1. In the src/bin folder, create a file called default.rs

2. Add the following code and run it with cargo run --bin default:

1  fn main() {
2    // There's a default value for nearly every primitive type
3    let foo: i32 = Default::default();
4    println!("foo: {}", foo); // Prints "foo: 0"
5 
6 
7    // A struct that derives from Default can be initialized like
     this
8    let pizza: PizzaConfig = Default::default();
9    // Prints "wants_cheese: false
10   println!("wants_cheese: {}", pizza.wants_cheese);
11 
12   // Prints "number_of_olives: 0"
13   println!("number_of_olives: {}", pizza.number_of_olives);
14 
15   // Prints "special_message: "
16   println!("special message: {}", pizza.special_message);
17 
18   let crust_type = match pizza.crust_type {
19     CrustType::Thin => "Nice and thin",
20     CrustType::Thick => "Extra thick and extra filling",
21   };
22   // Prints "crust_type: Nice and thin"
23   println!("crust_type: {}", crust_type);
24 
25 
26   // You can also configure only certain values
27   let custom_pizza = PizzaConfig {
28     number_of_olives: 12,
29     ..Default::default()
30   };
31 
32   // You can define as many values as you want
33   let deluxe_custom_pizza = PizzaConfig {
34     number_of_olives: 12,
35     wants_cheese: true,
36     special_message: "Will you marry me?".to_string(),
37     ..Default::default()
38   };
39
40 }
41 
42  #[derive(Default)]
43  struct PizzaConfig {
44    wants_cheese: bool,
45    number_of_olives: i32,
46    special_message: String,
47    crust_type: CrustType,
48  }
49 
50  // You can implement default easily for your own types
51  enum CrustType {
52    Thin,
53    Thick,
54  }
55  impl Default for CrustType {
56    fn default() -> CrustType {
57      CrustType::Thin
58    }
59  }