Working with JSON in C++: Parsing and Serialization in Practice

Working with JSON in C++: Parsing and Serialization in Practice

JSON (JavaScript Object Notation) is one of the most popular data interchange formats in modern applications. Despite the fact that C++ does not have built-in JSON support, there are many libraries that simplify working with this format. In this article, we will look at practical aspects of parsing and serializing JSON in C++ using the nlohmann/json library — one of the most popular and convenient to use.

Why choose nlohmann/json?

The nlohmann/json library (often called JSON for Modern C++) has gained popularity due to its intuitive API, reminiscent of working with STL containers. It is header-only (requires no compilation), supports C++11 and later, and provides high performance and type safety.

Installing the library is simple: just download one header file json.hpp or use a package manager like vcpkg, Conan, or CMake FetchContent.

#include <nlohmann/json.hpp>using json = nlohmann::json;

Basics of JSON Parsing in C++

JSON parsing is the process of converting a string into structured data. The nlohmann/json library provides several ways to parse.

Parsing from a String

The most common way is using the static method json::parse():

#include <iostream>#include <string>#include <nlohmann/json.hpp>

int main() {    std::string json_string = R"({        "name": "Ivan",        "age": 30,        "is_student": false,        "skills": ["C++", "Python", "JavaScript"],        "address": {            "city": "Moscow",            "zip": "101000"        }    })";

    try {        json data = json::parse(json_string);                // Accessing data        std::cout << "Name: " << data["name"] << std::endl;        std::cout << "Age: " << data["age"] << std::endl;        std::cout << "Student: " << (data["is_student"] ? "yes" : "no") << std::endl;                // Working with arrays        for (auto& skill : data["skills"]) {            std::cout << "Skill: " << skill << std::endl;        }                // Working with nested objects        std::cout << "City: " << data["address"]["city"] << std::endl;            } catch (json::parse_error& e) {        std::cerr << "Parse error: " << e.what() << std::endl;    }        return 0;}

Parsing from a File

For working with files, use C++ I/O streams:

#include <fstream>#include <nlohmann/json.hpp>

int main() {    std::ifstream file("data.json");    if (!file.is_open()) {        std::cerr << "Failed to open file" << std::endl;        return 1;    }        try {        json data = json::parse(file);        std::cout << "Data loaded: " << data.dump(2) << std::endl;    } catch (json::parse_error& e) {        std::cerr << "Error: " << e.what() << std::endl;    }        return 0;}

JSON Serialization in C++

Serialization is the reverse process: converting C++ data into a JSON string. The library offers the dump() method with formatting options.

Creating a JSON Object

You can create JSON objects directly, like regular associative arrays:

#include <nlohmann/json.hpp>#include <iostream>

int main() {    json person;    person["name"] = "Maria";    person["age"] = 25;    person["is_employed"] = true;    person["languages"] = {"Russian", "English", "German"};