# JSON::Serializable =\> custom initialize

**URL:** <https://forum.crystal-lang.org/t/json-serializable-custom-initialize/8459>\
**Category:** Help & Support\
**Created:** [October 7, 2025, 1:45am UTC](https://forum.crystal-lang.org/t/json-serializable-custom-initialize/8459 "2025-10-07T01:45:51Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![AlexR](https://avatars.discourse-cdn.com/v4/letter/a/8dc957/32.png) [@AlexR](https://forum.crystal-lang.org/u/AlexR)\
**Post date:** [October 7, 2025, 1:45am UTC](https://forum.crystal-lang.org/t/json-serializable-custom-initialize/8459/1 "2025-10-07T01:45:51Z")

</div>

Hi.

Given this…

```auto
# serial_tester.cr

require "sdl-crystal-bindings/sdl3-crystal-bindings"
require "json"

class SerialTester
  include JSON::Serializable

  property width : UInt32
  property height : UInt32
  
  @[JSON::Field(ignore: true)]
  getter renderer : LibSDL::Renderer*

  @[JSON::Field(ignore: true)]
  getter pixels : Array(UInt32)

  def initialize(@width, @height, @renderer)
    size = @width * @height
    @pixels = Array(UInt32).new(size, 0x00000000)
  end
end

```

```auto
# app.cr

@renderer = LibSDL::create_renderer etc etc...
...
st = SerialTester.new(500, 500, @renderer)
j = st.to_json()
puts j

# outputs {"width":500,"height":500} as expected

```

and that :

```auto
# app.cr

@renderer = LibSDL::create_renderer etc etc...

st = SerialTester.new(500, 500, @renderer)
j = st.to_json()
st2 = SerialTester.from_json(j)

# compilation error
In /opt/homebrew/Cellar/crystal/1.16.3/share/crystal/src/json/serialization.cr:181:9

 181 | def initialize(*, __pull_for_json_serializable pull : ::JSON::PullParser)
           ^---------
Error: instance variable '@renderer' of SerialTester was not initialized in this 'initialize', rendering it nilable

```

I couldn’t find a lot of JSON::Serializable examples for how to deal with custom initialize methods and am wondering what is the standard way to work with that.

---

<div class="post-metadata">

**Author:** ![Blacksmoke16](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/blacksmoke16/32/1241_2.png) [@Blacksmoke16](https://forum.crystal-lang.org/u/Blacksmoke16)\
**Post date:** [October 7, 2025, 3:24am UTC](https://forum.crystal-lang.org/t/json-serializable-custom-initialize/8459/2 "2025-10-07T03:24:15Z")

</div>

There is a `#after_initialize` method that gets called internally, if defined, after the instance is instantiated. However this won’t help you because there’d still be no way to set `@renderer` and keep it non-nilable. If you make it nilable, maybe using a non-public setter, you could use `getter!` to treat it as still not-nilable when you access it via `self.renderer`. Then a custom `.from_json` constructor to first call `.from_json` to get your instance, then `#renderer= ...` to set your renderer.

Another option would be to leverage the `converter` `JSON::Field` annotation to define _how_ to provide the renderer from the JSON. Given it’s not actually in the JSON I think you could just not do anything with the pull parser and just return the renderer somehow. Tho this may not be straightforward either depending on where/how the renderer comes into existance.

Maybe the best option would be to have a separate service/type that handles the rendering and keep this type as more of a DTO so you dont have to deal with it at all.

---

<div class="post-metadata">

**Author:** ![AlexR](https://avatars.discourse-cdn.com/v4/letter/a/8dc957/32.png) [@AlexR](https://forum.crystal-lang.org/u/AlexR)\
**Post date:** [October 7, 2025, 12:34pm UTC](https://forum.crystal-lang.org/t/json-serializable-custom-initialize/8459/3 "2025-10-07T12:34:47Z")

</div>

Thanks. Clear.