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jsoncons is a C++ library for the construction of JavaScript Object Notation (JSON). It supports parsing a JSON file or string into a tree structured json value, building a json value in C++ code, and serializing a json value to a file or string. It also provides an event-based API for reading and writing JSON documents that are too large to fit into available memory, somewhat analogously to SAX processing in the XML world. A brief introduction may be found in jsoncons: a C++ library for json construction.

The library is header-only: it consists solely of header files containing templates and inline functions, and requires no separately-compiled library binaries when linking. It has no dependence on other libraries.

jsoncons uses some features that are new to C++ 11, particularly move semantics, however, it has been written to be compatible with VC++ 10 SP1 (note that SP1 is required for VC++ 10, it fixes compiler bugs with move semantics.) It has been tested with MS Visual C++ 10 SP1, Intel C++ Studio XE 2013, clang 3.3 and GCC 4.8.

The code repository is on github, releases are on sourceforge. It is distributed under the Boost Software License

To install the jsoncons library, download the zip file, unpack the release, under src find the directory jsoncons, and copy it to your include directory. If you wish to use extensions, copy the jsoncons_ext directory as well.

0.93 Release

New features

  • Supports wide character strings and streams with wjson, wjson_reader etc. Assumes UTF16 encoding if sizeof(wchar_t)=2 and UTF32 encoding if sizeof(wchar_t)=4.
  • The empty class null_type is added to the jsoncons namespace, it replaces the member type json::null_type (json::null_type is typedefed to jsoncons::null_type for backward compatibility.)

Defect fixes:

  • The ascii character 0x7f (del) was not being considered a control character to be escaped, this is fixed.
  • Fixed two issues with serialization when the output format property escape_all_non_ascii is enabled. One, the individual bytes were being checked if they were non ascii, rather than first converting to a codepoint. Two, continuations weren't being handled when decoding.

0.92a Release

Includes contributed updates for valid compilation and execution in gcc and clang environments

0.92 Release

Breaking change (but only if you have subclassed json_input_handler or json_output_handler)

  • For consistency with other names, the input and output handler methods new to 0.91 - value_string, value_double, value_longlong, value_ulonglong and value_bool - have been renamed to string_value, double_value, longlong_value, ulonglong_value and bool_value.

Non breaking changes (previous features are deprecated but still work)

  • name_value_pair has been renamed to member_type (typedefed to previous name.)

  • as_string(output_format format) has been deprecated, use the existing to_string(output_format format) instead


  • json now has extensibilty, you can access and modify json values with new types, see the tutorial Extensibility

Preparation for allocator support:

  • The basic_json and related classes now have an Storage template parameter, which is currently just a placeholder, but will later provide a hook to allow users to control how json storage is allocated. This addition is transparent to users of the json and related classes.

0.91 Release

This release should be largely backwards compatible with 0.90 and 0.83 with two exceptions:

  1. If you have used object iterators, you will need to replace uses of std::pair with name_value_pair, in particular, first becomes name() and second becomes value().

  2. If you have subclassed json_input_handler, json_output_handler, or json_filter, and have implemented value(const std::string& ..., value(double ..., etc., you will need to modify the names to value_string(const std::string& ..., value_double(double ... (no changes if you are feeding existing implementations.)

The changes are

  • Replaced std::pair<std::string,json> with name_value_pair that has accessors name() and value()

  • In json_input_handler and json_output_handler, allowed for overrides of the value methods by making them non-virtual and adding virtual methods value_string, value_double, value_longlong, value_ulonglong, and value_bool

Other new features:

  • Changed implementation of is<T> and as<T>, the current implementation should be user extensible

  • make_multi_array<N> makes a multidimensional array with the number of dimensions specified as a template parameter. Replaces make_2d_array and make_3d_array, which are now deprecated.

  • Added support for is<std::vector<T>> and as<std::vector<T>>

  • Removed JSONCONS_NO_CXX11_RVALUE_REFERENCES, compiler must support move semantics

Incorporates a number of contributions from Pedro Larroy and the developers of the clearskies_core project:

  • build system for posix systems
  • GCC to list of supported compilers
  • Android fix
  • fixed virtual destructors missing in json_input_handler, json_output_handler and parsing_context
  • fixed const_iterator should be iterator in json_object implementation

To clean up the interface and avoid too much duplicated functionality, we've deprecated some json methods (but they still work)


Use json val(json::an_array) or json::make_multi_array<1>(...) instead (but make_array will continue to work)


Use make_multi_array<2> and make_multi_array<3> instead


Use as<std::vector<int>> etc. instead


Use as<int>, as<unsigned int>, and as<char> instead

Release 0.90a

Fixed issue affecting clang compile

Release 0.90

This release should be fully backwards compatible with 0.83.

