Defining an Enum
Where structs give you a way of grouping together related fields and data, like
a Rectangle
with its width
and height
, enums give you a way of saying a
value is one of a possible set of values. For example, we may want to say that
Rectangle
is one of a set of possible shapes that also includes Circle
and
Triangle
. To do this, Rust allows us to encode these possibilities as an enum.
Let’s look at a situation we might want to express in code and see why enums are useful and more appropriate than structs in this case. Say we need to work with IP addresses. Currently, two major standards are used for IP addresses: version four and version six. Because these are the only possibilities for an IP address that our program will come across, we can enumerate all possible variants, which is where enumeration gets its name.
Any IP address can be either a version four or a version six address, but not both at the same time. That property of IP addresses makes the enum data structure appropriate, because an enum value can only be one of its variants. Both version four and version six addresses are still fundamentally IP addresses, so they should be treated as the same type when the code is handling situations that apply to any kind of IP address.
We can express this concept in code by defining an IpAddrKind
enumeration and
listing the possible kinds an IP address can be, V4
and V6
. These are the
variants of the enum:
enum IpAddrKind { V4, V6, } fn main() { let four = IpAddrKind::V4; let six = IpAddrKind::V6; route(IpAddrKind::V4); route(IpAddrKind::V6); } fn route(ip_kind: IpAddrKind) {}
IpAddrKind
is now a custom data type that we can use elsewhere in our code.
Enum Values
We can create instances of each of the two variants of IpAddrKind
like this:
enum IpAddrKind { V4, V6, } fn main() { let four = IpAddrKind::V4; let six = IpAddrKind::V6; route(IpAddrKind::V4); route(IpAddrKind::V6); } fn route(ip_kind: IpAddrKind) {}
Note that the variants of the enum are namespaced under its identifier, and we
use a double colon to separate the two. This is useful because now both values
IpAddrKind::V4
and IpAddrKind::V6
are of the same type: IpAddrKind
. We
can then, for instance, define a function that takes any IpAddrKind
:
enum IpAddrKind { V4, V6, } fn main() { let four = IpAddrKind::V4; let six = IpAddrKind::V6; route(IpAddrKind::V4); route(IpAddrKind::V6); } fn route(ip_kind: IpAddrKind) {}
And we can call this function with either variant:
enum IpAddrKind { V4, V6, } fn main() { let four = IpAddrKind::V4; let six = IpAddrKind::V6; route(IpAddrKind::V4); route(IpAddrKind::V6); } fn route(ip_kind: IpAddrKind) {}
Using enums has even more advantages. Thinking more about our IP address type, at the moment we don’t have a way to store the actual IP address data; we only know what kind it is. Given that you just learned about structs in Chapter 5, you might be tempted to tackle this problem with structs as shown in Listing 6-1.
fn main() { enum IpAddrKind { V4, V6, } struct IpAddr { kind: IpAddrKind, address: String, } let home = IpAddr { kind: IpAddrKind::V4, address: String::from("127.0.0.1"), }; let loopback = IpAddr { kind: IpAddrKind::V6, address: String::from("::1"), }; }
Here, we’ve defined a struct IpAddr
that has two fields: a kind
field that
is of type IpAddrKind
(the enum we defined previously) and an address
field
of type String
. We have two instances of this struct. The first is home
,
and it has the value IpAddrKind::V4
as its kind
with associated address
data of 127.0.0.1
. The second instance is loopback
. It has the other
variant of IpAddrKind
as its kind
value, V6
, and has address ::1
associated with it. We’ve used a struct to bundle the kind
and address
values together, so now the variant is associated with the value.
However, representing the same concept using just an enum is more concise:
rather than an enum inside a struct, we can put data directly into each enum
variant. This new definition of the IpAddr
enum says that both V4
and V6
variants will have associated String
values:
fn main() { enum IpAddr { V4(String), V6(String), } let home = IpAddr::V4(String::from("127.0.0.1")); let loopback = IpAddr::V6(String::from("::1")); }
We attach data to each variant of the enum directly, so there is no need for an
extra struct. Here it’s also easier to see another detail of how enums work:
the name of each enum variant that we define also becomes a function that
constructs an instance of the enum. That is, IpAddr::V4()
is a function call
that takes a String
argument and returns an instance of the IpAddr
type. We
automatically get this constructor function defined as a result of defining the
enum.
