How to read json file using rapidjson and output to std::string?

How can I read a *.json file and put the output on a std::string?

I have this sample, but I always get null on std::string.

#include <rapidjson/document.h>
#include <rapidjson/istreamwrapper.h>
#include "rapidjson/writer.h"
#include "rapidjson/stringbuffer.h"
#include <rapidjson/ostreamwrapper.h>
#include <fstream>
#include <iostream>

using namespace rapidjson;
using namespace std;

void main()
{
    ifstream ifs("input.json");
    IStreamWrapper isw(ifs);
    Document d;
    d.ParseStream(isw);

    StringBuffer buffer;
    Writer<StringBuffer> writer(buffer);
    d.Accept(writer);

    std::string jsonStr(buffer.GetString());
    if(jsonStr == "null")
        std::cout << "is null..." << std::endl; //<--always here!
    else
    {
        std::cout << jsonStr.c_str() << std::endl;

        d["ip"] = "123456789";

        ofstream ofs("output.json");
        OStreamWrapper osw(ofs);
        Writer<OStreamWrapper> writer2(osw);
        d.Accept(writer2);
    }
}

This is my json file:

{
  "ip" :  "192.168.0.100",
  "angle x": 20,
  "angle y": 0,
  "angle z": 0
}