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Hjson Intro

  # specify rate in requests/second (because comments are helpful!)
  rate: 1000

  // prefer c-style comments?
  /* feeling old fashioned? */

  # did you notice that rate doesn't need quotes?
  hey: look ma, no quotes for strings either!

  # best of all
  notice: []
  anything: ?

  # yes, commas are optional!

The C++ implementation of Hjson is based on hjson-go. For other platforms see


The easiest way to use hjson-cpp is to simply include all of the files from the folders src and include into your own project. The only requirement is that your compiler fully supports C++11.

GCC 4.8 has the C++11 headers for regex, but unfortunately not a working implementation, so for GCC at least version 4.9 is required.


The second easiest way to use hjson-cpp is to either add it as a subfolder to your own Cmake project, or to install the hjson lib on your system by using Cmake. Works on Linux, Windows and MacOS. Your mileage may vary on other platforms. Cmake version 3.8 or newer is required.

Cmake subfolder

Instead of building a lib, you can choose to include hjson as a subfolder in your own Cmake project. That way, hjson will be built together with your own project, with the same settings. In Visual Studio that also means the hjson source code will be visible in a project in your own solution. Example CMakeLists.txt for your own project, if your executable is called myapp:

add_executable(myapp main.cpp)

add_subdirectory(../hjson-cpp ${CMAKE_BINARY_DIR}/hjson)
target_link_libraries(myapp hjson)

Cmake options

A list of Hjson Cmake options and their default values:

BUILD_WITH_STATIC_CRT=  # Can be set to Yes or No. Only used on Windows.
CMAKE_BUILD_TYPE=  # Set to Debug for debug symbols, or Release for optimization.
CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=ON  # Needed for shared libs on Windows. Introduced in Cmake 3.4.
HJSON_NUMBER_PARSER=StringStream  # Possible values are StringStream, StrToD and CharConv.
HJSON_VERSIONED_INSTALL=OFF  # Use version suffix on header and lib folders.

Linux lib

  1. First create a Makefile using Cmake.
$ cd hjson-cpp
$ mkdir build
$ cd build
  1. Then you can optionally run the tests.
$ make runtest
  1. Install the include files and lib to make them accessible system wide (optional).
$ sudo make install
  1. If you haven't done either step 2 or step 3, you must at least compile the code into a library by calling make.
$ make

Windows lib

The Cmake GUI is the most convenient way to generate a Visual Studio solution. Make sure that you tick the boxes for HJSON_ENABLE_TEST and HJSON_ENABLE_INSTALL if you want to run tests or make the hjson lib accessible system wide.

After generating the solution and opening it in Visual Studio you can run the tests by right-clicking the project runtest and selecting Build. The test results are shown in the Output window (the same window that shows the build result).

In order to make the hjson lib accessible system wide you must run Visual Studio as an administrator. In Windows 7 you can do that by right-clicking on the Visual Studio icon in the start menu and selecting Run as an administrator. Then open the hjson solution, right-click the INSTALL project and select Build. Make sure to do that for both the Debug and Release targets.


The hjson lib can now be used in your own Cmake projects, for example like this if your application is called myapp:

add_executable(myapp main.cpp)

find_package(hjson REQUIRED)
target_link_libraries(myapp hjson)

On Windows it's important that you compiled or installed both Debug and Release hjson libraries before Cmake-generating your own project. Otherwise the target type you didn't compile/install will not be available in your own project.

If you did not install hjson system wide, you can still use it like in the example above if you specify the location of the hjson build when running Cmake for your own project. Example:

$ cmake .. -Dhjson_DIR=../hjson-cpp/build

Usage in C++


The most important functions in the Hjson namespace are:

std::string Marshal(Value v);
Value Unmarshal(const char *data, size_t dataSize);
Value Unmarshal(const char *data);
Value Unmarshal(const std::string&);
Value Merge(const Value base, const Value ext);

Marshal is the output-function, transforming an Hjson::Value tree (represented by its root node) to a string that can be written to a file.

