Create a JSON array in C++

Question

I'm trying to create a JSON object in C++ dynamically. I want to add a timestamp and then an array with the data included.

This is what my JSON string would look like:

{
    "timestep": "2160.00",
    "vehicles": [
        {
            "id": "35092_35092_353",
            "x": "6.988270",
            "y": "50.872139",
            "angle": "-20.812787",
            "type": "passenger_P_14_1",
            "speed": "0.000000",
            "pos": "4.600000",
            "lane": "4.600000",
            "slope": "4.600000"
        },
        {
            "id": "35092_35092_353",
            "x": "6.988270",
            "y": "50.872139",
            "angle": "-20.812787",
            "type": "passenger_P_14_1",
            "speed": "0.000000",
            "pos": "4.600000",
            "lane": "4.600000",
            "slope": "4.600000"
        },
        {
            "id": "35092_35092_353",
            "x": "6.988270",
            "y": "50.872139",
            "angle": "-20.812787",
            "type": "passenger_P_14_1",
            "speed": "0.000000",
            "pos": "4.600000",
            "lane": "4.600000",
            "slope": "4.600000"
        }
    ]
}

I'm totally new to C++ and I'm using the Casablanca (C++ REST SDK) package. I'm having a really hard time producing the code and I cant find any working solutions. I found this on the wiki

Create a JSON object:

json::value obj;
obj[L"key1"] = json::value::boolean(false);
obj[L"key2"] = json::value::number(44);
obj[L"key3"] = json::value::number(43.6);
obj[L"key4"] = json::value::string(U("str"));

and that works for me. But how do I create an array?

I tried several things but nothing worked. Maybe there's a better package? But as far as I understood it's an official Microsoft package for JSON and HTTP.

Answer 1

The following sample json C++ array of strings works for me.

const char * const line_items_items =
"[\
    {\
        \"commodity_code\": \"44121903\",\
        \"description\": \"Miscellaneous goods\",\
        \"upc\": \"65100004327\",\
        \"quantity\": \"2\",\
        \"unit_of_measurement\": \"M62\",\
        \"unit_cost\": \"23.09\",\
        \"discount_amount\": \"10.03\",\
        \"total_amount\": \"50.03\",\
        \"tax_amount\": \"10.05\",\
        \"extended_amount\": \"76.04\",\
        \"debit_or_credit_indicator\": \"credit\",\
        \"net_or_gross_indicator\": \"net\"\
    },\
    {\
        \"commodity_code\": \"44121809\",\
        \"description\": \"Miscellaneous goods\",\
        \"upc\": \"65100007654\",\
        \"quantity\": \"4\",\
        \"unit_of_measurement\": \"M66\",\
        \"unit_cost\": \"35.09\",\
        \"discount_amount\": \"5.06\",\
        \"total_amount\": \"0.53\",\
        \"tax_amount\": \"8.07\",\
        \"extended_amount\": \"96.12\",\
        \"debit_or_credit_indicator\": \"debit\",\
        \"net_or_gross_indicator\": \"gross\"\
    }\
]";

Answer 2

If you wish to use the array as an answer on a received http_request (in case below it's a http_request request), you are free to use the following snippet of code as an example:

    json::value answer;
    auto array = answer.array();

    for (size_t i = 0; i < GenFile.GetNumberOfCurves(); i++)
    {
        web::json::value vehicle;

        vehicle[L"smth"] = web::json::value::number(WhatEverArray[i].whatever());

        array[i] = vehicle;
    }

    request.reply(status_codes::OK, array);

Answer 3

Here is another method to produce a json array in Casablanca:

int size = 3;
web::json::value yourJson;
yourJson[U("vehicles")] = web::json::value::array(size);

yourJson[U("vehicles")].as_array()[0] = web::json::value(U("some entry"));
yourJson[U("vehicles")].as_array()[1] = web::json::value(U("another entry"));
//...

Answer 4

Here's how to create an array dynamically using vector. Assume that you have 10 vehicles to add.

std::vector<web::json::value> arrayVehicles;
for(int i = 0; i < 10; i++)
{
    web::json::value vehicle;
    std::string vehicleID = "id_prefix_" + std::to_string(i);
    vehicle["id"] = web::json::value::string(vehicleID);
    vehicle["x"] = web::json::value::number(6.988270);
    vehicle["y"] = web::json::value::number(50.872139);

    arrayVehicles.push_back(vehicle);
}

web::json::value myJSON;
myJSON["vehicles"] = web::json::value::array(arrayVehicles);

Answer 5

You could put it like this:

json::value vehicle1;
vehicle1[L"id"] = json::value::string(L"35092_35092_353");
vehicle1[L"x"] = json::value::number(6.988270);
vehicle1[L"y"] = json::value::number(50.872139);

json::value vehicle2;
vehicle2[L"id"] = json::value::string(L"35092_35092_353");
vehicle2[L"x"] = json::value::number(1.23456);
vehicle2[L"y"] = json::value::number(6.78901);

json::value vehicles;
vehicles[L"timestamp"] = json::value::number(2160);
vehicles[L"vehicles"] = json::value::array({vehicle1, vehicle2});

Answer 6

There are 2 mechanisms. If you are used to std c++ libraries, this should look familiar. Element vector is derived from std::vector.

json::value::element_vector e;
// the first item in the pair is the array index, the second the value
e.push_back(std::make_pair(json::value(0), json::value(false)));
e.push_back(std::make_pair(json::value(1), json::value::string(U("hello"))));
json::value arr(e);

And, if you prefer a cleaner look, and can accept a less efficient compiled result:

json::value arr;
arr[0] = json::value(false);
arr[1] = json::value(U("hello"));

From your message you have tried a bunch of stuff. If you have tried mechanisms like these but they didn't work, give us a sample program that demontrates the failure and we'll have a crack at it.

To get the basic structure in your file above:

json::value vehicles;
vehicles[0] = // 1st vehicle object
vehicles[1] = // 2nd vehicle object
// etc
json::value root;
root[L"timestep"] = json::number(2160.0);
root[L"vehicles"] = vehicles;