jsoncons

C++ library for parsing and serializing JSON text

View the Project on GitHub danielaparker/jsoncons

jsoncons: a C++ library for json construction

Introduction

Reading JSON text from a file

Constructing json values in C++

Converting CSV files to json

Pretty print

Filters

JSONPath

About jsoncons::json

Wide character support

ojson and wojson

Introduction

jsoncons is a C++, header-only library for constructing JSON and JSON-like data formats such as CBOR. For each supported data format, it enables you to work with the data in a number of ways:

  • As a variant-like data structure, basic_json

  • As a strongly typed C++ data structure that implements json_type_traits

  • With cursor-level access to a stream of parse events, somewhat analogous to StAX pull parsing and push serializing in the XML world.

Compared to other JSON libraries, jsoncons has been designed to handle very large JSON texts. At its heart are SAX-style parsers and serializers. It supports reading an entire JSON text in memory in a variant-like structure. But it also supports efficient access to the underlying data using StAX-style pull parsing and push serializing. And it supports incremental parsing into a user’s preferred form, using information about user types provided by specializations of json_type_traits.

The jsoncons data model supports the familiar JSON types - nulls, booleans, numbers, strings, arrays, objects - plus byte strings. In addition, jsoncons supports semantic tagging of datetimes, epoch times, big integers, big decimals, big floats and binary encodings. This allows it to preserve these type semantics when parsing JSON-like data formats such as CBOR that have them.

For the examples below you need to include some header files and initialize a string of JSON data:

#include <jsoncons/json.hpp>
#include <iostream>
#include <cassert>

using namespace jsoncons; // for convenience

std::string data = R"(
    {
       "application": "hiking",
       "reputons": [
       {
           "rater": "HikingAsylum",
           "assertion": "advanced",
           "rated": "Marilyn C",
           "rating": 0.90,
           "confidence": 0.99
         }
       ]
    }
)";

jsoncons allows you to work with the data in a number of ways:

  • As a variant-like data structure, basic_json

  • As a strongly typed C++ data structure that implements json_type_traits

  • As a stream of parse events

As a variant-like data structure


int main()
{
    // Parse the string of data into a json value
    json j = json::parse(data);

    // Does object member reputons exist?
    std::cout << "(1) " << std::boolalpha << j.contains("reputons") << "\n\n";

    // Get a reference to reputons array 
    const json& v = j["reputons"]; 

    // Iterate over reputons array 
    std::cout << "(2)\n";
    for (const auto& item : v.array_range())
    {
        // Access rated as string and rating as double
        std::cout << item["rated"].as<std::string>() << ", " << item["rating"].as<double>() << "\n";
    }
    std::cout << "\n";

    // Select all "rated" with JSONPath
    std::cout << "(3)\n";
    json result = jsonpath::json_query(j,"$..rated");
    std::cout << pretty_print(result) << "\n\n";

    // Serialize back to JSON
    std::cout << "(4)\n" << pretty_print(j) << "\n\n";
}

Output:

(1) true

(2)
Marilyn C, 0.9

(3)
[
    "Marilyn C"
]

(4)
{
    "application": "hiking",
    "reputons": [
        {
            "assertion": "advanced",
            "confidence": 0.99,
            "rated": "Marilyn C",
            "rater": "HikingAsylum",
            "rating": 0.9
        }
    ]
}

As a strongly typed C++ data structure

jsoncons supports transforming JSON texts into C++ data structures. The functions decode_json and encode_json convert strings or streams of JSON data to C++ data structures and back. Decode and encode work for all C++ classes that have json_type_traits defined. jsoncons already supports many types in the standard library, and your own types will be supported too if you specialize json_type_traits in the jsoncons namespace.

