confused of paresr callback's behavior of null object: remove or discarded?

[gino0717]

hi sir,

I'm studying the document of Parser Callbacks

I replace some of the values to null of the example , then expect the parser would remove the object if the value is null.

the input goes like :

{
        "Image": {
            "Width":  null,
            "Height": 600,
            "Title":  "View from 15th Floor",
            "Thumbnail": {
                "Url":    "http://www.example.com/image/481989943",
                "Height": 125,
                "Width":  null
            },
            "Animated" : false,
            "IDs": [116, 943, 234, 38793]
        }
}

Both "Width" under "Image" and "Thumbnail" are null.

in version 2.1.1 which directly installed from apt-get gives the result just as I expected, both the "Width" disappear due to the null value:

{
    "Image": {
        "Animated": false,
        "Height": 600,
        "IDs": [
            116,
            943,
            234,
            38793
        ],
        "Thumbnail": {
            "Height": 125,
            "Url": "http://www.example.com/image/481989943"
        },
        "Title": "View from 15th Floor"
    }
}

but in version 3.10.5 it gives the result like:

{
    "Image": {
        "Animated": false,
        "Height": 600,
        "IDs": [
            116,
            943,
            234,
            38793
        ],
        "Thumbnail": {
            "Height": 125,
            "Url": "http://www.example.com/image/481989943",
            "Width": <discarded>
        },
        "Title": "View from 15th Floor"
    }
}

the second "Width" in Thumbnail becomes "discarded" , and the "discarded" itself cannot be parsed if I feed the filtered json back to parser by :
json parse_again=json::parse(j_filtered.dump());

it gives

terminate called after throwing an instance of 'nlohmann::detail::parse_error'
what(): [json.exception.parse_error.101] parse error at line 1, column 148: syntax error while parsing value - invalid literal; last read: '"Width":<'

When does the parser give the null object a "discarded" value or just remove the object if filtered by the parser callback? Could I chose which behavior it acts?

here's my original code:

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a JSON text
    auto text = R"(
    {
        "Image": {
            "Width":  null,
            "Height": 600,
            "Title":  "View from 15th Floor",
            "Thumbnail": {
                "Url":    "http://www.example.com/image/481989943",
                "Height": 125,
                "Width":  null
            },
            "Animated" : false,
            "IDs": [116, 943, 234, 38793]
        }
    }
    )";

    // parse and serialize JSON
    json j_complete = json::parse(text);
    std::cout << std::setw(4) << j_complete << "\n\n";

    // define parser callback
    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
    {
        // skip object elements with key "Thumbnail"
       // if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
         if (event == nlohmann::json::parse_event_t::value and parsed.is_null())
        {
            return false;
        }
        else
        {
            return true;
        }
    };

    // parse (with callback) and serialize JSON
    json j_filtered = json::parse(text, cb);
    std::cout << std::setw(4) << j_filtered << '\n';
    
    json parse_again=json::parse(j_filtered.dump());
}