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2017-07-23 Rustを続けてみて [長年日記]

_ [Rust] トレイト

これも理解しておくのです。

構造体にトレイト*1を追加してあげないとコンパイルエラーになっちゃう。


struct Color{
   r: u32,
   g: u32,
   b: u32,
}

fn main() {
   let x = Color{r :255, g: 255, b: 255};
   let y = x;
   println!("{:?}",y);
}

これだと


error[E0277]: the trait bound `Color: std::fmt::Debug` is not satisfied
 --> src/main.rs:11:21
  |
11 |     println!("{:?}",y);
  |                     ^ the trait `std::fmt::Debug` is not implemented for `Color`
  |
  = note: `Color` cannot be formatted using `:?`; if it is defined in your crate, add `#[derive(Debug)]` or manually implement it
  = note: required by `std::fmt::Debug::fmt`

error: aborting due to previous error

だそうなのでdebug traitを追加


#[derive(Debug)]
struct Color{
   r: u32,
   g: u32,
   b: u32,
}

fn main() {
   let x = Color{r :255, g: 255, b: 255};
   let y = x;
   println!("{:?}",y);
}

この時点ではCopy Traitを追加してないので


#[derive(Debug)]
struct Color{
   r: u32,
   g: u32,
   b: u32,
}

fn main() {
   let x = Color{r :255, g: 255, b: 255};
   let y = x;
   println!("{:?}",y);
   println!("{:?}",x);
}

だとコンパイルエラーです


error[E0382]: use of moved value: `x`
 --> src/main.rs:12:21
  |
10 |     let y = x;
  |         - value moved here
11 |     println!("{:?}",y);
12 |     println!("{:?}",x);
  |                     ^ value used here after move
  |
  = note: move occurs because `x` has type `Color`, which does not implement the `Copy` trait

error: aborting due to previous error

そこでCopyとClone Traitを追加してあげると通ります


#[derive(Debug,Copy,Clone)]
struct Color{
   r: u32,
   g: u32,
   b: u32,
}

fn main() {
   let x = Color{r :255, g: 255, b: 255};
   let y = x;
   println!("{:?}",y);
   println!("{:?}",x);
}

それかyのscopeを狭めて借用を使うんですかね。


#[derive(Debug)]
struct Color{
   r: u32,
   g: u32,
   b: u32,
}

fn main() {
   let x = Color{r :255, g: 255, b: 255};
   {
       let y = &x;
       println!("{:?}",y);
   }
   println!("{:?}",x);
}