Imitating Named Supertype/Union Class

[RespiteSage]

I'm working off of a JSON Schema that I and one of my coworkers are using for interprocess communication between programs in two different languages (Java and C++). We're autogenerating code using different tools (as my coworker's tool is Java-specific), and I recently realized that they were using the extends syntax, which has apparently been deprecated for quite a long time, to generate code for a superclass without any properties. Here's the relevant section of subschemas (from a file with definitions for a bunch of subschemas that are used in several other schemas):

{
  "distribution": {
    "title": "Distribution",
    "description": "A Probability Distribution",
    "type": "object"
  },
  "normalDistribution": {
    "title": "NormalDistribution",
    "description": "A Normal Probability Distribution",
    "type": "object",
    "extends": {
      "$ref": "#/distribution"
    },
    "additionalProperties": false,
    "required": [
      "mean",
      "standardDeviation"
    ],
    "properties": {
      "mean": {
        "description": "The mean (middle) of the distribution",
        "type": "number"
      },
      "standardDeviation": {
        "description": "The standard deviation (spread) of the distribution",
        "type": "number"
      }
    }
  },
  "triangularDistribution": {
    "title": "TriangularDistribution",
    "description": "A Triangular Probability Distribution",
    "type": "object",
    "extends": {
      "$ref": "#/distribution"
    },
    "additionalProperties": false,
    "required": [
      "maximum",
      "minimum",
      "peak",
      "ratio"
    ],
    "properties": {
      "maximum": {
        "description": "The maximum of the distribution",
        "type": "number"
      },
      "minimum": {
        "description": "The minimum of the distribution",
        "type": "number"
      },
      "peak": {
        "description": "The peak of the distribution",
        "type": "number"
      },
      "ratio": {
        "description": "The ratio of the distribution",
        "type": "number"
      }
    }
  },
  "uniformDistribution": {
    "title": "UniformDistribution",
    "description": "A Uniform Probability Distribution",
    "type": "object",
    "extends": {
      "$ref": "#/distribution"
    },
    "additionalProperties": false,
    "required": [
      "maximum",
      "minimum"
    ],
    "properties": {
      "maximum": {
        "description": "The maximum of the distribution",
        "type": "number"
      },
      "minimum": {
        "description": "The minimum of the distribution",
        "type": "number"
      }
    }
  }
}

The issue is that my tool (quicktype) does not implement the extends syntax (very understandably), so I'm hoping that there's a way around this other than using oneOf with references to all of the "subtype" schemas wherever a Distribution is needed. My current approach is to use oneOf in the "supertype":

{
  "distribution": {
    "title": "Distribution",
    "description": "A Probability Distribution",
    "type": "object",
    "oneOf": [
      {
        "$ref": "#/normalDistribution"
      },
      {
        "$ref": "#/triangularDistribution"
      },
      {
        "$ref": "#/uniformDistribution"
      }
    ],
    "additionalProperties": false
  },
  "normalDistribution": {
    "title": "NormalDistribution",
    "description": "A Normal Probability Distribution",
    "type": "object",
    "additionalProperties": false,
    "required": [
      "mean",
      "standardDeviation"
    ],
    "properties": {
      "mean": {
        "description": "The mean (middle) of the distribution",
        "type": "number"
      },
      "standardDeviation": {
        "description": "The standard deviation (spread) of the distribution",
        "type": "number"
      }
    }
  },
  "triangularDistribution": {
    "title": "TriangularDistribution",
    "description": "A Triangular Probability Distribution",
    "type": "object",
    "additionalProperties": false,
    "required": [
      "maximum",
      "minimum",
      "peak",
      "ratio"
    ],
    "properties": {
      "maximum": {
        "description": "The maximum of the distribution",
        "type": "number"
      },
      "minimum": {
        "description": "The minimum of the distribution",
        "type": "number"
      },
      "peak": {
        "description": "The peak of the distribution",
        "type": "number"
      },
      "ratio": {
        "description": "The ratio of the distribution",
        "type": "number"
      }
    }
  },
  "uniformDistribution": {
    "title": "UniformDistribution",
    "description": "A Uniform Probability Distribution",
    "type": "object",
    "additionalProperties": false,
    "required": [
      "maximum",
      "minimum"
    ],
    "properties": {
      "maximum": {
        "description": "The maximum of the distribution",
        "type": "number"
      },
      "minimum": {
        "description": "The minimum of the distribution",
        "type": "number"
      }
    }
  }
}

The code generated in C++ for this isn't particularly satisfactory, but I don't know whether this is an issue with my schema or something I could request from or contribute to the tool I'm using. Is there a better way to get what I want here?

Notably, this question differs from all the JSON schema "inheritance" questions I could find online because I don't want a supertype with properties. I essentially just want a named union subschema.

[gregsdennis]

I recently realized that they were using the extends syntax, which has apparently been deprecated for quite a long time...

extends was never part of the spec. There have been discussions around it, and at least one implementation supported it (not sure which), but it's not something that was ever official.

I realize this isn't really an answer, but you may be interested in https://github.com/json-schema-org/vocab-idl, which is exploring cases exactly like this in an effort to build a vocabulary so that it can be officially supported.

[RespiteSage]

It seems that the tool my coworker is using for generating Java from the schema is jsonschema2pojo. Apparently that tool specifically uses extends.

[awwright]

Actually "extends" existed for a while, but was removed after draft-03. I forget the exact reason... I think it became obsolete after $ref, and it wasn't implemented consistently.

[handrews]

I think one of the reasons that extends was dropped gets back to the nature of JSON Schema as a constraint system for validation rather than a data definition system.

Using draft-03's extends the way you show here did not mean that the extended schema (#/definitions/distribution) would function like the version that uses oneOf. With that oneOf approach #/definitions/distribution will validate instances that look like any of those three other schemas.**

However, with extends, #/definitions/distribution will validate anything and everything, because it has no local constraints, and it is unware of the extends in the other schema objects. Of course, people could implement tools that behaved differently, but that's not actually in the spec (this might have been part of the "wasn't consistently implemented" thing that Austin mentioned, idk, I didn't start using JSON Schema until draft-04).

JSON Schema processing generally works in a parent-to-child fashion, with "child" encompassing $ref targets at runtime. So you can incorporate the constraints of a child schema by $ref-ing it from the parent in a oneOf, but you can't magically have its effects incorporated by $ref-ing the "parent" from the "child". I put those in quotes because what really happens then is that your "parent" is treated as a child in terms of JSON Schema processing. So extends was always backwards from how the processing actually works. At least as best I can tell, years and years after extends was dropped.

**except actually it won't work at all, because "additionalProperties": false next to a oneOf doesn't do what you want it to do. You want "unevaluatedProperties": false, which wasn't introduced until draft 2019-09.