Accessing file location from Json::Value

[davidzchen]

For projects using jsoncpp to parse JSON configuration files, it would be useful to be able to obtain the actual file location (i.e. line, column) of a given JSON value. This way, it would be possible to print the line and column when reporting config validation errors, which would make such error messages more user/developer friendly.

If adding this functionality to the jsoncpp API is not desirable, is there an API to extend the parser to add such a feature?

[cdunn2001]

I'm not sure I understand. Maybe submit a PR to make this idea clear?

[davidzchen]

Internally, jsoncpp calls getLocationLineAndColumn in order to emit error messages at parsing time with the line and column of the token that is causing the error. I would like to be able to similarly print the line and column for a given JSON node while walking and validating the JSON tree that was parsed from a file. However, it seems that there is currently no way to obtain this information.

I can understand that this may not be a feature we would want to enable by default since it would require adding this location information to each Json::Value, but it would be sufficient for my use case if I could implement this feature through some kind of extension API.

[davidzchen]

To be more specific, what I am looking for is something like yaml-cpp's YAML::Mark class, which is set for each YAML::Node.

[cdunn2001]

Line and column info are are useful while a file is parsed, but not later, since a JSON object could be constructed from multiple files. The API would not be clear to me.

[KyleFromKitware]

I'm thinking we could add a new overload of Json::CharReader::parse() which accepts a pointer to a Json::Value. This Value would be recursively populated with line and column information from each token it encounters. This overload could even populate another Json::Value with more machine-readable error information.

As an example, the following JSON document:

{
  "name": "testDocument",
  "description": "This is a test document",
  "version": 1
}

could produce the following line/column information:

{
  "keys": {
    "name": {
      "line": 2,
      "startColumn": 3,
      "endColumn": 8
    },
    "description": {
      "line": 3,
      "startColumn": 3,
      "endColumn": 15
    },
    "version": {
      "line": 4,
      "startColumn": 3,
      "endColumn": 11
    }
  },
  "values": {
    "name": {
      "line": 2,
      "startColumn": 11,
      "endColumn": 24
    },
    "description": {
      "line": 3,
      "startColumn": 18,
      "endColumn": 42
    },
    "version": {
      "line": 4,
      "startColumn": 14,
      "endColumn": 14
    }
  }
}

[KyleFromKitware]

And I'm thinking the Json::CharReader::parse() overload could be something like:

bool parse(const char* beginDoc, const char* endDoc, Json::Value* root, Json::Value* lineInfo, Json::Value* errs)

[KyleFromKitware]

I just started attempting to implement my above proposal, and in the process discovered four methods in Json::Value that look relevant:

  // Accessors for the [start, limit) range of bytes within the JSON text from
  // which this value was parsed, if any.
  void setOffsetStart(ptrdiff_t start);
  void setOffsetLimit(ptrdiff_t limit);
  ptrdiff_t getOffsetStart() const;
  ptrdiff_t getOffsetLimit() const;

getLocationLineAndColumn() is currently private, but if that was made public and static, would that, in combination with the above methods, be sufficient to get location info from the parser?