namespace ns {
    enum class hiking_experience {beginner,intermediate,advanced};

    class hiking_reputon
    {
        std::string rater_;
        hiking_experience assertion_;
        std::string rated_;
        double rating_;
        std::optional<std::chrono::seconds> generated_; // assumes C++17, if not use jsoncons::optional
        std::optional<std::chrono::seconds> expires_;
    public:
        hiking_reputon(const std::string& rater,
                       hiking_experience assertion,
                       const std::string& rated,
                       double rating,
                       const std::optional<std::chrono::seconds>& generated = std::optional<std::chrono::seconds>(),
                       const std::optional<std::chrono::seconds>& expires = std::optional<std::chrono::seconds>())
            : rater_(rater), assertion_(assertion), rated_(rated), rating_(rating),
              generated_(generated), expires_(expires)
        {
        }

        const std::string& rater() const {return rater_;}
        hiking_experience assertion() const {return assertion_;}
        const std::string& rated() const {return rated_;}
        double rating() const {return rating_;}
        std::optional<std::chrono::seconds> generated() const {return generated_;}
        std::optional<std::chrono::seconds> expires() const {return expires_;}

        friend bool operator==(const hiking_reputon& lhs, const hiking_reputon& rhs)
        {
            return lhs.rater_ == rhs.rater_ && lhs.assertion_ == rhs.assertion_ && 
                   lhs.rated_ == rhs.rated_ && lhs.rating_ == rhs.rating_ &&
                   lhs.confidence_ == rhs.confidence_ && lhs.expires_ == rhs.expires_;
        }

        friend bool operator!=(const hiking_reputon& lhs, const hiking_reputon& rhs)
        {
            return !(lhs == rhs);
        };
    };

    class hiking_reputation
    {
        std::string application_;
        std::vector<hiking_reputon> reputons_;
    public:
        hiking_reputation(const std::string& application, 
                          const std::vector<hiking_reputon>& reputons)
            : application_(application), 
              reputons_(reputons)
        {}

        const std::string& application() const { return application_;}
        const std::vector<hiking_reputon>& reputons() const { return reputons_;}
    };

} // namespace ns

// Declare the traits. Specify which data members need to be serialized.

JSONCONS_ENUM_TRAITS(ns::hiking_experience, beginner, intermediate, advanced)
// First four members listed are mandatory, generated and expires are optional
JSONCONS_N_CTOR_GETTER_TRAITS(ns::hiking_reputon, 4, rater, assertion, rated, rating, 
                              generated, expires)

// All members are mandatory
JSONCONS_ALL_CTOR_GETTER_TRAITS(ns::hiking_reputation, application, reputons)

int main()
{
    // Decode the string of data into a c++ structure
    ns::hiking_reputation v = decode_json<ns::hiking_reputation>(data);

    // Iterate over reputons array value
    std::cout << "(1)\n";
    for (const auto& item : v.reputons())
    {
        std::cout << item.rated() << ", " << item.rating();
        if (item.generated())
        {
            std::cout << ", " << (*item.generated()).count();
        }
        std::cout << "\n";
    }

    // Encode the c++ structure into a string
    std::string s;
    encode_json_pretty(v, s);
    std::cout << "(2)\n";
    std::cout << s << "\n";
}

Output:

(1)
Marilyn C, 0.9, 1514862245
(2)
{
    "application": "hiking",
    "reputons": [
        {
            "assertion": "advanced",
            "generated": 1514862245,
            "rated": "Marilyn C",
            "rater": "HikingAsylum",
            "rating": 0.9
        }
    ]
}

This example makes use of the convenience macros JSONCONS_ENUM_TRAITS and JSONCONS_ALL_CTOR_GETTER_TRAITS to specialize the json_type_traits for the enum type ns::hiking_experience and the classes ns::hiking_reputon and ns::hiking_reputation. The macro JSONCONS_ENUM_TRAITS generates the code from the enum values, and the macro JSONCONS_ALL_CTOR_GETTER_TRAITS generates the code from the get functions and a constructor. These macro declarations must be placed outside any namespace blocks.