There’s another advantage to using an enum rather than a struct: each variant
can have different types and amounts of associated data. Version four type IP
addresses will always have four numeric components that will have values
between 0 and 255. If we wanted to store V4
addresses as four u8
values but
still express V6
addresses as one String
value, we wouldn’t be able to with
a struct. Enums handle this case with ease:
fn main() { enum IpAddr { V4(u8, u8, u8, u8), V6(String), } let home = IpAddr::V4(127, 0, 0, 1); let loopback = IpAddr::V6(String::from("::1")); }
We’ve shown several different ways to define data structures to store version
four and version six IP addresses. However, as it turns out, wanting to store
IP addresses and encode which kind they are is so common that the standard
library has a definition we can use! Let’s look at how
the standard library defines IpAddr
: it has the exact enum and variants that
we’ve defined and used, but it embeds the address data inside the variants in
the form of two different structs, which are defined differently for each
variant:
#![allow(unused)] fn main() { struct Ipv4Addr { // --snip-- } struct Ipv6Addr { // --snip-- } enum IpAddr { V4(Ipv4Addr), V6(Ipv6Addr), } }
This code illustrates that you can put any kind of data inside an enum variant: strings, numeric types, or structs, for example. You can even include another enum! Also, standard library types are often not much more complicated than what you might come up with.
Note that even though the standard library contains a definition for IpAddr
,
we can still create and use our own definition without conflict because we
haven’t brought the standard library’s definition into our scope. We’ll talk
more about bringing types into scope in Chapter 7.
Let’s look at another example of an enum in Listing 6-2: this one has a wide variety of types embedded in its variants.
enum Message { Quit, Move { x: i32, y: i32 }, Write(String), ChangeColor(i32, i32, i32), } fn main() {}
This enum has four variants with different types:
Quit
has no data associated with it at all.Move
has named fields like a struct does.Write
includes a singleString
.ChangeColor
includes threei32
values.
Defining an enum with variants such as the ones in Listing 6-2 is similar to
defining different kinds of struct definitions, except the enum doesn’t use the
struct
keyword and all the variants are grouped together under the Message
type. The following structs could hold the same data that the preceding enum
variants hold:
struct QuitMessage; // unit struct struct MoveMessage { x: i32, y: i32, } struct WriteMessage(String); // tuple struct struct ChangeColorMessage(i32, i32, i32); // tuple struct fn main() {}
But if we used the different structs, which each have their own type, we
couldn’t as easily define a function to take any of these kinds of messages as
we could with the Message
enum defined in Listing 6-2, which is a single type.
There is one more similarity between enums and structs: just as we’re able to
define methods on structs using impl
, we’re also able to define methods on
enums. Here’s a method named call
that we could define on our Message
enum:
fn main() { enum Message { Quit, Move { x: i32, y: i32 }, Write(String), ChangeColor(i32, i32, i32), } impl Message { fn call(&self) { // method body would be defined here } } let m = Message::Write(String::from("hello")); m.call(); }
The body of the method would use self
to get the value that we called the
method on. In this example, we’ve created a variable m
that has the value
Message::Write(String::from("hello"))
, and that is what self
will be in the
body of the call
method when m.call()
runs.
Let’s look at another enum in the standard library that is very common and
useful: Option
.
The Option
Enum and Its Advantages Over Null Values
This section explores a case study of Option
, which is another enum defined
by the standard library. The Option
type encodes the very common scenario in
which a value could be something or it could be nothing.
For example, if you request the first of a list containing items, you would get a value. If you request the first item of an empty list, you would get nothing. Expressing this concept in terms of the type system means the compiler can check whether you’ve handled all the cases you should be handling; this functionality can prevent bugs that are extremely common in other programming languages.
Programming language design is often thought of in terms of which features you include, but the features you exclude are important too. Rust doesn’t have the null feature that many other languages have. Null is a value that means there is no value there. In languages with null, variables can always be in one of two states: null or not-null.
In his 2009 presentation “Null References: The Billion Dollar Mistake,” Tony Hoare, the inventor of null, has this to say:
I call it my billion-dollar mistake. At that time, I was designing the first comprehensive type system for references in an object-oriented language. My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn’t resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.
The problem with null values is that if you try to use a null value as a not-null value, you’ll get an error of some kind. Because this null or not-null property is pervasive, it’s extremely easy to make this kind of error.
However, the concept that null is trying to express is still a useful one: a null is a value that is currently invalid or absent for some reason.
