Create a json array in C++ - c++

Question

So im trying to create a json Object in c++ dynamically. I want to add a timestamp and then an array with the data included.
So thats 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"
}
]
}
Im totally new to C++ and im using the Casablanca ( C++ REST SDK) package.
So im 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 theres a better package? But as far as i understood its an official micorosft package for json and http.
Help would be really nice!

Answer 1

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;

Answer 2

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 3

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 4

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 5

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 6

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\"\
}\
]";