See examples for other ways of specializing json_type_traits.

With cursor-level access

int main()
{
    json_cursor cursor(data);
    for (; !cursor.done(); cursor.next())
    {
        const auto& event = cursor.current();
        switch (event.event_type())
        {
            case staj_event_type::begin_array:
                std::cout << event.event_type() << " " << "\n";
                break;
            case staj_event_type::end_array:
                std::cout << event.event_type() << " " << "\n";
                break;
            case staj_event_type::begin_object:
                std::cout << event.event_type() << " " << "\n";
                break;
            case staj_event_type::end_object:
                std::cout << event.event_type() << " " << "\n";
                break;
            case staj_event_type::key:
                // Or std::string_view, if supported
                std::cout << event.event_type() << ": " << event.get<jsoncons::string_view>() << "\n";
                break;
            case staj_event_type::string_value:
                // Or std::string_view, if supported
                std::cout << event.event_type() << ": " << event.get<jsoncons::string_view>() << "\n";
                break;
            case staj_event_type::null_value:
                std::cout << event.event_type() << "\n";
                break;
            case staj_event_type::bool_value:
                std::cout << event.event_type() << ": " << std::boolalpha << event.get<bool>() << "\n";
                break;
            case staj_event_type::int64_value:
                std::cout << event.event_type() << ": " << event.get<int64_t>() << "\n";
                break;
            case staj_event_type::uint64_value:
                std::cout << event.event_type() << ": " << event.get<uint64_t>() << "\n";
                break;
            case staj_event_type::double_value:
                std::cout << event.event_type() << ": " << event.get<double>() << "\n";
                break;
            default:
                std::cout << "Unhandled event type: " << event.event_type() << " " << "\n";
                break;
        }
    }    
}

Output:

Marilyn C
begin_object
key: application
string_value: hiking
key: reputons
begin_array
begin_object
key: rater
string_value: HikingAsylum
key: assertion
string_value: advanced
key: rated
string_value: Marilyn C
key: rating
double_value: 0.9
key: confidence
double_value: 0.99
end_object
end_array
end_object

Reading JSON text from a file

Input JSON file books.json:

[
    {
        "title" : "Kafka on the Shore",
        "author" : "Haruki Murakami",
        "price" : 25.17
    },
    {
        "title" : "Women: A Novel",
        "author" : "Charles Bukowski",
        "price" : 12.0
    },
    {
        "title" : "Cutter's Way",
        "author" : "Ivan Passer"
    }
]

It consists of an array of book elements, each element is an object with members title, author, and price.

Read the JSON text into a json value,

std::ifstream is("books.json");
json books = json::parse(is);

Loop through the book array elements, using a range-based for loop

for (const auto& book : books.array_range())
{
    std::string author = book["author"].as<std::string>();
    std::string title = book["title"].as<std::string>();
    std::cout << author << ", " << title << std::endl;
}

or begin-end iterators

for (auto it = books.array_range().begin(); 
     it != books.array_range().end();
     ++it)
{
    std::string author = (*it)["author"].as<std::string>();
    std::string title = (*it)["title"].as<std::string>();
    std::cout << author << ", " << title << std::endl;
} 

or a traditional for loop

for (std::size_t i = 0; i < books.size(); ++i)
{
    json& book = books[i];
    std::string author = book["author"].as<std::string>();
    std::string title = book["title"].as<std::string>();
    std::cout << author << ", " << title << std::endl;
}

Output:

Haruki Murakami, Kafka on the Shore
Charles Bukowski, Women: A Novel
Ivan Passer, Cutter's Way

Loop through the members of the third book element, using a range-based for loop

for (const auto& member : books[2].object_range())
{
    std::cout << member.key() << "=" 
              << member.value() << std::endl;
}

or begin-end iterators:

for (auto it = books[2].object_range().begin(); 
     it != books[2].object_range().end();
     ++it)
{
    std::cout << (*it).key() << "=" 
              << (*it).value() << std::endl;
} 

Output:

author=Ivan Passer
title=Cutter's Way

Note that the third book, Cutter’s Way, is missing a price.

