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

Question

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
}

Solution

You need to check for all the errors before converting to std::string. Make sure that the file is open for reading / writing and the parsing is successful i.e. the JSON is valid. GetParseError() and GetErrorOffset() are the functions to validate parsing.

I've used your example and enhanced it. Hope you won't mind. :-)

Here's a working example:

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

int main()
{
    using namespace rapidjson;

    std::ifstream ifs { R"(C:\Test\Test.json)" };
    if ( !ifs.is_open() )
    {
        std::cerr << "Could not open file for reading!\n";
        return EXIT_FAILURE;
    }

    IStreamWrapper isw { ifs };

    Document doc {};
    doc.ParseStream( isw );

    StringBuffer buffer {};
    Writer<StringBuffer> writer { buffer };
    doc.Accept( writer );

    if ( doc.HasParseError() )
    {
        std::cout << "Error  : " << doc.GetParseError()  << '\n'
                  << "Offset : " << doc.GetErrorOffset() << '\n';
        return EXIT_FAILURE;
    }

    const std::string jsonStr { buffer.GetString() };

    std::cout << jsonStr << '\n';

    doc[ "ip" ] = "127.0.0.1";

    std::ofstream ofs { R"(C:\Test\NewTest.json)" };
    if ( !ofs.is_open() )
    {
        std::cerr << "Could not open file for writing!\n";
        return EXIT_FAILURE;
    }

    OStreamWrapper osw { ofs };
    Writer<OStreamWrapper> writer2 { osw };
    doc.Accept( writer2 );

    return EXIT_SUCCESS;
}

Comments

[Heavy]

What are you trying to achieve exactly? If you want to read the whole file into string, you don't need a rapidjson for that.

[waas1919]

@Azeem Thanks! You were correct. Can you write your last comment as the answer for this thread so I can close. The buffer.GetString() was indeed showing that the file was empty and it was printing nothing. I screw up somewhere and didn't notice that the file was wrong. Your clues were very helpful :)

[Azeem]

@waas1919: You are welcome! :) I'm glad that it helped. I've posted the code as answer with explanation.