Jsony newHook拥有“`SIGSEGV:非法存储访问”。(尝试从nil读取？)当反序列化为ref对象时

Question

我正在编写一个web应用程序，并通过索尼反序列化为规范模型对象类型.范数-模型类型总是引用对象.不知何故，我的代码(非常类似于jsony的github文档中的默认示例)没有编译。相反，我收到错误SIGSEGV: Illegal storage access. (Attempt to read from nil?)。

看这里我的代码示例

import std/[typetraits, times]
import norm/[pragmas, model]
import jsony

const OUTPUT_TIME_FORMAT* = "yyyy-MM-dd'T'HH:mm:ss'.'ffffff'Z'"
type Character* {.tableName: "wikientries_character".} = ref object of Model
    name*: string
    creation_datetime*: DateTime
    update_datetime*: DateTime

proc parseHook*(s: string, i: var int, v: var DateTime) =
    ##[ jsony-hook that is automatically called to convert a json-string to datetime
    
    ``s``: The full JSON string that needs to be serialized. Your type may only be a part of this
    ``i``: The index on the JSON string where the next section of it starts that needs to be serialized here
    ``v``: The variable to fill with a proper value]##
    var str: string
    s.parseHook(i, str)
    v = parse(s, OUTPUT_TIME_FORMAT, utc())

proc newHook*(entry: var Character) =
  let currentDateTime: DateTime = now()
  entry.creation_datetime = currentDateTime # <-- This line is listed as the reason for the sigsev
  entry.update_datetime = currentDateTime
  entry.name = ""
  
var input = """ {"name":"Test"} """

let c = input.fromJson(Character)

我不明白这里的问题是什么，因为它的github页面上的jsony示例看起来非常相似：

type
  Foo5 = object
    visible: string
    id: string
proc newHook*(foo: var Foo5) =
  # Populates the object before its fully deserialized.
  foo.visible = "yes"

var s = """{"id":"123"}"""
var v = s.fromJson(Foo5)
doAssert v.id == "123"
doAssert v.visible == "yes"

我怎么才能解决这个问题？

Answer 1

答案在于，规范对象类型是ref对象，而不是普通(值)对象(感谢ElegantBeef、Rika和Yardanico从nim-不和谐中指出这一点)！如果您没有在某一点上显式地“创建”一个ref类型，那么它的内存就不会被分配，因为代码不会为您分配内存，而不像使用值类型那样！

因此，您必须先初始化/创建一个ref对象，然后才能使用它，而Jsony不负责初始化！

因此，编写上述newHook的正确方法如下所示：

proc newHook*(entry: var Character) =
  entry = new(Character)
  let currentDateTime: DateTime = now()
  entry.creation_datetime = currentDateTime
  entry.update_datetime = currentDateTime
  entry.name = ""