You have a choice of object member accessors:

  • book["price"] will throw std::out_of_range if there is no price.
  • book.at("price") will throw std::out_of_range if there is no price.
  • book.at_or_null("price") will return a null json value if there is no price.
  • book.get_value_or<std::string>("price","n/a") will return the price as std::string if available, otherwise "n/a".

Or, you can check if book has a member “price” with the member function contains,

if (book.contains("price"))
{
    double price = book["price"].as<double>();
    std::cout << price;
}
else
{
    std::cout << "n/a";
}

Constructing json values in C++

The default json constructor produces an empty json object. For example

json image_sizing;
std::cout << image_sizing << std::endl;

produces

{}

To construct a json object with members, take an empty json object and set some name-value pairs

image_sizing.insert_or_assign("Resize To Fit",true);  // a boolean 
image_sizing.insert_or_assign("Resize Unit", "pixels");  // a string
image_sizing.insert_or_assign("Resize What", "long_edge");  // a string
image_sizing.insert_or_assign("Dimension 1",9.84);  // a double
image_sizing.insert_or_assign("Dimension 2",json::null());  // a null value

Or, use an object initializer-list:

json file_settings(json::object_arg, {
    {"Image Format", "JPEG"},
    {"Color Space", "sRGB"},
    {"Limit File Size", true},
    {"Limit File Size To", 10000}
});

To construct a json array, initialize with the array type

json color_spaces(json_array_arg);

and add some elements

color_spaces.push_back("sRGB");
color_spaces.push_back("AdobeRGB");
color_spaces.push_back("ProPhoto RGB");

Or, use an array initializer-list:

json image_formats(json_array_arg, {"JPEG","PSD","TIFF","DNG"});

The operator[] provides another way for setting name-value pairs.

json file_export;
file_export["File Format Options"]["Color Spaces"] = 
    std::move(color_spaces);
file_export["File Format Options"]["Image Formats"] = 
    std::move(image_formats);
file_export["File Settings"] = std::move(file_settings);
file_export["Image Sizing"] = std::move(image_sizing);

Note that if file_export["File Format Options"] doesn’t exist, the statement

file_export["File Format Options"]["Color Spaces"] = std::move(color_spaces)

creates "File Format Options" as an object and puts "Color Spaces" in it.

Serializing

std::cout << pretty_print(file_export) << std::endl;

produces

{
    "File Format Options": {
        "Color Spaces": ["sRGB","AdobeRGB","ProPhoto RGB"],
        "Image Formats": ["JPEG","PSD","TIFF","DNG"]
    },
    "File Settings": {
        "Color Space": "sRGB",
        "Image Format": "JPEG",
        "Limit File Size": true,
        "Limit File Size To": 10000
    },
    "Image Sizing": {
        "Dimension 1": 9.84,
        "Dimension 2": null,
        "Resize To Fit": true,
        "Resize Unit": "pixels",
        "Resize What": "long_edge"
    }
}

Converting CSV files to json

Example CSV file (tasks.csv):

project_id, task_name, task_start, task_finish
4001,task1,01/01/2003,01/31/2003
4001,task2,02/01/2003,02/28/2003
4001,task3,03/01/2003,03/31/2003
4002,task1,04/01/2003,04/30/2003
4002,task2,05/01/2003,

You can read the CSV file into a json value with the decode_csv function.

