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Understanding RapidJson

_no_rules_ profile image Subhendu Ghosh Originally published at codeflu.blog on ・3 min read

With new technologies softwares need to evolve and adapt. My new task is to make cppagent generate output in Json (JavaScript Object Notation) format. Last week i spent sometime to try out different libraries and finally settled on using Rapidjson.   Rapidjson is a json manipulation library  for c++ which is fast, simple and has compatibility with different c++ compilers in different platforms. In this post we will be looking at example codes to generate, parse and manipulate json  data. For people who want to use this library i would highly recommend them to play with and understand the example codes first.

First we will write a simple program to write a sample json as below (the same simplewriter.cpp as in example) :

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

To generate a Json output you need:

  • a StringBuffer object, a buffer object to write the Json output.
  • Writer  object to write Json to the buffer. Here i have used PrettyWriter object to write human-readable and properly indented json output.
  • functions StartObject/EndObject to start and close a json object parenthesis “ { ” and  “ } ” respectively.
  • functions StartArray/EndArray to start and end a json array object i.e “[” and “]“.
  • functions String(), Uint(), Bool(), Null() , Double()  are called on writer object to write string, unsigned integer, boolean, null, floating point numbers respectively.
#include "rapidjson/stringbuffer.h"
#include "rapidjson/prettywriter.h"
#include <iostream>

using namespace rapidjson;
using namespace std;

template <typename Writer>
void display(Writer& writer );

int main() {
 StringBuffer s; 
 PrettyWriter<StringBuffer> writer(s);
 display(writer);
 cout << s.GetString() << endl; // GetString() stringify the Json
 }

template <typename Writer>
void display(Writer& writer){
 writer.StartObject(); // write "{"
 writer.String("hello"); // write string "hello"
 writer.String("world");
 writer.String("t"); 
 writer.Bool(true); // write boolean value true
 writer.String("f");
 writer.Bool(false);
 writer.String("n");
 writer.Null(); // write null
 writer.String("i");
 writer.Uint(123); // write unsigned integer value
 writer.String("pi");
 writer.Double(3.1416); // write floating point numbers
 writer.String("a");
 writer.StartArray(); // write "["
 for (unsigned i = 0; i < 4; i++)
 writer.Uint(i);
 writer.EndArray(); // End Array "]"
 writer.EndObject(); // end Object "}"
}

Next we will manipulate the Json document and change the value for key “Hello” to “C++” ,

To manipulate:

  • first you need to parse your json data into a  Document  object.
  • Next you may use a Value  reference to the value of the desired node/key or you can directly access them as doc_object[‘key’] .
  • Finally you need to call the Accept method passing the Writer object to write the document to the StringBuffer object.

Below function changes the keywords for “hello” , “t”, “f” to “c++” , false , true respectively.

template <typename Document>
void changeDom(Document& d){
// any of methods shown below can be used to change the document
Value& node = d["hello"]; // using a reference
node.SetString("c++"); // call SetString() on the reference
d["f"] = true; // access directly and change
d["t"].SetBool(false); // best way
}

Now to put it all together:

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

The final code to display the above output:

#include "rapidjson/stringbuffer.h"
#include "rapidjson/prettywriter.h"
#include "rapidjson/document.h"
#include <iostream>

using namespace rapidjson;
using namespace std;

template <typename Writer> 
void display(Writer& writer);

template <typename Document>
void changeDom(Document& d);

int main() {
 StringBuffer s;
 Document d;
 PrettyWriter<StringBuffer> writer(s);
 display(writer);
 cout << "Before Manupulation\n" << s.GetString() << endl ;
 d.Parse(s.GetString());
 changeDom(d);
 s.Clear(); // clear the buffer to prepare for a new json document
 writer.Reset(s); // resetting writer for a fresh json doc
 d.Accept(writer); // writing parsed document to buffer
 cout << "After Manupulation\n" << s.GetString() << endl;
 }

template <typename Document>
void changeDom(Document& d){
Value& node = d["hello"];
node.SetString("c++");
d["f"] = true;
d["t"].SetBool(false);
}

template <typename Writer>
void display(Writer& writer){
 writer.StartObject();
 writer.String("hello");
 writer.String("world");
 writer.String("t");
 writer.Bool(true);
 writer.String("f");
 writer.Bool(false);
 writer.String("n");
 writer.Null();
 writer.String("i");
 writer.Uint(123);
 writer.String("pi");
 writer.Double(3.1416);
 writer.String("a");
 writer.StartArray();
 for (unsigned i = 0; i < 4; i++)
 writer.Uint(i);
 writer.EndArray();
 writer.EndObject();
}

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