Understanding RapidJson

With new technologies, software must evolve and adapt. The task was to make cppagent generate output in JSON (JavaScript Object Notation) format. After evaluating different libraries, RapidJSON was selected, a C++ JSON manipulation library offering speed, simplicity, and cross-platform compiler compatibility. This post explores example code for generating, parsing, and manipulating JSON data.

Basic JSON Generation

The following sample JSON demonstrates the target output structure:

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

To generate JSON output, you need:

• A StringBuffer object to buffer the JSON output
• A Writer object (or PrettyWriter for human-readable, indented output)
• StartObject/EndObject functions for JSON object braces
• StartArray/EndArray functions for array brackets
• Type-specific functions like String(), Uint(), Bool(), Null(), and Double()

Sample Writer Code

#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 "}"
}

Manipulating JSON Documents

To modify JSON data:

• Parse the JSON into a Document object
• Use a Value reference or direct access via doc['key'] to modify nodes
• Call the Accept method with a Writer to output the modified document

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
}

Complete Example: Before and After Manipulation

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
    ]
}

Complete Implementation

#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();
}

Note: more complex examples were added in a follow-up post, Understanding RapidJson – Part 2.