Includes performance enhancements to json_reader and json_deserializer

Fixes issues with column numbers reported with exceptions

Incorporates a number of patches contributed by Marc Chevrier:

  • Fixed issue with set member on json object when a member with that name already exists
  • clang port
  • cmake build files for examples and test suite
  • json template method is<T> for examining the types of json values
  • json template method as<T> for accessing json values

Release 0.83

Optimizations (very unlikely to break earlier code)

  • get(const std::name& name) const now returns const json& if keyed value exists, otherwise a const reference to json::null

  • get(const std::string& name, const json& default_val) const now returns const json (actually a const proxy that evaluates to json if read)

Bug fixes

  • Line number not incremented within multiline comment - fixed

Deprecated features removed

  • Removed deprecated output_format properties (too much bagage to carry around)

Release 0.82a

  • The const version of the json operator[](const std::string& name) didn't need to return a proxy, the return value has been changed to const json& (this change is transparent to the user.)

  • get(const std::name& name) has been changed to return a copy (rather than a reference), and json::null if there is no member with that name (rather than throw.) This way both get methods return default values if no keyed value exists.

  • non-const and const methods json& at(const std::name& name) have been added to replace the old single argument get method. These have the same behavior as the corresponding operator[] functions, but the non-const at is more efficient.

Release 0.81

  • Added accessor and modifier methods floatfield to output_format to provide a supported way to set the floatfield format flag to fixed or scientific with a specified number of decimal places (this can be done in older versions, but only with deprecated methods.)

  • The default constructor now constructs an empty object (rather than a null object.) While this is a change, it's unlikely to break exisitng code (all test cases passed without modification.)

This means that instead of

json obj(json::an_object);
obj["field"] = "field";

you can simply write

json obj;
obj["field"] = "field";

The former notation is still supported, though.

  • Added a version of 'resize_array' to json that resizes the array to n elements and initializes them to a specified value.

  • Added a version of the static method json::make_array that takes no arguments and makes an empty json array

Note that

json arr(json::an_array);

is equivalent to

json arr = json::make_array();


json arr(json::an_array);

is equivalent to

json arr = json::make_array(10,0.0);

For consistency the json::make_array notation is now favored in the documentation.

Release 0.71

  • Added resize_array method to json for resizing json arrays

  • Fixed issue with remove_range method (templated code failed to compile if calling this method.)

  • Added remove_member method to remove a member from a json object

  • Fixed issue with multiline line comments, added test case

  • Fixed issue with adding custom data to a json array using add_custom_data, added examples.

Release 0.70

  • Since 0.50, jsoncons has used snprintf for default serialization of double values to string values. This can result in invalid json output when running on a locale like German or Spanish. The period character (‘.’) is now always used as the decimal point, non English locales are ignored.

  • The output_format methods that support alternative floating point formatting, e.g. fixed, have been deprecated.

  • Added a template method as_vector<T> to the json class. If a json value is an array and conversion is possible to the template type, returns a std::vector of that type, otherwise throws an std::exception. Specializations are provided for std::string, bool, char, int, unsigned int, long, unsigned long, long long, unsigned long long, and double. For example

    std::string s("[0,1,2,3]");

    json val = json::parse_string(s);

    std::vector<int> v = val.as_vector<int>();

  • Undeprecated the json member function precision

Release 0.60b

This release (0.60b) is fully backwards compatible with 0.50.

A change introduced with 0.60 has been reversed. 0.60 introduced an alternative method of constructing a json arrray or object with an initial default constructor, a bug with this was fixed in 0.60a, but this feature and related documentation has been removed because it added complexity but no real value.


  • Added swap member function to json

  • Added add and add_custom_data overrides to json that take an index value, for adding a new element at the specified index and shifting all elements currently at or above that index to the right.

  • Added capacity member functions to json

0.60 extensions

  • csv_serializer has been added to the csv extension

Release 0.50

This release is fully backwards compatible with 0.4*, and mostly backwards compatible to 0.32 apart from the two name changes in 0.41

Bug fixes

  • When reading the escaped characters "\b", "\f", "\r" and "\t" appearing in json strings, json_reader was replacing them with the linefeed character, this has been fixed.


