类型提示结构作为特征
Type hinting structs as traits
我有一个方法 (Journal.next_command()
),其签名 return 是 Command
特征。在方法中,我试图 return Jump
结构的一个实例,它实现了 Command
特征:
trait Command {
fn execute(&self);
}
struct Jump {
height: u32,
}
impl Command for Jump {
fn execute(&self) {
println!("Jumping {} meters!", self.height);
}
}
struct Journal;
impl Journal {
fn next_command(&self) -> Command {
Jump { height: 2 }
}
}
fn main() {
let journal = Journal;
let command = journal.next_command();
command.execute();
}
编译失败,出现以下错误:
src/main.rs:19:9: 19:27 error: mismatched types:
expected `Command`,
found `Jump`
(expected trait Command,
found struct `Jump`) [E0308]
src/main.rs:19 Jump { height: 2 }
^~~~~~~~~~~~~~~~~~
如何通知编译器 Jump
实现了 Command
?
您目前无法 return 拆箱特征,它们需要包装在某种容器中。
fn next_command(&self) -> Box<Command> {
Box::new(Jump { height: 2 })
}
这有效。
我有一个方法 (Journal.next_command()
),其签名 return 是 Command
特征。在方法中,我试图 return Jump
结构的一个实例,它实现了 Command
特征:
trait Command {
fn execute(&self);
}
struct Jump {
height: u32,
}
impl Command for Jump {
fn execute(&self) {
println!("Jumping {} meters!", self.height);
}
}
struct Journal;
impl Journal {
fn next_command(&self) -> Command {
Jump { height: 2 }
}
}
fn main() {
let journal = Journal;
let command = journal.next_command();
command.execute();
}
编译失败,出现以下错误:
src/main.rs:19:9: 19:27 error: mismatched types:
expected `Command`,
found `Jump`
(expected trait Command,
found struct `Jump`) [E0308]
src/main.rs:19 Jump { height: 2 }
^~~~~~~~~~~~~~~~~~
如何通知编译器 Jump
实现了 Command
?
您目前无法 return 拆箱特征,它们需要包装在某种容器中。
fn next_command(&self) -> Box<Command> {
Box::new(Jump { height: 2 })
}
这有效。