Ignore nullptr values on constructing json object from a container

[vassilisw]

I use void to_json(json& j, const Bar& b) function. In some cases I would like this function to just assign nullptr to j. For example...

void to_json(nlohmann::json& j, const Bar& b) {
    if (b.value == __FLT_MAX__) {
        j = nullptr;
        return;
    }
    j["name"] = b.name;
    j["value"] = b.value;
}
...

    std::vector<Bar> bars;
    bars.emplace_back("b1", 1.234);
    bars.emplace_back("b2", __FLT_MAX__);
    bars.emplace_back("b3", __FLT_MAX__);

    nlohmann::json j(bars);
    std::cout << j.dump(4) << std::endl;

Output...

[
    {
        "name": "b1",
        "value": 1.232
    },
    null,
    null
]

Is there a way for nullptr objects to just get ignored when the json object is created?
Such that the object to be...

[
    {
        "name": "b1",
        "value": 1.232
    }
]

I am aware of the parser_callback_t, anything similar for this?

[nlohmann]

Unfortunately, this is not possible at the moment (maybe @theodelrieu has an idea).

In the meantime, you can employ the erase-remove idiom to remove null values, just like it is possible for std::vector:

j.erase(std::remove(j.begin(), j.end(), nullptr), j.end());

[FrancoisChabot]

Another way to do this would be to provide an explicit to_json() for std::vector<Bar>, and apply the filter there:

void to_json(nlohmann::json& j, const Bar& b) {
    j["name"] = b.name;
    j["value"] = b.value;
}

void to_json(nlohmann::json& j, const std::vector<Bar>& b) {
  for(const auto& i : b) {
    if(i.value == __FLT_MAX__) {continue;}
    j.push_back(i);
  }
}

[vassilisw]

@FrancoisChabot very nice workaround indeed!

[nlohmann]

@vassilisw Does this solve your issue?

[vassilisw]

Yes it does, thanks (sorry for not closing).

[TomGarden]

Unfortunately, this is not possible at the moment (maybe @theodelrieu has an idea).

In the meantime, you can employ the erase-remove idiom to remove null values, just like it is possible for std::vector:

j.erase(std::remove(j.begin(), j.end(), nullptr), j.end());

@nlohmann
Hello , I try the code , I get err data . my nlohmann lib version '3.11.2'

• I what to filter json data , del null element , get not null element json obj .
• my env
  • gcc (Debian 10.2.1-6) 10.2.1 20210110
  • Debian GNU/Linux 11 (bullseye)
  • C++ STL 20
• my test code : https://godbolt.org/z/WY98be9v3
• no build err , data err .

you have any suggest ?

thanks .

  nlohmann::json jObj = R"(
    {
      "callback": null,
      "city": null,
      "citylimit": null,
      "datatype": null,
      "key": "123123123",
      "keywords": "123123123",
      "location": "1.1.123",
      "output": "JSON",
      "sig": null,
      "type": null
    }
    )"_json;
  jObj.erase(std::remove(jObj.begin(), jObj.end(), nullptr), jObj.end());
  std::cout << jObj.dump(2) << std::endl;

/* expected output */
{
      "key": "123123123",
      "keywords": "123123123",
      "location": "1.1.123",
      "output": "JSON",
}

/* actual output */
{
      "callback": "123123123",
      "city": "123123123",
      "citylimit": "1.1.123",
      "datatype": "JSON"
}   

[TomGarden]

my last code

  nlohmann::json& clearNull(nlohmann::json& jObj) {

    nlohmann::json jClone = jObj;
    jObj.clear();

    for (const auto& item : jClone.items()) {
      if (item.value().is_object()) {
        nlohmann::json jSubObj = clearNull(item.value());
        if (!jSubObj.empty()) jObj[item.key()] = jSubObj;
      } else if (!item.value().is_null()) {
        jObj[item.key()] = item.value();
      }
    }

    return jObj;
  }

[nlohmann]

@TomGarden

In order to asses your issue, we need the following information:

• What is the issue you have?

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

• What is the expected behavior?

• And what is the actual behavior instead?

• Which compiler and operating system are you using? Is it a supported compiler?

• Did you use a released version of the library or the version from the develop branch?

• If you experience a compilation error: can you compile and run the unit tests?

[lingxd]

my last code

  nlohmann::json& clearNull(nlohmann::json& jObj) {

    nlohmann::json jClone = jObj;
    jObj.clear();

    for (const auto& item : jClone.items()) {
      if (item.value().is_object()) {
        nlohmann::json jSubObj = clearNull(item.value());
        if (!jSubObj.empty()) jObj[item.key()] = jSubObj;
      } else if (!item.value().is_null()) {
        jObj[item.key()] = item.value();
      }
    }

    return jObj;
  }

nlohmann::json &clear_null(nlohmann::json &j)
{
    nlohmann::json j_clone = j;
    j.clear();

    for (const auto& item : j_clone.items()) {
        if (item.value().is_object()) {
            nlohmann::json jSubObj = clear_null(item.value());
            if (!jSubObj.empty()) j[item.key()] = jSubObj;
        } else if (item.value().is_array()) {
            nlohmann::json jSubArray = item.value();
            jSubArray.erase(std::remove_if(jSubArray.begin(), jSubArray.end(), [](const nlohmann::json& el) {
                                return el.is_null(); 
                            }), jSubArray.end());
            for (json::iterator it = jSubArray.begin(); it != jSubArray.end(); ++it) {
                clear_null(*it);
            }
            if (!jSubArray.empty()) j[item.key()] = jSubArray;
        } else if (!item.value().is_null()) {
            j[item.key()] = item.value();
        }
    }

    return j;
}

if array is null need to erase.

[lingxd]

Unfortunately, this is not possible at the moment (maybe @theodelrieu has an idea).

In the meantime, you can employ the erase-remove idiom to remove null values, just like it is possible for std::vector:

j.erase(std::remove(j.begin(), j.end(), nullptr), j.end());

thanks, I use version 3.11.3 to use boost::optional, this is my code

NLOHMANN_JSON_NAMESPACE_BEGIN
template <typename T>
void to_json(json& j, const boost::optional<T>& opt) {
    if (opt) {
        j = *opt;
    } else {
        std::cout << j.dump() << std::endl;
    }
}

template <typename T>
void from_json(const json& j, boost::optional<T>& opt) {
    if (j.is_null()) {
        opt = boost::none;
    } else {
        opt = j.template get<T>();
    }
}

NLOHMANN_JSON_NAMESPACE_END

What I want to ask is that in the case of opt::none, will the value of null be filled in the default?

{
    "json": null
}

[gregmarr]

nlohmann::json &clear_null(nlohmann::json &j)
{
    nlohmann::json j_clone = j;
    j.clear();

That's an unnecessary copy. This just uses moves:

nlohmann::json &clear_null(nlohmann::json &j)
{
    nlohmann::json j_clone;
    std::swap(j, j_clone);