Understanding RapidJson – Part 2

In my previous blog on Rapidjson, many people asked for a detailed example in the comments, so here is part 2 of Understanding Rapidjson with a slightly detailed example.

We will improve on the previous example and modify the changeDom function to add more complex objects to the DOM tree.

template <typename Document>
void changeDom(Document& d){
    Value& node = d["hello"];
    node.SetString("c++");
    Document subdoc(&d.GetAllocator());
    subdoc.SetObject(); // starting the object
    Value arr(kArrayType); // the innermost array
    Value::AllocatorType allocator;
    for (unsigned i = 0; i < 10; i++)
        arr.PushBack(i, allocator); // adding values to array
    // adding the array to its parent object and so on
    subdoc.AddMember("New", Value(kObjectType).Move().AddMember("Numbers", arr, allocator), subdoc.GetAllocator());
    d.AddMember("testing", subdoc, d.GetAllocator()); // finally adding the sub document to the main doc object
    d["f"] = true;
    d["t"].SetBool(false);
}

Here Value objects of type kArrayType and kObjectType are created and appended to their parent node from innermost to outermost.

Before Manipulation

{
    "hello": "world",
    "t": true,
    "f": false,
    "n": null,
    "i": 123,
    "pi": 3.1416,
    "a": [
        0,
        1,
        2,
        3
    ]
}

After Manipulation

{
    "hello": "c++",
    "t": false,
    "f": true,
    "n": null,
    "i": 123,
    "pi": 3.1416,
    "a": [
        0,
        1,
        2,
        3
    ],
    "testing": {
        "New": {
            "Numbers": [
                0,
                1,
                2,
                3,
                4,
                5,
                6,
                7,
                8,
                9
            ]
        }
    }
}

The changeDom function can also be written using the PrettyWriter object as follows:

template <typename Document>
void changeDom(Document& d){
    Value& node = d["hello"];
    node.SetString("c++");
    Document subdoc(&d.GetAllocator()); // sub-document
    // old school write the json element by element
    StringBuffer s;
    PrettyWriter<StringBuffer> writer(s);
    writer.StartObject();
        writer.String("New");
        writer.StartObject();
            writer.String("Numbers");
            writer.StartArray();
            for (unsigned i = 0; i < 10; i++)
                writer.Uint(i);
            writer.EndArray();
        writer.EndObject();
    writer.EndObject();
    subdoc.Parse(s.GetString()); // Parsing the string written to buffer to form a sub DOM

    d.AddMember("testing", subdoc, d.GetAllocator()); // Attaching the Sub DOM to the Main DOM object
    d["f"] = true;
    d["t"].SetBool(false);
}

Happy Coding! Cheers.

More reads: https://stackoverflow.com/questions/32896695/rapidjson-add-external-sub-document-to-document