JSON Creation

[fkicenko]

Hello,
I'm trying to create a JSON structure that contains Arrays and Objects that are hierarchical. Could spend a considerable amount of time but thought I would post my questions first. I'm having issues creating Objects (key:value) pairs eg: "modelSeries":"520", within an Array. Also creating Arrays within Arrays. I'm attaching a JSON file that i'm working on and below is the code i've created so far. Hopefully someone can shed some light on the easiest way to do this.

Code:

// create an empty structure 
	json j;
	// Update the template configuration version
	std::string standardString = marshal_as<std::string>(hsConfig->getTemplateVersion());
	j["headsetConfig"]["templateConfiguration"]["configTemplateVersion"] = standardString;
	// Update the Timestamp
	hsConfig->setUpdateTime(strTemp);
	standardString = marshal_as<std::string>(hsConfig->getUpdateTime());
	j["headsetConfig"]["templateConfiguration"]["updatedTime"] = standardString;
	// Update the ReportId
	standardString = marshal_as<std::string>(hsConfig->getReportId());
	j["headsetConfig"]["templateConfiguration"]["reportId"] = standardString;
	// Add the Model Specific array of models
	json mSpecificSettings = j.array();
	// Loop through the Models
	// Set all the models for this series
	json models = { "models" };
	json mModelSeries;
	for each(ModelSpecific^ ms in hsConfig->ModelSpecificList)
	{
		String^ s = ms->getModelSeries();
		standardString = marshal_as<std::string>(s);
		mModelSeries = { {"modelSeries", standardString } };

		mSpecificSettings.push_back({ {"modelSeries", standardString } });
		// Build the models array eg:521, 522, 523
		json models250 = j.array();
		for each(String^ mds in ms->modelList)
		{
			standardString = marshal_as<std::string>(mds);
			models250.insert(models250.end(), standardString);
		}
		mSpecificSettings.push_back(models250);
		//mModelSeries.insert(mModelSeries.end(), models250);
	}
	j["headsetConfig"]["templateConfiguration"]["modelSpecificSettings"] = mModelSeries;
	// write prettified JSON to another file
	std::ofstream o("pretty.json");
	o << std::setw(4) << j << std::endl;

headsetJSON.zip

[arrtchiu]

Relying on the brace-initialization is a little bit funny in this library and might vary depending on your compiler (see #1359). I'd be explicit about use of object vs array, and also break it up into smaller pieces (functions/lambdas depending on your style). Doing it like this also help avoid typo bugs (which still compile fine) by not repeating the keys several times.

json j = json::object();
j["headsetConfig"] = [] {
  json headsetConfig = json::object(); // note I am explicitly specifying I want this to be an object
  headsetConfig["templateConfiguration"] = [] {
    json templateConfiguration = json::object();
    templateConfiguration["configTemplateVersion"] = "1";
    // ...
    templateConfiguration["someArray"] = [] {
      json someArray = json::array(); // note I am explicitly specifying I want this to be an array
      for (auto const& thing : collection) {
        someArray.push_back(JsonObjectForThing(thing));
      }
    }();
    return templateConfiguration;
  }();
  return headsetConfig;
}();

For even cleaner code (those lambdas are pretty ugly!) you can provide your own overloads to to_json.

[nlohmann]

@fkicenko Do you need further assistance?

[fkicenko]

Hey... the lambda's simply don't work here... Is there a more elegant way to do this with this library? The JSON structure is really simple enough. Thanks for the help!

[arrtchiu]

IMO overloading to_json is your best bet. It's simple and elegant. The documentation is also very good: https://github.com/nlohmann/json#basic-usage

Here's an example with nesting:

#include <iostream>
#include <iomanip>

#include <nlohmann/json.hpp>

struct Foo {
	std::string value;
};

struct Bar {
	int value;
};

struct Nested {
	Foo foo;
	Bar bar;
};

using json = nlohmann::json;

void to_json(json& j, Foo const& f) {
	j["foo_value"] = f.value;
}

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

void to_json(json& j, Nested const& n) {
	j["foo"] = n.foo;
	j["bar"] = n.bar;
}

int main(int argc, const char** argv) {
	Nested serialiseMe{{"myname"}, {42}};
	json serialised(serialiseMe);
	std::cout << std::setw(2) << serialised << std::endl;
	return 0;
}

[fkicenko]

Ok this is good... If you can give me an example of nesting an Array within an Object I think i'm good to go. This example gives me 2 objects but just fuzzy on now nesting an Array in there. Thanks!!!

[nlohmann]

Can you provide an example of the input, the current output, and the expected output?