In Rust programming, when we have to select a value from a list of possible variants we use enumeration data types. An enumerated type is declared using the enum keyword. Following is the syntax of enum −

enum enum_name {
   variant1,
   variant2,
   variant3
}

Illustration: Using an Enumeration

The example declares an enum − GenderCategory, which has variants as Male and Female. The print! macro displays value of the enum. The compiler will throw an error the trait std::fmt::Debug is not implemented for GenderCategory. The attribute #[derive(Debug)] is used to suppress this error.

// The `derive` attribute automatically creates the implementation
// required to make this `enum` printable with `fmt::Debug`.
#[derive(Debug)]
enum GenderCategory {
   Male,Female
}
fn main() {
   let male = GenderCategory::Male;
   let female = GenderCategory::Female;

   println!("{:?}",male);
   println!("{:?}",female);
}

Output

Male
Female

Struct and Enum

The following example defines a structure Person. The field gender is of the type GenderCategory (which is an enum) and can be assigned either Male or Female as value.

// The `derive` attribute automatically creates the 
implementation
// required to make this `enum` printable with 
`fmt::Debug`.

#[derive(Debug)]
enum GenderCategory {
   Male,Female
}

// The `derive` attribute automatically creates the implementation
// required to make this `struct` printable with `fmt::Debug`.
#[derive(Debug)]
struct Person {
   name:String,
   gender:GenderCategory
}

fn main() {
   let p1 = Person {
      name:String::from("Mohtashim"),
      gender:GenderCategory::Male
   };
   let p2 = Person {
      name:String::from("Amy"),
      gender:GenderCategory::Female
   };
   println!("{:?}",p1);
   println!("{:?}",p2);
}

The example creates objects p1 and p2 of type Person and initializes the attributes, name and gender for each of these objects.

Output

Person { name: "Mohtashim", gender: Male }
Person { name: "Amy", gender: Female }

Option Enum

Option is a predefined enum in the Rust standard library. This enum has two values − Some(data) and None.

Syntax

enum Option<T> {
   Some(T),      //used to return a value
   None          // used to return null, as Rust doesn''t support 
   the null keyword
}

Here, the type T represents value of any type.

Rust does not support the null keyword. The value None, in the enumOption, can be used by a function to return a null value. If there is data to return, the function can return Some(data).

Let us understand this with an example −

The program defines a function is_even(), with a return type Option. The function verifies if the value passed is an even number. If the input is even, then a value true is returned, else the function returns None.

fn main() {
   let result = is_even(3);
   println!("{:?}",result);
   println!("{:?}",is_even(30));
}
fn is_even(no:i32)->Option<bool> {
   if no%2 == 0 {
      Some(true)
   } else {
      None
   }
}

Output

None
Some(true)

Match Statement and Enum

The match statement can be used to compare values stored in an enum. The following example defines a function, print_size, which takes CarType enum as parameter. The function compares the parameter values with a pre-defined set of constants and displays the appropriate message.

enum CarType {
   Hatch,
   Sedan,
   SUV
}
fn print_size(car:CarType) {
   match car {
      CarType::Hatch => {
         println!("Small sized car");
      },
      CarType::Sedan => {
         println!("medium sized car");
      },
      CarType::SUV =>{
         println!("Large sized Sports Utility car");
      }
   }
}
fn main(){
   print_size(CarType::SUV);
   print_size(CarType::Hatch);
   print_size(CarType::Sedan);
}

Output

Large sized Sports Utility car
Small sized car
medium sized car

Match with Option

The example of is_even function, which returns Option type, can also be implemented with match statement as shown below −

fn main() {
   match is_even(5) {
      Some(data) => {
         if data==true {
            println!("Even no");
         }
      },
      None => {
         println!("not even");
      }
   }
}
fn is_even(no:i32)->Option<bool> {
   if no%2 == 0 {
      Some(true)
   } else {
      None
   }
}

Output

not even

Match & Enum with Data Type

It is possible to add data type to each variant of an enum. In the following example, Name and Usr_ID variants of the enum are of String and integer types respectively. The following example shows the use of match statement with an enum having a data type.

// The `derive` attribute automatically creates the implementation
// required to make this `enum` printable with `fmt::Debug`.
#[derive(Debug)]
enum GenderCategory {
   Name(String),Usr_ID(i32)
}
fn main() {
   let p1 = GenderCategory::Name(String::from("Mohtashim"));
   let p2 = GenderCategory::Usr_ID(100);
   println!("{:?}",p1);
   println!("{:?}",p2);

   match p1 {
      GenderCategory::Name(val)=> {
         println!("{}",val);
      }
      GenderCategory::Usr_ID(val)=> {
         println!("{}",val);
      }
   }
}

Output

Name("Mohtashim")
Usr_ID(100)
Mohtashim