  • Deprecated modifiers precision and fixed_decimal_places from output_format. Use set_floating_point_format instead.
  • Deprecated constructor that takes indenting parameter from output_format. For pretty printing with indenting, use the pretty_print function or pass the indenting parameter in json_serializer.


  • When serializing floating point values to a stream, previous versions defaulted to default floating point precision with a precision of 16. This has been changed to truncate trailing zeros but keep one if immediately after a decimal point.

New features

  • For line reporting in parser error messages, json_reader now recognizes \r\n, \n alone or \r alone (\r alone is new.)
  • Added set_floating_point_format methods to output_format to give more control over floating point notation.

Non functional enhancements

  • json_reader now estimates the minimum capacity for arrays and objects, and reports that information for the begin_array and begin_object events. This greatly reduces reallocations.

Release 0.42

  • Fixed another bug with multi line /**/ comments
  • Minor fixes to reporting line and column number of errors
  • Added fixed_decimal_places setter to output_format
  • Added version of as_string to json that takes output_format as a parameter
  • Reorganization of test cases and examples in source tree

Release 0.41

  • Added non-member overload swap(json& a, json& b)
  • Fixed bug with multi line /**/ comments
  • Added begin_json and end_json methods to json_output_handler
  • json_deserializer should now satisfy basic exception safety (no leak guarantee)
  • Moved csv_reader.hpp to jsoncons_ext/csv directory
  • Changed csv_reader namespace to jsoncons_ext::csv
  • json::parse_file no longer reads the entire file into memory before parsing (it now uses json_reader default buffering)

Release 0.40

  • json_listener renamed to json_input_handler
  • json_writer renamed to json_output_handler

  • Added json_filter class

  • json get method that takes default argument now returns a value rather than a reference

  • Issue in csv_reader related to get method issue fixed
  • Issue with const json operator[] fixed
  • Added as_char method to json
  • Improved exception safety, some opportunites for memory leaks in the presence of exceptions removed

Release 0.33

Added reserve method to json

Added static make_3d_array method to json

json_reader now configured for buffered reading

Added csv_reader class for reading CSV files and producing JSON events

Fixed bug with explicitly passing output_format in pretty_print.

Release 0.32

Added remove_range method, operator== and operator!= to proxy and json objects

Added static methods make_array and make_2d_array to json

Release 0.31

error_handler method content_error renamed to error

Added error_code to warning, error and fatal_error methods of error_handler

Release 0.30

json_in_stream renamed to json_listener

json_out_stream renamed to json_writer

Added buffer accessor method to parsing_context

Release 0.20

Added parsing_context class for providing information about the element being parsed.

error_handler methods take message and parsing_context parameters

json_in_stream handlers take parsing_context parameter

Release 0.19

Added error_handler class for json_reader

Made json_exception a base class for all json exceptions

Added root() method to json_deserializer to get a reference to the json value

Removed swap_root() method from json_deserializer

Release 0.18

Renamed serialize() class method to to_stream() in json

Custom data serialization supported through template function specialization of serialize (reverses change in 0.17)

Release 0.17

Added is_custom() method to json and proxy

get_custom() method renamed to custom_data() in json and proxy

Added clear() method to json and proxy

set_member() method renamed to set()

set_custom() method renamed to set_custom_data()

push_back() method renamed to add() in json and proxy

Added add_custom_data method() in json and proxy

Custom data serialization supported through template class specialization of custom_serialization (replaces template function specialization of serialize)

Release 0.16

Change to json_out_stream and json_serializer:

void value(const custom_data& value)


Free function serialize replaces free function to_stream for serializing custom data.

pretty print tidied up for nested arrays

Release 0.15

Made eof() method on json_reader public, to support reading multiple JSON objects from a stream.

Release 0.14

Added pretty_print class

Renamed json_stream_writer to json_serializer, implements pure virtual class json_out_stream

Renamed json_stream_listener to json_deserializer implements pure virtual class json_in_stream

Renamed json_parser to json_reader, parse to read.

Changed indenting so object and array members start on new line.

Added support for storing user data in json object, and serializing to JSON.

Release 0.13

Replaced simple_string union member with json_string that wraps std::basic_string<Char>

name() and value() event handler methods on basic_json_stream_writer take const std::basic_string<Char>& rather than const Char* and length.

Release 0.12

Implemented operator<< for json::proxy

Added to_stream methods to json::proxy

Release 0.11

Added members to json_parser to access and modify the buffer capacity

Added checks when parsing integer values to determine overflow for long long and unsigned long long, and if overflow, parse them as doubles.

Source: README.md, updated 2014-03-07