rust存取一个含有borrowed域的结构体

原创

寻找繁星 2022-10-14 15:09:22 博主文章分类：rust ©著作权

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直接存取不可以：

use std::fs::File;
use std::io::{BufWriter, BufReader};

use serde::{Serialize, Deserialize};

extern crate bincode;

#[derive(Debug, Serialize, Deserialize)]
struct A<'a> {
    a: &'a [u8]
}

fn main() {
    let a: &[u8] = &[2, 3, 5, 7, 11, 13];
    let b = A{a};
    let writer = BufWriter::new(File::create("test").unwrap());
    bincode::serialize_into(&mut writer, &b).unwrap();
    drop(writer);

    let mut reader = BufReader::new(File::open("test").unwrap());
    let c = bincode::deserialize_from::<_, A>(&mut reader).unwrap();
    println!("{:?}", c);
}

会报错：

error: implementation of `Deserialize` is not general enough
  --> src/main.rs:21:13
   |
21 |     let c = bincode::deserialize_from::<_, A>(&mut reader).unwrap();
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Deserialize` is not general enough
   |
   = note: `A<'_>` must implement `Deserialize<'0>`, for any lifetime `'0`...
   = note: ...but `A<'_>` actually implements `Deserialize<'1>`, for some specific lifetime `'1`

error: aborting due to previous error

这是因为BufWriter里的缓存的所有权不是调用者的，可能会被释放，这样构造含有borrowed域的结构体时就不能安全地借用读出来的数据了。解决方法就是把这个结构体对应的文件数据读出来存到一个变量里，然后用bincode::deserialize来把结构体构造出来。一种简单的方法是构造一个wrapper:

#[derive(Debug, Serialize, Deserialize)]
struct Wrapper {
    data: Box<[u8]>,
}

然后先把结构体序列化到这个wrapper里，再将wrapper序列化到文件里。读的时候先把这个wrapper反序列化出来，这样这个wrapper就拿到了结构体的文件数据的所有权了，再用bincode::deserialize来把结构体构造出来。完整代码如下。

use std::fs::File;
use std::io::{Write, BufWriter, BufReader};

use serde::{Serialize, Deserialize};

extern crate bincode;

#[derive(Debug, Serialize, Deserialize)]
struct Wrapper {
    data: Box<[u8]>,
}

#[derive(Debug, Serialize, Deserialize)]
struct A<'a> {
    a: &'a [u8]
}

pub fn serialize_into_file_wrapped<W, T: ?Sized>(
    writer: W, value: &T
) -> std::result::Result<(), Box<bincode::ErrorKind>>
where
    W: Write,
    T: Serialize,
{
    let data = Box::from(bincode::serialize(value)?);
    bincode::serialize_into(writer, &Wrapper{data})
}

fn main() {
    let a: &[u8] = &[2, 3, 5, 7, 11, 13];
    let b = A{a};
    let mut writer = BufWriter::new(File::create("test").unwrap());
    serialize_into_file_wrapped(&mut writer, &b).unwrap();
    drop(writer);

    let mut reader = BufReader::new(File::open("test").unwrap());
    let wrapper: Wrapper = bincode::deserialize_from(&mut reader).unwrap();
    let c: A = bincode::deserialize(&wrapper.data).unwrap();
    println!("{:?}", c);
}