Unmarshal is the input-function, transforming a string to a Hjson::Value tree. The string is expected to be UTF8 encoded. Other encodings might work too, but have not been tested. The function comes in three flavors: char pointer with or without the dataSize parameter, or std::string. For a char pointer without dataSize parameter the data parameter must be null-terminated (like all normal strings).

Merge returns an Hjson::Value tree that is a cloned combination of the input Hjson::Value trees base and ext, with values from ext used whenever both base and ext has a value for some specific position in the tree. The function is convenient when implementing an application with a default configuration (base) that can be overridden by input parameters (ext).

Two more functions exist, allowing adjustments to the output formatting when creating an Hjson string:

EncoderOptions DefaultOptions();
std::string MarshalWithOptions(Value v, EncoderOptions options);

Stream operator

An Hjson::Value can be inserted into a stream, for example like this:

Hjson::Value myValue = 3.0;
std::cout << myValue;

The stream operator marshals the Hjson::Value using standard options, so this code will produce the exact same result:

Hjson::Value myValue = 3.0;
std::cout << Hjson::Marshal(myValue);


Input strings are unmarshalled into a tree representation where each node in the tree is an object of the type Hjson::Value. The class Hjson::Value mimics the behavior of Javascript in that you can assign any type of primitive value to it without casting. Examples:

Hjson::Value myValue(true);
Hjson::Value myValue2 = 3.0;
myValue2 = "A text.";

An Hjson::Value can behave both like a vector (array) and like a map (Object in Javascript):

Hjson::Value map;
map["down1"]["down2"]["down3"] = "three levels deep!";
map["down1"]["number"] = 7;

Hjson::Value arr;
std::string myString = arr[0];

If you try to access a map element that doesn't exist, an Hjson::Value of type Hjson::Value::UNDEFINED is returned. But if you try to access a vector element that doesn't exist, an Hjson::index_out_of_bounds exception is thrown.

These are the possible types for an Hjson::Value:


The default constructor creates an Hjson::Value of the type Hjson::Value::UNDEFINED.

Number representations

The C++ implementation of Hjson can both read and write 64-bit integers. But since functions and operators overloaded in C++ cannot differ on the return value alone, Hjson::Value is treated as double in arithmetic operations. That works fine up to 52 bits of integer precision. For the full 64-bit integer precision the function Hjson::Value::to_int64() can be used.

The Hjson::Value constructor for 64-bit integers also requires a special solution, in order to avoid ambiguous overload errors for some compilers. An empty struct is used as the second parameter so that all ambiguity is avoided. An Hjson::Value created using the 64-bit constructor will be of the type Hjson::Value::DOUBLE, but has the full 64-bit integer precision internally.


Hjson::Value myValue(9223372036854775807, Hjson::Int64_tag{});
assert(myValue.to_int64() == 9223372036854775807);

The function Hjson::Value::is_int64() returns true if the Hjson::Value in question internally contains a 64-bit integer. All integers are stored with 64-bit precision. The only other way that a number is stored in an Hjson::Value is in the form of a double precision floating point representation.

An Hjson::Value that has been unmarshalled from a string that contains a decimal point (for example the string "1.0"), or a string containing a number that is bigger or smaller than what can be represented by an std::int64_t variable (bigger than 9223372036854775807 or smaller than -9223372036854775808) will be stored as a double precision floating point number internally in the Hjson::Value.

Any number stored internally as a double will be represented by a string containing a decimal point when marshalled (for example "1.0"), so that the string can be unmarshalled back into an Hjson::Value containing a double, i.e. no information is lost in the marshall-unmarshall cycle.

Order of map elements

Iterators for an Hjson::Value of type Hjson::Value::MAP are always ordered by the keys in alphabetic order. That is also the default ordering in the output from Hjson::Marshal(). But when editing a configuration file you might instead want the output to have the same order of elements as the file you read for input. That can be achieved by setting the option preserveInsertionOrder to true in the call to Hjson::MarshalWithOptions(), like this:

auto opt = Hjson::DefaultOptions();
opt.preserveInsertionOrder = true;
auto out = Hjson::MarshalWithOptions(root, opt);

The elements in an Hjson::Value of type Hjson::Value::MAP can be accessed directly using the bracket operator with either the string key or the insertion index as input parameter.

