Nesting additional data in json object

[Chrums]

v3.3.0
gcc 7.3.0

Inexperienced C++ user here, so I may just be overlooking something.
My goal is to nest additional, context specific data inside a json object, such that when a json reference is returned by a member function, the data is passed to the child.

To be more specific, I have a Scene class that is deserializing a json file to instantiate a prefab. Within this prefab json file, there are IDs that can be locally referenced, but when the Scene is deserializing it, it generates a new ID for each ID in the json, so that if a prefab is instantiated multiple times, each instance won't have the same IDs.

{
  "components": {
    "hierarchy": {
      "0": {
        "parent": null,
        "children": ["1"]
      },
      "1": {
        "parent": "0",
        "children": []
      }
    }
  }
}

So in the preceding example, 0 and 1 are the local IDs, and this deserialization "context" would create a map of local IDs to real IDs that would look something like...

{  
  "0": "89207bbc-3e9a-4310-897b-aef7628ee063",
  "1": "79322b6c-4a2b-ab91-897b-f8f7626edf64"
}

The Scene doesn't know how to deserialize Hierarchy components, so it calls deserialize on each component, passing in the data that it needs to deserialize itself, such as...

{
  "parent": null,
  "children": ["1"]
}

But now the Hierarchy component knows nothing of the 'real ID' that "1" maps to.

I tried creating a class extending json, and storing this map in there as a shared_ptr, and wrapping all of the accessors, but quickly realized this wouldn't work because they return references to json objects, and I can't return a reference to my type.

The end goal is to allow the deserialization for the Hierarchy component to look something along the lines of...

void deserialize(json j) {
  parent = j["parent"].get<optional<Entity>>();
  children = j["children"].get<vector<Entity>>();
}

Which would call a specialized template on the json object for optional and vector that would look in the ID map, and return an Entity with the context specific ID.

The only thing I can think of is modifying the entire basic_json class to hold a void* or something, that it passes down to each child, and then my wrapper would utilize that void* to store the context specific data.

[nlohmann]

I am sorry, but I have not fully understood the issue.

• Right now, it is not possible to extend the JSON class - each object from it models a single JSON value with a type like null, boolean, number, etc.
• If you want to read/write other types from/to JSON, please have a look at this paragraph from the README. This can allow a deserialize function as you sketched it above.

Can you provide more information?

[stale]

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