Read data from JSON file in C++

In this tutorial, we will learn how to read a JSON file in C++.

Firstly, what is a JSON file?

JSON is short for JavaScript Object Notation. A JSON file stores data structures and objects in an organised and easy to access manner in JavaScript Object Notation (JSON) format. It is commonly used for transmitting data between web application and server.

Reading data from a JSON file

In this tutorial, I shall be using the boost library to read the JSON file. Within the boost library, we have a sublibrary called property tree. Property tree allows us to handle the tree of properties and is used to represent various files like XML, INI, JSON files etc.

In the following program, we first make a root of the property tree type and load the JSON file into it. Then, we use the declare variables roll, name, class1 to store the values of “roll no”, “name” and “class” respectively. Thereafter, we display all the values. For the values of the “address“, we use for loop. In the loop, the different values of the vector are displayed.

The JSON file with file name “file.json” , that we will be using is :

{
    "name": "Aryan",
    "class": "Masters in Maths with Comp Sc.",
    "roll no": 3526,
  "address": {
    "city": "Delhi",
    "house no": "432-B",
    "locality": "Omaxe City"
  }
}

 

Program: Read data from a JSON File in C++

#include <iostream>
#include <string.h>
#include <exception>
#include <boost/property_tree/json_parser.hpp>
#include <boost/property_tree/ptree.hpp>
#include "json.hpp"
using namespace std;
using json = nlohmann::json;
namespace pt = boost::property_tree;

int main()
{
  
  pt::ptree root;

  pt::read_json("file.json", root);	// Load the json file in this ptree
  int roll = root.get<int>("roll no");	//read and save the roll no in *roll*
  string  name = root.get<string>("name");	//read and save the name in *name*
  string class1 = root.get<string>("class");	//read and save the class in *class1*

  cout << "name : " << name << endl;			//getting the output of all
  cout << "roll no : " << roll << endl;
  cout << "class : " << class1 << endl << "address : " << endl << endl;
  for (pt::ptree::value_type & v : root.get_child("address"))
  {
    cout << v.first << endl;
    cout << "  "<<v.second.data() << endl;
  }

  return 0;
}

Output:

name : Aryan
roll no : 3526
class : Masters in Maths with Comp Sc.
address :

city
  Delhi
house no
  432-B
locality
  Omaxe City

You can also make a JSON object within the program and read it similarly. In the below program, we first make an object of JSON type and read its contents. Then, using the for loop, we get the output. Below is a program depicting the same :

#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>
#include <iostream>
  
using namespace std;
namespace pt = boost::property_tree;
int main()
{
    stringstream s;
    s << "{ \"root\": { \"values\": [1, 2, 3, 4, 5 ] } }";

    pt::ptree root;			// Creates a root
    pt::read_json(s, root);	// Loads the json file in this ptree

    for (pt::ptree::value_type& v : root.get_child("root.values"))
    {
      cout << v.second.data() << endl;		//gives the output
    }
    return 0;

Output :

1
2
3
4
5

Hope this was helpful. Enjoy coding!

Also learn :

Nested Class Constructor in C++

Multiple ways to split a string in C++

4 responses to “Read data from JSON file in C++”

1. Vinod Akula says:

   May 10, 2020 at 1:00 am

   Hi. Thanks for the article. But there is an issue…
   Instead of #include “json.hpp” we should use :
   #include

   And you should have mentioned to download nlohmann-json also. I downloaded it from powershell using vcpkg.

   Reply
2. Vinod Akula says:

   May 10, 2020 at 1:02 am

   It’s actually #include

   Reply
3. Vinod Akula says:

   May 10, 2020 at 1:05 am

   I mean instead of “json.hpp” we should use nlohmann/json.hpp placed between “”symbols

   Reply
4. Ashish says:

   July 27, 2020 at 12:50 pm

   How to take values of the “values” array into another array out of json, so that it can later on be used to perform further operations??
   Please help!

   Reply