6

我需要将 JSON 反序列化为具有Vec<Vec<f64>>字段的结构。JSON 具有数字字符串，因此我需要一个自定义反序列化器f64在反序列化期间将字符串转换为。

我想反序列化的示例 JSON：

{
  "values": [["2", "1.4"], ["8.32", "1.5"]]
}

我的结构是这样的：

#[derive(Deserialize)]
struct Payload {
    #[serde(default, deserialize_with = "from_array_of_arrays_of_strs")]
    values: Vec<Vec<f64>>,
}

我在 Serde 的示例中看到您可能可以对访问者执行此操作，因此我实现了此访问者：

fn from_array_of_arrays_of_strs<'de, T, D>(deserializer: D) -> Result<Vec<Vec<f64>>, D::Error>
where
    T: Deserialize<'de>,
    D: Deserializer<'de>,
{
    struct F64Visitor(PhantomData<fn() -> Vec<Vec<f64>>>);

    impl<'de> Visitor<'de> for F64Visitor {
        type Value = Vec<Vec<f64>>;

        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.write_str("a nonempty sequence of numbers")
        }

        #[inline]
        fn visit_str<E>(self, value: &str) -> Result<f64, E>
        where
            E: serde::de::Error,
        {
            self.visit_string(String::from(value))
        }

        #[inline]
        fn visit_string<E>(self, value: String) -> Result<f64, E> {
            Ok(value.parse::<f64>().unwrap())
        }

        #[inline]
        fn visit_seq<V, T>(self, mut visitor: V) -> Result<Vec<T>, V::Error>
        where
            V: SeqAccess<'de>,
        {
            let mut vec = Vec::new();

            while let Some(elem) = try!(visitor.next_element()) {
                vec.push(elem);
            }

            Ok(vec)
        }
    }

    let visitor = F64Visitor(PhantomData);
    deserializer.deserialize_seq(visitor)
}

操场

编译器抱怨visit_str并visit_string有一个不兼容的特征类型：

error[E0053]: method `visit_str` has an incompatible type for trait
  --> src/main.rs:32:9
   |
32 | /         fn visit_str<E>(self, value: &str) -> Result<f64, E>
33 | |             where
34 | |             E: serde::de::Error,
35 | |         {
36 | |             self.visit_string(String::from(value))
37 | |         }
   | |_________^ expected struct `std::vec::Vec`, found f64
   |
   = note: expected type `fn(from_array_of_arrays_of_strs::F64Visitor, &str) -> std::result::Result<std::vec::Vec<std::vec::Vec<f64>>, E>`
              found type `fn(from_array_of_arrays_of_strs::F64Visitor, &str) -> std::result::Result<f64, E>`

error[E0053]: method `visit_string` has an incompatible type for trait
  --> src/main.rs:40:9
   |
40 | /         fn visit_string<E>(self, value: String) -> Result<f64, E> {
41 | |             Ok(value.parse::<f64>().unwrap())
42 | |         }
   | |_________^ expected struct `std::vec::Vec`, found f64
   |
   = note: expected type `fn(from_array_of_arrays_of_strs::F64Visitor, std::string::String) -> std::result::Result<std::vec::Vec<std::vec::Vec<f64>>, E>`
              found type `fn(from_array_of_arrays_of_strs::F64Visitor, std::string::String) -> std::result::Result<f64, E>`

error[E0049]: method `visit_seq` has 2 type parameters but its trait declaration has 1 type parameter
  --> src/main.rs:45:21
   |
45 |         fn visit_seq<V, T>(self, mut visitor: V) -> Result<Vec<T>, V::Error>
   |                     ^^^^^^ found 2 type parameters, expected 1

我想我对访问者的工作方式没有正确的理解。我可以只有一个访问者来反序列化字符串数组，还是需要一个访问者来反序列化数组，而需要一个访问者来反序列化字符串f64？

我读了：

4

2 回答 2

5

如如何在使用 serde 反序列化之前转换字段中所述？，最简单的解决方案是为您的字符串作为浮点值引入一个新类型。然后，您可以Deserialize利用现有的实现Deserialize和字符串解析来实现：

extern crate serde;
#[macro_use]
extern crate serde_derive;
extern crate serde_json;

use serde::de::{Deserialize, Deserializer, Error, Unexpected};

#[derive(Debug, Deserialize)]
struct Payload {
    #[serde(default)]
    values: Vec<Vec<Value>>,
}

