借入的价值没有足够长的循环寿命 [英] borrowed value does not live long enough in loop
问题描述
我正在尝试解析文件并从函数中返回 Vec< Vec< str>>
.但是在推送到向量时,我在文件读取循环中遇到借入值错误.
I'm trying to parse a file and return Vec<Vec<&str>>
from a function. But I'm getting borrowed value error inside the file read loop while pushing to the vector.
use std::io::{self, BufReader, prelude::*};
use std::fs::File;
fn read() -> Vec<Vec<&'static str>> {
let file = File::open("~/test").expect("failed to read file");
let reader = BufReader::new(file);
let mut main_vector: Vec<Vec<&str>> = Vec::new();
for line in reader.lines() {
match line {
Ok(v) => {
let mut sub_vector: Vec<&str> = Vec::new();
for element in v.split_whitespace().collect::<Vec<&str>>() {
sub_vector.push(element);
}
main_vector.push(sub_vector);
},
Err(e) => panic!("failed to parse: {:?}", e),
}
}
//return main_vector;
}
这是编译器错误:
error[E0597]: `v` does not live long enough
--> src/main.rs:67:32
|
67 | for element in v.split_whitespace().collect::<Vec<&str>>() {
| ^ borrowed value does not live long enough
...
70 | main_vector.push(sub_vector);
| -------------- borrow later used here
71 | },
| - `v` dropped here while still borrowed
我认为这与参考资料和借阅有关,但是我仍然很难弄清这一点.
I think it's about the references and borrowing but still I'm having hard time to figure this out.
推荐答案
This question is similar to Return local String as a slice (&str). And the easiest solution is the same as in that question - use String, not &str. These questions are different as that answer specifically talks about returning from a function, and you have no function listed.
要解决生命周期使代码失败的原因,请尝试一个更简单的示例
To address why lifetimes make your code fail, try a simpler example
fn main() {
let mut v:Vec<&str> = Vec::new();
{
let chars = [b'x', b'y', b'z'];
let s:&str = std::str::from_utf8(&chars).unwrap();
v.push(&s);
}
println!("{:?}", v);
}
和编译器输出
let s:&str = std::str::from_utf8(&chars).unwrap();
^^^^^^ borrowed value does not live long enough
这不起作用的原因正是编译器所说的. chars
是在块内创建的,因此它具有与该块相关联的生存期,并且当您的程序退出该块时,chars可能不再存在.引用 chars
的任何内容都可能具有悬空指针.Rust通过将其设为非法来避免悬空指针.在我的示例中,Rust不允许这样做是很愚蠢的,但是在您看来,Rust可以通过从 v.split_whitespace()中删除旧的
. str
s使堆栈变小.每次循环时都收集::< Vec< str>>()
The reason this doesn't work is exactly what the compiler says. chars
is created inside the block, so it gets a lifetime associated with that block, and when your program exits that block, chars might not exist anymore. Anything that referred to chars
might have a dangling pointer. Rust avoids dangling pointers by making this illegal. In my example it seems silly that Rust doesn't allow this, but in yours it makes sense - Rust can keep the stack small by deleting the old str
s from v.split_whitespace().collect::<Vec<&str>>()
every time through the loop.
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