我一直在读《Rust编程语言》第17章的Trait对象,并尝试在我的代码中使用Trait对象。
请问为什么函数test2
不能编译,而其他函数可以?
trait Print {
fn print(&self) -> String;
}
impl Print for i32 {
fn print(&self) -> String {
return format!("{}", &self);
}
}
impl Print for &str {
fn print(&self) -> String {
return format!("'{}'", &self);
}
}
pub fn test1() {
let mut v: Vec<(usize, Box<dyn Print>)> = Vec::new();
let bxx = Box::new(0);
let idx = 1;
v.push((idx, bxx));
for (idx, val) in &v {
println!("{} - {}", idx, val.print());
}
}
pub fn test2() {
let mut v: Vec<(usize, Box<dyn Print>)> = Vec::new();
let bxx = Box::new(0);
let idx = 2;
let t = (idx, bxx);
v.push(t);
for (idx, val) in &v {
println!("{} - {}", idx, val.print());
}
}
pub fn test3() {
let mut v: Vec<(usize, Box<dyn Print>)> = Vec::new();
v.push((3, Box::new("a")));
for (idx, val) in &v {
println!("{} - {}", idx, val.print());
}
}
fn main() {
test1();
test2();
test3();
}
v.push((idx, bxx));
和let t = (idx, bxx); v.push(t);
,后者失败了。这是因为当let t = (idx, bxx);
时,元组的类型由编译器确定了吗? - rustyhu