How to create an array of Objects?

[brunohkbx]

Its possible to create the folowing structure?
Using something like jsonObject["objects"]["object"]

{
    "objects": [
      {
        "object": {
          "address": "xxx"
        }},
      {
        "object": {
          "address": "xxx"
        }},
      {
        "object": {
          "address": "xxx"
        }}]
}

[nlohmann]

Yes, please have a look at the documentation: https://github.com/nlohmann/json#json-as-first-class-data-type

[fnc12]

json j = {
    { "objects" , {
        { "object" , {
            { "address", "adr1" },
        }, },
        { "object" , {
            { "address", "adr2" },
        }, },
        { "object" , {
            { "address", "adr3" },
        }, },
    },
};

[brunohkbx]

Thanks @nlohmann
@fnc12 I'm trying to do something like this :

json JsonObjects = json::array();
for (int i = 0; i <= 5; i++)
{
	JsonObjects.append(JsonObject)
}

Is it possible to append/Insert an object in the array?

[fnc12]

Yes. Just like std::vector

auto jsonObjects = json::array();
for(auto i = 0; i <= 5; ++i) {
    jsonObjects.push_back(json::object());
}

[nlohmann]

Of course - it uses (more or less) the same API as std::vector, so you can use push_back. See https://nlohmann.github.io/json/ for an overview.

[brunohkbx]

Thanks everyone.
Got it.

[moodboom]

Excellent. Would it be common enough to get mentioned in the main README doc? Would have helped me I know. Thanks nlohmann, love the class.

[arwinder-jaspal]

Yes. Just like std::vector

auto jsonObjects = json::array();
for(auto i = 0; i <= 5; ++i) {
    jsonObjects.push_back(json::object());
}

hi I am new to C++ and I am trying to achieve the same thing here.

Can anyone provide sample code for this problem?

Thank you in advance.

[code-blooded245]

Can you please tell how to iterate over this type of json array ?

{
    "objects": [
        {
            "object": {
                "address": "xxx"
            }
        },
        {
            "object": {
                "address": "xxx"
            }
        },
        {
            "object": {
                "address": "xxx"
            }
        }
    ]
}

[nlohmann]

// assuming you parse the JSON above to variable j

for (const auto& element : j["objects"])
{
    std::cout << element << std::endl;
}

This should output

{"object":{"address":"xxx"}}
{"object":{"address":"xxx"}}
{"object":{"address":"xxx"}}

Inside the loop, you can access the other elements with

element["object"]["address"]

[code-blooded245]

Thanks!

[siltau]

Hi,
I'm having some trouble trying to access an element of an array of objects. The Json look like this:

{
"parentX":
    {
        "key1": "value1",
        "key2": "value2",
        "list":
        [
        {"0":{"keyA":"valueA"}},
        {"1":{"keyB":"valueB"}}
        ]
    },
"parentY":
    {
        "key1": "value1",
        "key2": "value2",
        "list":
        [
        {"0":{"keyA":"valueA"}},
        {"1":{"keyB":"valueB"}}
        ]
    }
}

Now I would like to accessand modify a specific element of "list" but I can't figure out how to do it.
I tried:

json["parentX"]["list"]["0"]["keyA"]          // WRONG
json["parentX"]["list"].at("0")["keyA"]      // WRONG

but they crash and probably I have misunderstood the documentation.
I'm doing correctly or is there another way to perform such operation?

[nlohmann]

The example you posted ins not valid JSON - there are commas missing. However, if you pass strings to operator[] or at(), an object is assumed. This should be the description of the exception that you should be getting. To access elements of arrays, pass numbers. For instance, json["parentX"]["list"][0] accessed the first element of the JSON array at json["parentX"]["list"].

[siltau]

Thanks for the fast answer, and you are right, the Json was not valid as written but I just wrote it by hand to give an overview of what I was doing (I edited, should be fine now).

In the begin my objective was to access a specific element in an array, which I'm sure is present but I don't know the position, by using the underlying search/find features (like those used by operator[] or at() ) without scanning whole array, for both code clearness and possible performance issues. In my case the array is used as a circular buffer/FIFO, with parameterized size, so it can eventually grow.

In the end I realized that what I was trying is not possible so find out some option:

• Using find() but I'm not sure is recursive (by trying seems not working the way I'm using it):

auto elementX = json["parentX"].find("0");
*elementX["keyA"] = "new_value";

auto elementY = json["parentY"].find("0");
*elementY["keyA"] = "new_value";

• Using a for cicle scanning the whole array:

// 1
for (const auto& element : json["parentX"]["list"])
{
    // Have to check the key somehow and then access the value
    std::cout << element << std::endl;
}

// 2
for (auto it = json["parentX"]["list"].begin(); it !=json["parentX"]["list"].end(); ++it)
{
    std::cout << it.key() << " | " << it.value() << "\n";
}

// 3
for ( auto it:  json["parentX"]["list"].items() )
{
    std::cout << it.key() << " | " << it.value() << "\n";
}

But from information gatered seems I should scan the array twice, first time to find the position comparing the key, then again to modify the value.

Still testing, as soon as I find the solution I will post it.

[siltau]

I've got what I was doing wrong and for sure I have to learn/deal more about APIs return values and how they work.
That is the solution to my problem:

for (auto& element :  json["parentX"]["list"])
{
    if(element.contains("0")==true)
    {
        element.at("0")["keyA"] = "newValue";
        break;
    }
}

maybe there is a clean/better way to do it because it seems a little bit ugly to me.

In my humble opinion the operator[] should also works on array when a string is passed hiding this search, in order to be "homogenous" with all value access.