The problem isn’t really with the concept but with the particular
implementation. As such, Rust does not have nulls, but it does have an enum
that can encode the concept of a value being present or absent. This enum is
Option<T>
, and it is defined by the standard library
as follows:
#![allow(unused)] fn main() { enum Option<T> { None, Some(T), } }
The Option<T>
enum is so useful that it’s even included in the prelude; you
don’t need to bring it into scope explicitly. Its variants are also included in
the prelude: you can use Some
and None
directly without the Option::
prefix. The Option<T>
enum is still just a regular enum, and Some(T)
and
None
are still variants of type Option<T>
.
The <T>
syntax is a feature of Rust we haven’t talked about yet. It’s a
generic type parameter, and we’ll cover generics in more detail in Chapter 10.
For now, all you need to know is that <T>
means the Some
variant of the
Option
enum can hold one piece of data of any type, and that each concrete
type that gets used in place of T
makes the overall Option<T>
type a
different type. Here are some examples of using Option
values to hold number
types and string types:
fn main() { let some_number = Some(5); let some_char = Some('e'); let absent_number: Option<i32> = None; }
The type of some_number
is Option<i32>
. The type of some_char
is
Option<char>
, which is a different type. Rust can infer these types because
we’ve specified a value inside the Some
variant. For absent_number
, Rust
requires us to annotate the overall Option
type: the compiler can’t infer the
type that the corresponding Some
variant will hold by looking only at a
None
value. Here, we tell Rust that we mean for absent_number
to be of type
Option<i32>
.
When we have a Some
value, we know that a value is present and the value is
held within the Some
. When we have a None
value, in some sense, it means
the same thing as null: we don’t have a valid value. So why is having
Option<T>
any better than having null?
In short, because Option<T>
and T
(where T
can be any type) are different
types, the compiler won’t let us use an Option<T>
value as if it were
definitely a valid value. For example, this code won’t compile because it’s
trying to add an i8
to an Option<i8>
:
fn main() {
let x: i8 = 5;
let y: Option<i8> = Some(5);
let sum = x + y;
}
If we run this code, we get an error message like this:
$ cargo run
Compiling enums v0.1.0 (file:///projects/enums)
error[E0277]: cannot add `Option<i8>` to `i8`
--> src/main.rs:5:17
|
5 | let sum = x + y;
| ^ no implementation for `i8 + Option<i8>`
|
= help: the trait `Add<Option<i8>>` is not implemented for `i8`
= help: the following other types implement trait `Add<Rhs>`:
<&'a f32 as Add<f32>>
<&'a f64 as Add<f64>>
<&'a i128 as Add<i128>>
<&'a i16 as Add<i16>>
<&'a i32 as Add<i32>>
<&'a i64 as Add<i64>>
<&'a i8 as Add<i8>>
<&'a isize as Add<isize>>
and 48 others
For more information about this error, try `rustc --explain E0277`.
error: could not compile `enums` due to previous error
Intense! In effect, this error message means that Rust doesn’t understand how
to add an i8
and an Option<i8>
, because they’re different types. When we
have a value of a type like i8
in Rust, the compiler will ensure that we
always have a valid value. We can proceed confidently without having to check
for null before using that value. Only when we have an Option<i8>
(or
whatever type of value we’re working with) do we have to worry about possibly
not having a value, and the compiler will make sure we handle that case before
using the value.
In other words, you have to convert an Option<T>
to a T
before you can
perform T
operations with it. Generally, this helps catch one of the most
common issues with null: assuming that something isn’t null when it actually
is.
Eliminating the risk of incorrectly assuming a not-null value helps you to be
more confident in your code. In order to have a value that can possibly be
null, you must explicitly opt in by making the type of that value Option<T>
.
Then, when you use that value, you are required to explicitly handle the case
when the value is null. Everywhere that a value has a type that isn’t an
Option<T>
, you can safely assume that the value isn’t null. This was a
deliberate design decision for Rust to limit null’s pervasiveness and increase
the safety of Rust code.
So, how do you get the T
value out of a Some
variant when you have a value
of type Option<T>
so you can use that value? The Option<T>
enum has a large
number of methods that are useful in a variety of situations; you can check
them out in its documentation. Becoming familiar with
the methods on Option<T>
will be extremely useful in your journey with Rust.
In general, in order to use an Option<T>
value, you want to have code that
will handle each variant. You want some code that will run only when you have a
Some(T)
value, and this code is allowed to use the inner T
. You want some
other code to run if you have a None
value, and that code doesn’t have a T
value available. The match
expression is a control flow construct that does
just this when used with enums: it will run different code depending on which
variant of the enum it has, and that code can use the data inside the matching
value.