如何解决当你在 Rust 中可变地阴影一个 Vector 时会发生什么?
我目前正在学习 rustlings 课程,当这个关于 移动语义 的练习出现时,我感到很困惑。这是它的要点:
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.4.4</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
<version>${spring.boot.version}</version>
</dependency>
据我所知,不可变的 fn main(){
let vec0 = Vec::new();
let mut vec1 = fill_vec(vec0);
//stuff happens
}
fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
let mut vec = vec;
//do stuff
vec
}
正在作为参数 vec0
移动到 fill_vec
,然后由 {{1} 可变地隐藏 }}。由于 Vector 是对堆存储的引用,这是否意味着 vec
由于阴影而发生,还是相同的指针被重用并且只是可变?为什么要在这种情况下使用阴影?
解决方法
是否重复使用了相同的指针并使其可变?
是的。完全相同的对象,在运行时它本质上是一个空操作。
为什么要在这种情况下使用阴影?
有些人喜欢通过阴影临时“锁定”和“解锁”绑定的模式,例如
let a = …;
…
let mut a = a;
// mutate `a` in-place
let a = a; // lock to readonly again
这真的只是个人喜好。这里:
- 本可以引入一个新变量,但它真的会更有用吗?会叫什么?
vec_mut
?不像源可以重复使用,它被移走了。 - 或者,输入参数可以直接为
mut vec: …
。
这主要取决于个人选择。
,从语义上讲,它是同一个对象,只是在您的情况下更改了名称。
但在当前实现中,它会将您的堆栈数据复制到堆栈中的新位置,因此 Vec 对象的地址将更改(但堆指针保持不变)。 这样做是因为你可以用新对象隐藏旧名称:
let v = make_first_vec();
let mut v = make_second_vec();
// old v still exists and it would be dropped only at end of block.
更好的例子(你可以运行它):
struct Droppable(i32);
impl Drop for Droppable{
fn drop(&mut self){
println!("Dropping {}",self.0);
}
}
fn main(){
// Same object moved
// Changes stack location but still same object
// dropped only once
println!("Same object moved begin");
{
let a = Droppable(1);
let old_ref = &a;
println!("Address is {}",&a as *const _ as usize);
let mut a = a;
println!("Address is {}",&a as *const _ as usize);
let a = a;
println!("Address is {}",&a as *const _ as usize);
// Cannot use old reference because old object is moved
// Compile error if uncommented
// println!("Old object is still alive and has {}",old_ref.0);
}
println!("Same object moved end");
// Different object hides
// dropped in reverse order
println!("Different object hides begin");
{
let a = Droppable(2);
let old_ref = &a;
println!("Address is {}",&a as *const _ as usize);
let a = Droppable(3);
println!("Address is {}",&a as *const _ as usize);
println!("Old object is still alive and has {}",old_ref.0);
}
println!("Different object hides end");
// Different object overrides
// old object dropped when overrided
println!("Different object override begin");
{
let mut a = Droppable(4);
let old_ref = &a;
println!("Address is {}",&a as *const _ as usize);
a = Droppable(5);
println!("Address is same {}",&a as *const _ as usize);
// Cannot use old reference because old object destroyed
// Compile error if uncommented
// println!("Old object is still alive and has {}",old_ref.0);
}
println!("Different object override end");
}
它打印:
Same object moved begin
Address is 140736088967924
Address is 140736088967888
Address is 140736088967892
Dropping 1
Same object moved end
Different object hides begin
Address is 140736088967888
Address is 140736088967892
Old object is still alive and has 2
Dropping 3
Dropping 2
Different object hides end
Different object override begin
Address is 140736088967892
Dropping 4
Address is same 140736088967892
Dropping 5
Different object override end
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