#include <fstream>
#include <jsoncons/json.hpp>
#include <jsoncons_ext/csv/csv.hpp>

using namespace jsoncons;

int main()
{
    std::ifstream is("input/tasks.csv");

    csv::csv_options options;
    options.assume_header(true)
           .trim(true)
           .ignore_empty_values(true) 
           .column_types("integer,string,string,string");
    ojson tasks = csv::decode_csv<ojson>(is, options);

    std::cout << "(1)\n" << pretty_print(tasks) << "\n\n";

    std::cout << "(2)\n";
    csv::encode_csv(tasks, std::cout);
}

Output:

(1)
[
    {
        "project_id": 4001,
        "task_name": "task1",
        "task_start": "01/01/2003",
        "task_finish": "01/31/2003"
    },
    {
        "project_id": 4001,
        "task_name": "task2",
        "task_start": "02/01/2003",
        "task_finish": "02/28/2003"
    },
    {
        "project_id": 4001,
        "task_name": "task3",
        "task_start": "03/01/2003",
        "task_finish": "03/31/2003"
    },
    {
        "project_id": 4002,
        "task_name": "task1",
        "task_start": "04/01/2003",
        "task_finish": "04/30/2003"
    },
    {
        "project_id": 4002,
        "task_name": "task2",
        "task_start": "05/01/2003"
    }
]

There are a few things to note about the effect of the parameter settings.

  • assume_header true tells the csv parser to parse the first line of the file for column names, which become object member names.
  • trim true tells the parser to trim leading and trailing whitespace, in particular, to remove the leading whitespace in the column names.
  • ignore_empty_values true causes the empty last value in the task_finish column to be omitted.
  • The column_types setting specifies that column one (“project_id”) contains integers and the remaining columns strings.

Pretty print

The pretty_print function applies stylistic formatting to JSON text. For example

json j;

j["verts"] = json(json_array_arg, {1, 2, 3});
j["normals"] = json(json_array_arg, {1, 0, 1});
j["uvs"] = json(json_array_arg, {0, 0, 1, 1});

std::cout << pretty_print(j) << std::endl;

produces

{
    "normals": [1,0,1],
    "uvs": [0,0,1,1],
    "verts": [1,2,3]
}

By default, within objects, arrays of scalar values are displayed on the same line.

The pretty_print function takes an optional second parameter, basic_json_options, that allows custom formatting of output. To display the array scalar values on a new line, set the object_array_line_splits property to line_split_kind::new_line. The code

json_options options;
format.object_array_line_splits(line_split_kind::new_line);
std::cout << pretty_print(val,options) << std::endl;

produces

{
    "normals": [
        1,0,1
    ],
    "uvs": [
        0,0,1,1
    ],
    "verts": [
        1,2,3
    ]
}

To display the elements of array values on multiple lines, set the object_array_line_splits property to line_split_kind::multi_line. The code

json_options options;
format.object_array_line_splits(line_split_kind::multi_line);
std::cout << pretty_print(val,options) << std::endl;

produces

{
    "normals": [
        1,
        0,
        1
    ],
    "uvs": [
        0,
        0,
        1,
        1
    ],
    "verts": [
        1,
        2,
        3
    ]
}

Filters

You can rename object member names with the built in filter rename_object_key_filter

#include <sstream>
#include <jsoncons/json.hpp>
#include <jsoncons/json_filter.hpp>

using namespace jsoncons;

int main()
{
    std::string s = R"({"first":1,"second":2,"fourth":3,"fifth":4})";    

    json_stream_encoder encoder(std::cout);

    // Filters can be chained
    rename_object_key_filter filter2("fifth", "fourth", encoder);
    rename_object_key_filter filter1("fourth", "third", filter2);

    // A filter can be passed to any function that takes
    // a json_visitor ...
    std::cout << "(1) ";
    std::istringstream is(s);
    json_reader reader(is, filter1);
    reader.read();
    std::cout << std::endl;

    // or a json_visitor    
    std::cout << "(2) ";
    ojson j = ojson::parse(s);
    j.dump(filter1);
    std::cout << std::endl;
}

Output:

(1) {"first":1,"second":2,"third":3,"fourth":4}
(2) {"first":1,"second":2,"third":3,"fourth":4}

Or define and use your own filters. See basic_json_filter for details.

