如何解决使用Box对结构进行反序列化
包含Box作为成员变量的结构:
type Item = dyn Fn() -> Result<(),Box<dyn std::error::Error + Send + Sync>>;
struct Inner {
pub data: Box<Item>,}
// a function,like the type: Item
fn parse() -> Result<(),Box<dyn std::error::Error + Send + Sync>> {
println!("parse called");
Ok(())
}
现在出现的问题是,自定义实现对结构Inner
进行反序列化的反序列化:
let s = r#"{"data": "parse"}"#;
插入内部结构
{
data: Box::new(parse)
}
我知道serde
不会为Box<T>
实现反序列化,而必须手动实现反序列化,这是我的代码,遵循docmentaion给出的示例:
use serde::de::{self,Deserialize,Deserializer,MapAccess,Visitor};
use std::fmt;
type Item = dyn Fn() -> Result<(),}
fn parse() -> Result<(),Box<dyn std::error::Error + Send + Sync>> {
println!("parse called");
Ok(())
}
impl<'de> Deserialize<'de> for Inner {
fn deserialize<D>(deserializer: D) -> Result<Self,D::Error>
where
D: Deserializer<'de>,{
enum Field {
Data,};
impl<'de> Deserialize<'de> for Field {
fn deserialize<D>(deserializer: D) -> Result<Field,D::Error>
where
D: Deserializer<'de>,{
struct FieldVisitor;
impl<'de> Visitor<'de> for FieldVisitor {
type Value = Field;
fn expecting(&self,formatter: &mut fmt::Formatter) -> fmt::Result {
formatter.write_str("`data` only")
}
fn visit_str<E>(self,value: &str) -> Result<Field,E>
where
E: de::Error,{
match value {
"data" => Ok(Field::Data),_ => Err(de::Error::unkNown_field(value,FIELDS)),}
}
}
deserializer.deserialize_identifier(FieldVisitor)
}
}
struct InnerVisitor;
impl<'de> Visitor<'de> for InnerVisitor {
type Value = Inner;
fn expecting(&self,formatter: &mut fmt::Formatter) -> fmt::Result {
formatter.write_str("struct Inner")
}
fn visit_map<V>(self,mut map: V) -> Result<Inner,V::Error>
where
V: MapAccess<'de>,{
let mut data = None;
while let Some(key) = map.next_key()? {
match key {
Field::Data => {
if data.is_some() {
return Err(de::Error::duplicate_field("data"));
}
data = Some(map.next_value()?);
}
}
}
let data = data.ok_or_else(|| de::Error::missing_field("data"))?;
//
// do something on the `data` and finally return a Item-like function (***)
//
Ok(Inner {
data: Box::new(parse),}) // (***)
}
}
const FIELDS: &'static [&'static str] = &["data"];
deserializer.deserialize_struct("Inner",FIELDS,InnerVisitor)
}
}
fn main() {
let s = r#"{"data": "parse"}"#;
let inner: Inner = serde_json::from_str(s).unwrap();
}
但是,当我运行这些代码时,它会出错:
Compiling playground v0.0.1 (/playground)
warning: unused variable: `inner`
--> src/main.rs:93:9
|
93 | let inner: Inner = serde_json::from_str(s).unwrap();
| ^^^^^ help: if this is intentional,prefix it with an underscore: `_inner`
|
= note: `#[warn(unused_variables)]` on by default
warning: unused variable: `data`
--> src/main.rs:76:21
|
76 | let data = data.ok_or_else(|| de::Error::missing_field("data"))?;
| ^^^^ help: if this is intentional,prefix it with an underscore: `_data`
warning: 2 warnings emitted
Finished dev [unoptimized + debuginfo] target(s) in 1.12s
Running `target/debug/playground`
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error("invalid type: string \"parse\",expected unit",line: 1,column: 16)',src/main.rs:93:48
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
大致上,我认为问题出在该区块的(***)
上,但不知道如何解决。如何为我的用例正确实现Deserialize
?
解决方法
警告提示您:您没有使用数据,因此Rust不在乎Option内的类型。其实我很惊讶它正在编译。由于使用serdes默认类型,Rust假设您期望()
,而您期望String
。根本不会消耗您的数据,只需这样做:
let mut data: Option<&str> = None;
此外,我建议您将所有Field
实现替换为:
#[derive(Deserialize)]
#[serde(field_identifier,rename_all = "lowercase")]
enum Field {
Data,}
(此堆栈在playground上由于未知原因而溢出,但是我希望这是操场上的问题)
此外,您可以只使用deserialize_with:
use serde::{Deserialize,Deserializer};
type Item = dyn Fn() -> Result<(),Box<dyn std::error::Error + Send + Sync>>;
#[derive(Deserialize)]
struct Inner {
#[serde(deserialize_with = "deserialize_data")]
pub data: Box<Item>,}
fn deserialize_data<'de,D>(d: D) -> Result<Box<Item>,D::Error>
where
D: Deserializer<'de>,{
let data = <&str>::deserialize(d)?;
println!("{}",data);
Ok(Box::new(parse))
}
fn parse() -> Result<(),Box<dyn std::error::Error + Send + Sync>> {
println!("parse called");
Ok(())
}
fn main() {
let s = r#"{"data": "parse"}"#;
let inner: Inner = serde_json::from_str(s).unwrap();
}
这是更好的方法,因为通过Serde宏实现比纯朴的实现要好得多。
,与Stargateurs类似的技巧是在#[serde(from = "FromType")]
类上使用Inner
。这样,您就可以通过实现From
特性来遍历中介结构。
#[derive(Deserialize)]
struct InnerRepr {
pub data: String,}
#[derive(Deserialize)]
#[serde(from = "InnerRepr")]
struct Inner {
pub data: Box<Item>,}
impl From<InnerRepr> for Inner {
fn from(repr: InnerRepr) -> Inner {
Inner{ data: Box::new(parse) }
}
}
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