Rust {:?} vs {} 知多少

{} 指示符

{} 需要实现std::fmt::Display

{:?} 指示符

{:?} 需要实现std::fmt::Debug

案例展示

struct Rectangle {width: u32,height: u32,
}fn main() {let rect1 = Rectangle {width: 30,height: 50,};println!("rect1 is {}", rect1);
}

编译报错：

error[E0277]: `Rectangle` doesn't implement `std::fmt::Display`

---

提示我们没有实现Display trait

---

改成如下代码：

fn main() {let rect1 = Rectangle {width: 30,height: 50,};println!("rect1 is {:?}", rect1);
}

编译报错:

error[E0277]: `Rectangle` doesn't implement `Debug`

---

提示没有实现Debug trait

---

通过上面报错提示，我们在实现struct时候要根据自己的需要来实现Display或者Debug Trait。

自定义实现Display trait

use std::fmt;
struct Rectangle {width: u32,height: u32,
}
impl fmt::Display for Rectangle{// This trait requires `fmt` with this exact signature.fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {// Write strictly the first element into the supplied output// stream: `f`. Returns `fmt::Result` which indicates whether the// operation succeeded or failed. Note that `write!` uses syntax which// is very similar to `println!`.write!(f, "{} {}", self.width,self.height)}
}
fn main() {let rect1 = Rectangle {width: 30,height: 50,};println!("rect1 is {}", rect1);
}

输出如下：

rect1 is 30 50

Debug Trait

#[derive(Debug)]
struct Person {name: String,age: u32,
}fn main() {let person = Person {name: String::from("Alice"),age: 30,};println!("Person: {:?}", person);println!("Person: {:#?}", person);
}

输出如下：

Person: Person { name: "Alice", age: 30 }
Person: Person {name: "Alice",age: 30,
}

一般我们可以通过#[derive(Debug)] 在编译阶段，编译器帮忙我们自动实现Debug trait。

总结

本文通过一些实例介绍了{:?} vs {} 的区别，以及实现Display和Debug Trait