Rust 一个struct是否可以扩展现有struct，保留所有字段

没有任何东西能完全符合这一点.我想到了两个概念.

1. struct 组成

   struct Person {
       age: u8,
   }
   
   struct Child {
       person: Person,
       has_toy: bool,
   }
   
   impl Person {
       fn new(age: u8) -> Self {
           Person { age: age }
       }
   
       fn age(&self) -> u8 {
           self.age
       }
   }
   
   impl Child {
       fn new(age: u8, has_toy: bool) -> Self {
           Child { person: Person::new(age), has_toy: has_toy }
       }
   
       fn age(&self) -> u8 {
           self.person.age()
       }
   }
   
   fn main() {
       let p = Person::new(42);
       let c = Child::new(7, true);
   
       println!("I am {}", p.age());
       println!("My child is {}", c.age());
   }
   

   您可以简单地将一个 struct 嵌入到另一个 struct 中.内存布局很好且紧凑，但您必须手动将所有方法从Person委派到Child，或者将一个&Person委派出go .

2. 特点

   trait SayHi {
       fn say_hi(&self);
   }
   
   struct Person {
       age: u8,
   }
   
   struct Child {
       age: u8,
       has_toy: bool,
   }
   
   impl SayHi for Person {
       fn say_hi(&self) {
           println!("Greetings. I am {}", self.age)
       }
   }
   
   impl SayHi for Child {
       fn say_hi(&self) {
           if self.has_toy {
               println!("I'm only {}, but I have a toy!", self.age)
           } else {
               println!("I'm only {}, and I don't even have a toy!", self.age)
           }
       }
   }
   
   fn greet<T>(thing: T)
       where T: SayHi
   {
       thing.say_hi()
   }
   
   fn main() {
       let p = Person { age: 42 };
       let c = Child { age: 7, has_toy: true };
   
       greet(p);
       greet(c);
   }
   

当然，你可以把这两个概念结合起来.

As DK. mentions, you could choose to implement Deref or DerefMut. However, I do not agree that these traits should be used in this manner. My argument is akin to the argument that using classical object-oriented inheritance simply for code reuse is the wrong thing. "Favor composition over inheritance" => "favor composition over Deref". However, I do hold out hope for a language feature that enables succinct delegation, reducing the annoyance of composition.