Hjson::Value val1;
val1["zeta"] = 1;
assert(val1[0] == 1);
val1[0] = 99;
assert(val1["zeta"] == 99);

The key for a given insertion index can be viewed using the function Hjson::Value::key(int index).

Hjson::Value val1;
val1["zeta"] = 1;
assert(val1.key(0) == "zeta");

The insertion order can be changed using the function Hjson::Value::move(int from, int to). If the input parameter from is lower than the input parameter to, the value will end up at index to - 1.

Hjson::Value val1;
val1["zeta"] = 1;
val1["y"] = 2;
val1.move(0, 2);
assert(val1.key(1) == "zeta");
assert(val1[0] == 2);

The insertion order is kept when cloning or merging Hjson::Value maps.


Hjson is not much optimized for speed. But if you require fast(-ish) execution, escpecially in a multithreaded application, you can experiment with different values for the Cmake option HJSON_NUMBER_PARSER. The default value StringStream uses C++ string streams with the locale classic imbued to ensure that dots are used as decimal separators rather than commas.

The value StrToD gives better performance, especially in multi threaded applications, but will use whatever locale the application is using. If the current locale uses commas as decimal separator Hjson will umarshal floating point numbers into strings, and will use commas in floating point numbers in the marshal output, which means that other parsers will treat that value as a string.

Setting HJSON_NUMBER_PARSER to CharConv gives the best performance, and uses dots as comma separator regardless of the application locale. Using CharConv will automatically cause the code to be compiled using the C++17 standard (or a newer standard if required by your project). Unfortunately neither GCC 10.1 or Clang 10.0 implement the required features of C++17 (std::from_chars() for double). It does work in Visual Studio 17 and later.

Example code

#include <hjson.h>

int main() {

  // Now let's look at decoding Hjson data into Hjson::Value.
  std::string sampleText = R"(
  # specify rate in requests/second
  rate: 1000

  // Decode. Throws Hjson::syntax_error on failure.
  Hjson::Value dat = Hjson::Unmarshal(sampleText.c_str(), sampleText.size());

  // Values can be assigned directly without casting.
  int rate = dat["rate"];
  printf("%d\n", rate);

  // Sometimes it's difficult for the compiler to know
  // what primitive type to convert the Hjson::Value to.
  // Then an explicit cast can be used.
  printf("%s\n", static_cast<const char*>(dat["array"][0]));

  // To encode to Hjson with default options:
  Hjson::Value sampleMap;
  sampleMap["apple"] = 5;
  sampleMap["lettuce"] = 7;
  std::string hjson = Hjson::Marshal(sampleMap);
  // this is short for:
  // auto options = Hjson::DefaultOptions();
  // std::string hjson = Hjson::MarshalWithOptions(sampleMap, options);
  printf("%s\n", hjson.c_str());

Iterating through the elements of an Hjson::Value of type Hjson::Value::VECTOR:

for (int index = 0; index < int(arr.size()); ++index) {
  std::cout << arr[index] << std::endl;

Iterating through the elements of an Hjson::Value of type Hjson::Value::MAP:

for (auto it = map.begin(); it != map.end(); ++it) {
  std::cout << "key: " << it->first << "  value: " << it->second << std::endl;

Having a default configuration:

#include <hjson.h>
#include <cstdio>

static const char *_szDefaultConfig = R"(
  imageSource: NO DEFAULT
  showImages: true
  writeImages: true
  printFrameIndex: false
  printFrameRate: true

Hjson::Value GetConfig(const char *szInputConfig) {
  Hjson::Value defaultConfig = Hjson::Unmarshal(_szDefaultConfig);

  Hjson::Value inputConfig
  try {
    inputConfig = Hjson::Unmarshal(szInputConfig);
  } catch(std::exception e) {
    std::fprintf(stderr, "Error: Failed to unmarshal input config\n");
    std::fprintf(stdout, "Default config:\n");
    std::fprintf(stdout, _szDefaultConfig);

    return Hjson::Value();

  return Hjson::Merge(defaultConfig, inputConfig);


see releases

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