JSON library
- C++ course
- Advanced
- Lesson 51/83
- 5 min
Learn JSON serialization with nlohmann/json.
What is nlohmann/json?
nlohmann/json is a popular header only JSON library for C++. You use it with a single #include <nlohmann/json.hpp>. JSON objects are represented by the nlohmann::json type; fields are added as j["key"] = value;.
Example: nlohmann::json j; j["name"] = "Alex"; j["age"] = 30;. You add to an array with j["tags"] = {"a", "b"};. The j.dump() method returns the JSON as a string. j.parse(str) builds a JSON object from a string. This library offers a more structured way than text serialization.
Creating JSON and dump
After creating a nlohmann::json object you add fields with square brackets. The j.dump() method returns the JSON as a compact string. j.dump(2) produces a readable, 2 space indented string.
Example: nlohmann::json j; j["name"] = "Alex"; j["age"] = 30; std::cout << j.dump();. Output: {"age":30,"name":"Alex"}. Keys are sorted alphabetically because the library uses std::map by default. To read a field, j["name"] is enough; the return type is deduced. This is safer than text serialization for structured data.
Practice Create a JSON object and print it
Add #include <nlohmann/json.hpp>. Define nlohmann::json j;. Set j["name"] = "Alex"; and j["age"] = 30;. Print with std::cout << j.dump();.
Starter code
#include <iostream>
#include <string>
// Add the nlohmann/json header and JSON object
int main() {
// Create JSON and print it
return 0;
}Sample solution
#include <iostream>
#include <nlohmann/json.hpp>
int main() {
nlohmann::json j;
j["name"] = "Alex";
j["age"] = 30;
std::cout << j.dump();
return 0;
}Expected output
{"age":30,"name":"Alex"}
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is nlohmann/json included?
- #include <json>
- #include <std::json>
- #include <nlohmann/json.hpp> Correct answer
- #include <json/json.h>
Explanation: #include <nlohmann/json.hpp> enables the library with one header.
2 Complete the JSON type
____ j;nlohmann::json j;Explanation: nlohmann::json defines a JSON object.
3 Match each piece to its role
| Term | Meaning |
|---|---|
j["name"] = "Alex" |
Adds a field |
j.dump() |
Produces a string |
j.dump(2) |
Produces indented string |
nlohmann::json |
JSON type |