Userstruct definition in Listing 5-1, we used the ownedStringtype rather than the&strstring 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’s possible for structs to store references to data owned by something else, but to do so requires the use of lifetimes, a Rust feature that is discussed in Chapter 10. Lifetimes ensure that the data referenced by a struct is valid for as long as the struct is. Let’s say you try to store a reference in a struct without specifying lifetimes, like this:
Filename: src/main.rs
struct User { username: &str, email: &str, sign_in_count: u64, active: bool, } fn main() { let user1 = User { email: "[email protected]", username: "someusername123", active: true, sign_in_count: 1, }; }The compiler will complain that it needs lifetime specifiers:
error[E0106]: missing lifetime specifier --> | 2 | username: &str, | ^ expected lifetime parameter error[E0106]: missing lifetime specifier --> | 3 | email: &str, | ^ expected lifetime parameterWe’ll discuss how to fix these errors so you can store references in structs in Chapter 10, but for now, we’ll fix errors like these using owned types like
Stringinstead of references like&str.