simpleson

Lightweight C++ JSON parser & serializer that is C++98 compatible with no dependencies

Why simpleson?

Simpleson is built under the following requirements:

• One header and one source file only
• No external dependencies
• ISO/IEC 14882:1998 (aka C++98) compatible
• Cross-platform

A primary use case for simpleson is in an memory-constrained embedded system.

Building simpleson

Simpleson was intentionally built such that a developer could simply copy json.h into the target project's inc folder, copy json.cpp into the src folder, and then compile the target project. No linking -> no drama.

Building the library and tests follows the standard build chain via CMake:

mkdir build
cd build
cmake ../
make

Unit tests can then be run by executing make test

Quickstart

// Create the input
std::string input = "{ \"hello\": \"world\" }";

// Parse the input
json::jobject result = json::jobject::parse(input);

// Get a value
std::string value = (std::string)result.get_entry("hello").value

// Add entries
json::jobject example;
example["int"] = 123;
example["float"] = 12.3f;
example["string"] = "test string";
int test_array[3] = { 1, 2, 3 };
example["array"] = std::vector<int>(test_array, test_array + 3);
std::string test_string_array[2] = { "hello", "world" };
example["strarray"] = std::vector<std::string>(test_string_array, test_string_array + 2);
example["emptyarray"] = std::vector<std::string>();
example["boolean"].set_boolean(true);
example["null"].set_null();

// Reference values
std::string hello = example["strarray"][0];
std::string world = example["strarray"][1];

// Serialize the new object
std::string serial = (std::string)result;

Using simpleson

The namespace of simpleson is simply json. JSON objects can be parsed by calling json::jobject::parse(), which takes a string and returns a jobject. The array operators are overloaded for jobject, meaning you can assign and access entries using the [] operators like many other software languages. Other useful methods of jobject include:

• has_key("key") - Returns true if the key exists in the jobject
• remove("key") - Removes they entry associated with the key if the key exists in the jobject
• clear() - Removes all entries in the jobject
• ["key"].set_boolean(true) - Sets the key to the boolean value
• ["key"].set_null() - Sets the value associated with the key to null
• ["key"].is_true() - Returns true if the boolean value associated with the key is true
• ["key"].is_null() - Returns true if the value associated with the key is null

An instance of jobject can be searlized by casting it to a std::string. Note that an instance of jobject does not retain it's original formatting (it drops tabs, spaces outside strings, and newlines).

Arrays

Simpleson supports arrays as the root object:

json::jobject example = json::jobject::parse("[1,2,3]");
int one = example[0]; // Value is 1
int two = example[1]; // Value is 2
int three = example[2]; // Value is 3

Arrays are stored in a jobject. You can determine whether a jobject is holding an array through the method is_array().

Multi-level Access

To access elements several levels down, the get(key) and array(index) can be used for objects and arrays, respectively:

std::string music_desired = example.array(0).get("hobbies").array(1).get("music");

See the full example here.

A note on booleans

Booleans are handled a bit differently than other data types. Since everything can be cast to a boolean, having an implicit boolean operator meant everything goes to a boolean! Instead, boolean values are set by using the set_boolean() method. If you do not use this method and instead directly create/assign a boolean to a jobject array entry, then the boolean will be cast to an int with a value of 0 or 1. Similarly, you can check if a value is set to true or false using the is_true() method.