#[derive(Debug)]
struct Value(f64);

impl<'de> Deserialize<'de> for Value {
    fn deserialize<D>(deserializer: D) -> Result<Value, D::Error>
        where D: Deserializer<'de>
    {
        let s: &str = Deserialize::deserialize(deserializer)?;
        s.parse()
            .map(Value)
            .map_err(|_| D::Error::invalid_value(Unexpected::Str(s), &"a floating point number as a string"))
    }
}

fn main() {
    let input = r#"
{
  "values": [["2", "1.4"], ["8.32", "1.5"]]
}
"#;

    let out: Payload = serde_json::from_str(input).unwrap();

    println!("{:?}", out);
}

我更喜欢这种解决方案，因为在许多情况下，我希望这种新类型在我的系统中发挥作用。

如果你真的，真的需要反序列化一次并且正好是 a Vec<Vec<f64>>，你必须实现两个访问者。一个将反序列化外部Vec，一个将反序列化内部Vec。我们将重用之前的Valuenewtype，但内部访问者会将其剥离。外部访问者将对围绕内部访问者的新类型执行相同的操作：

extern crate serde;
#[macro_use]
extern crate serde_derive;
extern crate serde_json;

use serde::de::{Deserialize, Deserializer, Error, SeqAccess, Unexpected, Visitor};
use std::fmt;

#[derive(Debug, Deserialize)]
struct Payload {
    #[serde(default, deserialize_with = "from_array_of_arrays_of_strs")]
    values: Vec<Vec<f64>>,
}

fn from_array_of_arrays_of_strs<'de, D>(deserializer: D) -> Result<Vec<Vec<f64>>, D::Error>
where
    D: Deserializer<'de>,
{
    struct OuterVisitor;

    impl<'de> Visitor<'de> for OuterVisitor {
        type Value = Vec<Vec<f64>>;

        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.write_str("a nonempty sequence of a sequence of numbers")
        }

        #[inline]
        fn visit_seq<V>(self, mut visitor: V) -> Result<Self::Value, V::Error>
        where
            V: SeqAccess<'de>,
        {
            let mut vec = Vec::new();

            while let Some(Inner(elem)) = try!(visitor.next_element()) {
                vec.push(elem);
            }

            Ok(vec)
        }
    }

    deserializer.deserialize_seq(OuterVisitor)
}

struct Inner(Vec<f64>);

impl<'de> Deserialize<'de> for Inner {
    fn deserialize<D>(deserializer: D) -> Result<Inner, D::Error>
    where
        D: Deserializer<'de>,
    {
        struct InnerVisitor;

        impl<'de> Visitor<'de> for InnerVisitor {
            type Value = Inner;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("a nonempty sequence of numbers")
            }

            #[inline]
            fn visit_seq<V>(self, mut visitor: V) -> Result<Inner, V::Error>
            where
                V: SeqAccess<'de>,
            {
                let mut vec = Vec::new();

                while let Some(Value(elem)) = try!(visitor.next_element()) {
                    vec.push(elem);
                }

                Ok(Inner(vec))
            }
        }

        deserializer.deserialize_seq(InnerVisitor)
    }
}

struct Value(f64);

impl<'de> Deserialize<'de> for Value {
    fn deserialize<D>(deserializer: D) -> Result<Value, D::Error>
    where
        D: Deserializer<'de>,
    {
        let s: &str = Deserialize::deserialize(deserializer)?;
        s.parse().map(Value).map_err(|_| {
            D::Error::invalid_value(Unexpected::Str(s), &"a floating point number as a string")
        })
    }
}

fn main() {
    let input = r#"
{
  "values": [["2", "1.4"], ["8.32", "1.5"]]
}
"#;

    let out: Payload = serde_json::from_str(input).unwrap();

    println!("{:?}", out);
}
于 2018-01-17T03:29:28.330 回答
1

无需自己编写访问者，也可以使用字符串而不是数字来解析 JSON 文件。

use serde_with::{serde_as, DisplayFromStr};

#[serde_as]
#[derive(Debug, serde::Deserialize)]
struct Payload {
    #[serde_as(as = "Vec<Vec<DisplayFromStr>>")]
    #[serde(default)]
    values: Vec<Vec<f64>>,
}

let j = serde_json::json!({
  "values": [["2", "1.4"], ["8.32", "1.5"]]
});

let p: Payload = serde_json::from_value(j)?;
assert_eq!(p.values, vec![vec![2.0, 1.4], vec![8.32, 1.5]]);

注释意味着我们有一个Vec内部 aVec并且最里面的元素应该使用反FromStr序列化和序列化使用Display。注解支持任何带有Display和FromStr实现的类型，因此它也可以用于类型上u64或Url类型上。

于 2021-04-25T13:45:06.670 回答