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trait Double {
fn double(self) -> usize;
}
impl<‘a> Double for &‘a String {
fn double(self) -> usize { self.len()}
}
impl<‘a, ‘b, ‘c> Double for &‘a &‘b &‘c String {
fn double(self) -> usize { self.len() * 2 }
}
pub fn main() {
let x = "hello".to_string(); println!("x:String => {}", x.double());
let x = &x; println!("x:&String => {}", x.double());
let x = &x; println!("x:&&String => {}", x.double());
let x = &x; println!("x:&&&String => {}", x.double());
let x = &x; println!("x:&&&&String => {}", x.double());
}
OUTPUT:
x:String => 5 x:&String => 5 x:&&String => 10 x:&&&String => 10 x:&&&&String => 10
how the compiler decide the ord of auto-deref or auto-ref?
if we remove &&&String version of impl:
trait Double {
fn double(self) -> usize;
}
impl<‘a> Double for &‘a String {
fn double(self) -> usize { self.len()}
}
pub fn main() {
let x = "hello".to_string(); println!("x:String => {}", x.double());
let x = &x; println!("x:&String => {}", x.double());
let x = &x; println!("x:&&String => {}", x.double());
let x = &x; println!("x:&&&String => {}", x.double());
let x = &x; println!("x:&&&&String => {}", x.double());
}
OUTPUT:
x:String => 5 x:&String => 5 x:&&String => 5 x:&&&String => 5 x:&&&&String => 5
if we remove the &String version of impl:
trait Double {
fn double(self) -> usize;
}
impl<‘a, ‘b, ‘c> Double for &‘a &‘b &‘c String {
fn double(self) -> usize { self.len() * 2 }
}
pub fn main() {
let x = "hello".to_string(); println!("x:String => {}", x.double()); // error
let x = &x; println!("x:&String => {}", x.double()); // error
let x = &x; println!("x:&&String => {}", x.double());
let x = &x; println!("x:&&&String => {}", x.double());
let x = &x; println!("x:&&&&String => {}", x.double());
}
总结：在做x的方法搜索时，最多只会做一次自动引用，然后就一直做自动解引用。当为一个变量x找搜索方法f时，具体算法如下：
1、当前x上是否有方法f，有调用之；
2、没有，则在x上自动引用一次变成&x，&x上如果有方法f，则调用之；
3、没有，则尝试*x，一直重复3步，直到找到或失败。
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原文地址：http://my.oschina.net/zengsai/blog/376123
