如何在其多態類型上調用Display特性？

Question

我：

use std::fmt; 
struct TeamMember { 
    name: String, 
    age: u32, 
} 

struct Manager { 
    name: String, 
    age: u32, 
} 

impl fmt::Display for TeamMember { 
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { 
     write!(f, "TeamMember; name => {}, age => {}", self.name, self.age) 
     } 
} 

impl fmt::Display for Manager { 
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { 
     write!(f, "Manager; name => {}, age => {}", self.name, self.age) 
     } 
} 

trait Employee {} 

impl fmt::Display for Employee { 
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { 
     write!(f, "{}", *self) 
     } 
} 

fn main() { 
    let t = TeamMember { name: "abc".to_string(), age: 23 }; 
    let t2 = Manager { name: "xyz".to_string(), age: 18 }; 

    let mut v: Vec<&Employee> = Vec::new(); 
    v.push(&t); 
    v.push(&t2); 

    for it in &v { 
     println!("i am a {}", *it); 
    } 
} 

我想多態調用2種具體類型特徵的矢量對象引用的顯示器。我得到以下編譯錯誤：

<std macros>:2:21: 2:52 error: the trait `core::marker::Sized` is not implemented for the type `Employee` [E0277] 
<std macros>:2 $ dst . write_fmt (format_args ! ($ ($ arg) *))) 
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
<std macros>:2:21: 2:52 note: in this expansion of format_args! 

矢量迭代應該呼籲的具體類型Display功能...

Answer 1

當你這樣寫：

write!(f, "{}", *self) 

你承擔的類型*self（即Employee）實現了Display，這不受類型約束的保證。所以，你應該做的：

trait Employee: fmt::Display {} 

其中，作爲一個副作用，讓您免去您的impl fmt::Display for Employee。

Answer 2

這最後的工作，感謝：

use std::fmt; 

struct TeamMember { 
    name: String, 
    age: u32, 
} 
struct Manager { 
    name: String, 
    age: u32, 
} 

impl fmt::Display for TeamMember { 
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { 
     write!(f, "TeamMember!; name => {}, age => {}", self.name, self.age) 
    } 
} 

impl fmt::Display for Manager { 
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { 
     write!(f, "Manager!; name => {}, age => {}", self.name, self.age) 
    } 
} 

trait Employee: fmt::Display {} 
impl Employee for TeamMember {} 
impl Employee for Manager {} 

fn main() { 
    let t = TeamMember { 
     name: "abc".to_string(), 
     age: 23, 
    }; 
    let t2 = Manager { 
     name: "xyz".to_string(), 
     age: 18, 
    }; 

    let mut v: Vec<&Employee> = Vec::new(); 
    v.push(&t); 
    v.push(&t2); 

    for it in &v { 
     println!("i am a {}", *it); 
    } 
} 

Answer 3

由於Employee沒有方法，沒有必要創建另一個特點。你可以只用Display直接：

use std::fmt::Display; 

fn main() { 
    let t = TeamMember { name: "abc".to_string(), age: 23 }; 
    let t2 = Manager { name: "xyz".to_string(), age: 18 }; 

    let v = vec![&t as &Display, &t2]; 
    for it in &v { 
     println!("i am a {}", *it); 
    } 
} 

如果你想有一個Employee特點是沒有要求Display中實現（如在Valentin Lorentz' answer是必需的），你可以創建一個既需要另一個特點：

use std::fmt::Display; 

trait Employee {} 
impl Employee for TeamMember {} 
impl Employee for Manager {} 

trait PrintableEmployee: Employee + Display {} 
impl<T> PrintableEmployee for T where T: Employee + Display {} 

fn main() { 
    let t = TeamMember { name: "abc".to_string(), age: 23 }; 
    let t2 = Manager { name: "xyz".to_string(), age: 18 }; 

    let v = vec![&t as &PrintableEmployee, &t2]; 

    for it in &v { 
     println!("i am a {}", *it); 
    } 
}