rapidjson: How can I split a Document object to a smaller Document object?

Question

I am working on a C++ project and I use rapidjson for JSON parsing. I have this JSON:

{
  "a": "valA",
  "b": {
    "ba": "valBA",
    "bb": "valBB",
    "bc": "valBC"
  },
  "c": "valC"
}

I parse the whole JSON and I get a Document object containing all values. What I want is to somehow process this Document object and extract only the b part. As if I was parsing this JSON:

{
  "b": {
    "ba": "valBA",
    "bb": "valBB",
    "bc": "valBC"
  }
}

I thought of parsing the Document object myself but I was wondering if there is an easier/faster way of doing that. Any ideas?

Answer 1

"b" element can be extracted and put into a new document this way:

#include <iostream>
#include <rapidjson/document.h>
#include <rapidjson/stringbuffer.h>
#include <rapidjson/writer.h>

using namespace rapidjson;

int main(void)
{
    const char* json = "{\"a\": \"valA\",\"b\": {\"ba\": \"valBA\",\"bb\": \"valBB\",\"bc\": \"valBC\"},\"c\": \"valC\"}";

    Document d;
    d.Parse<0>(json);

    Value& data = d["b"];

    Document d2;
    d2.SetObject();
    d2.AddMember("b", data, d2.GetAllocator());

    rapidjson::StringBuffer buffer;
    rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
    d2.Accept(writer);

    std::cout << buffer.GetString() << std::endl;

    return 0;
}

Output:

{"b":{"ba":"valBA","bb":"valBB","bc":"valBC"}}