Using JSON with C++

Description

JSON formatted data is a great asset for sending and receiving formatted information. In C++ the nlohmann library provides an easy way to create and display JSON formatted data.

Documentation: https://github.com/nlohmann/json

Installation

Homebrew (MacOS & Linux)

brew install nlohmann-json

Linking

Link with CMakeLists.txt (CMake):

find_package(nlohmann_json 3.2.0 REQUIRED)
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

CPM-Make (Package manager):

Check in releases for the latest version.

You will need the URL specific to that release version and the SHA256 key.

Only get the include folder as we don't want to download all the test examples.

CPMAddPackage(
	NAME nlohmann_json
	VERSION 3.9.0
	# Just get the latest (3.9.0) version, and only the include folder as the rep is big
	URL https://github.com/nlohmann/json/releases/download/v3.9.0/include.zip
	URL_HASH SHA256=5b9b819aed31626aefe2eace23498cafafc1691890556cd36d2a8002f6905009
)

Usage

Initialisation

#include "nlohmann/json.hpp"
using json = nlohmann::json;

Construct JSON Class

json j;

Add values

j["value_name"] = "value"; // Single value
j["value_name1"]["value_name2"] = "value"; // Nested value

Convert string to JSON object

std::string input = "{"happy":true,"pi":3.141}";
j = input;
std::cout << j.dump(4) << std::endl;

int n = 4;
// n represents an int for the number of spaces to indent
std::cout << j.dump(n) << std::endl;

Example output

{
	"value_name": "value", 
	"value_name1" {
		"value_name2": "value"
	}
}