7.14. Rust enumerated class

7.14.1. Example #

#[derive(Debug)] enum Book { Papery, Electronic } fn main() { let book = Book::Papery; println!("{:?}", book); } 

Running result:

Papery 

Books are divided into paper books and electronic books.

If you are currently developing a library management system, you need to describe the different attributes of the two books (paper books have call numbers, e-books only URL), and you can add tuple attribute descriptions to enumerated class members:

enum Book { Papery(u32), Electronic(String), } let book = Book::Papery(1001); let ebook = Book::Electronic(String::from("url://...")); 

If you want to name the attribute, you can use the structure syntax:

enum Book { Papery { index: u32 }, Electronic { url: String }, } let book = Book::Papery{index: 1001}; 

Although you can name it this way, note that you cannot access the properties bound to the enumerated class as you would a structural body field. The method of access is in match in grammar.

7.14.2. Example #

fn main() { enum Book { Papery {index: u32}, Electronic {url: String}, } let book = Book::Papery{index: 1001}; let ebook = Book::Electronic{url: String::from("url...")}; match book { Book::Papery { index } => { println!("Papery book {}", index); }, Book::Electronic { url } => { println!("E-book {}", url); } } } 

Running result:

Papery book 1001 

match blocks can also be treated as function expressions, and it can also have a return value:

 Match Enumeration Class Instance{ Category 1=>Return value expression, Category 2=>Return value expression, ... } 

But all return value expressions must be of the same type!

If you define the additional properties of the enumerated class as tuples, in the match a name needs to be temporarily specified in the block:

7.14.3. Example #

enum Book { Papery(u32), Electronic {url: String}, } let book = Book::Papery(1001); match book { Book::Papery(i) => { println!("{}", i); }, Book::Electronic { url } => { println!("{}", url); } } 

In addition to branch selection for enumerated classes, match can also branch selection for data of integer, floating-point, character, and string slice reference (& str) types. Among them, although it is legal for floating point type to be selected by branch, it is not recommended because precision problems may lead to branch errors.

When selecting branches for non-enumerated classes, you must pay attention to handling exceptions, even if there is nothing to do in exceptional cases.Use an underscore as an exception \_ indicates:

7.14.4. Example #

fn main() { let t = "abc"; match t { "abc" => println!("Yes"), \_ => {}, } } 

enum Option<T> { Some(T), None, } 

let opt = Option::Some("Hello"); 

7.14.5. Example #

fn main() { let opt = Option::Some("Hello"); match opt { Option::Some(something) => { println!("{}", something); }, Option::None => { println!("opt is nothing"); } } } 

Running result:

Hello 

If your variable is null at first, consider the compiler. How does it know what type of variable is when the value is not null?

So the initial value is empty. Option type must be clear:

7.14.6. Example #

fn main() { let opt: Option<&str> = Option::None; match opt { Option::Some(something) => { println!("{}", something); }, Option::None => { println!("opt is nothing"); } } } 

Running result:

opt is nothing 

This design makes null programming difficult, but it is exactly what is needed to build a stable and efficient system. Due to Option is introduced by default by the Rust compiler and can be omitted when in use Option:: write directly None or Some() .

Option is a special enumeration class that can contain value branch selections:

7.14.7. Example #

fn main() { let t = Some(64); match t { Some(64) => println!("Yes"), \_ => println!("No"), } } 

7.14.8. Example #

let i = 0; match i { 0 => println!("zero"), \_ => {}, } 

Put the running result of the main function:

zero 

The purpose of this program is to determine whether I is the number 0, and if so, print it. zero .

Use it now if let syntax shortens this code:

let i = 0; if let 0 = i { println!("zero"); } 

The `` if let``syntax format is as follows:

 If let matching value=source variable{ Statement block } 

You can add one later else block to handle exceptions.

if let grammar can be thought of as distinguishing only two cases. The match “grammatical sugar” of a statement (grammatical sugar refersto a convenient substitute with the same principle as a grammar).

It still applies to enumerated classes:

7.14.9. Example #

fn main() { enum Book { Papery(u32), Electronic(String) } let book = Book::Electronic(String::from("url")); if let Book::Papery(index) = book { println!("Papery {}", index); } else { println!("Not papery book"); } }