JSONPath

Stefan Goessner’s JSONPath is an XPATH inspired query language for selecting parts of a JSON structure.

Example JSON file (booklist.json):

{ "store": {
    "book": [ 
      { "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century",
        "price": 8.95
      },
      { "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour",
        "price": 12.99
      },
      { "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": 8.99
      },
      { "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": 22.99
      }
    ]
  }
}

JSONPath examples:

#include <jsoncons_ext/jsonpath/jsonpath.hpp>

using jsoncons::jsonpath::json_query;

std::ifstream is("./input/booklist.json");
json booklist = json::parse(is);

// The authors of books that are cheaper than $10
json result1 = json_query(booklist, "$.store.book[?(@.price < 10)].author");
std::cout << "(1) " << result1 << std::endl;

// The number of books
json result2 = json_query(booklist, "$..book.length");
std::cout << "(2) " << result2 << std::endl;

// The third book
json result3 = json_query(booklist, "$..book[2]");
std::cout << "(3)\n" << pretty_print(result3) << std::endl;

// All books whose author's name starts with Evelyn
json result4 = json_query(booklist, "$.store.book[?(@.author =~ /Evelyn.*?/)]");
std::cout << "(4)\n" << pretty_print(result4) << std::endl;

// The titles of all books that have isbn number
json result5 = json_query(booklist, "$..book[?(@.isbn)].title");
std::cout << "(5) " << result5 << std::endl;

// All authors and titles of books
json result6 = json_query(booklist, "$['store']['book']..['author','title']");
std::cout << "(6)\n" << pretty_print(result6) << std::endl;

Output:

(1) ["Nigel Rees","Herman Melville"]
(2) [4]
(3)
[
    {
        "author": "Herman Melville",
        "category": "fiction",
        "isbn": "0-553-21311-3",
        "price": 8.99,
        "title": "Moby Dick"
    }
]
(4)
[
    {
        "author": "Evelyn Waugh",
        "category": "fiction",
        "price": 12.99,
        "title": "Sword of Honour"
    }
]
(5) ["Moby Dick","The Lord of the Rings"]
(6)
[
    "Nigel Rees",
    "Sayings of the Century",
    "Evelyn Waugh",
    "Sword of Honour",
    "Herman Melville",
    "Moby Dick",
    "J. R. R. Tolkien",
    "The Lord of the Rings"
]

About jsoncons::json

The json class is an instantiation of the basic_json class template that uses char as the character type and sorts object members in alphabetically order.

typedef basic_json<char,
                   ImplementationPolicy = sorted_policy,
                   Allocator = std::allocator<char>> json;

If you prefer to retain the original insertion order, use ojson instead.

The library includes an instantiation for wide characters as well, wjson

typedef basic_json<wchar_t,
                   ImplementationPolicy = sorted_policy,
                   Allocator = std::allocator<wchar_t>> wjson;

If you prefer to retain the original insertion order, use wojson instead.

Note that the allocator type allows you to supply a custom allocator. For example, you can use the boost fast_pool_allocator:

#include <boost/pool/pool_alloc.hpp>
#include <jsoncons/json.hpp>

typedef jsoncons::basic_json<char, boost::fast_pool_allocator<char>> my_json;

my_json o;

o.insert_or_assign("FirstName","Joe");
o.insert_or_assign("LastName","Smith");

This results in a json value being constucted with all memory being allocated from the boost memory pool. (In this particular case there is no improvement in performance over std::allocator.)

Note that the underlying memory pool used by the boost::fast_pool_allocator is never freed.

Wide character support

jsoncons supports wide character strings and streams with wjson and wjson_reader. It supports UTF16 encoding if wchar_t has size 2 (Windows) and UTF32 encoding if wchar_t has size 4. You can construct a wjson value in exactly the same way as a json value, for instance:

using jsoncons::wjson;

wjson j;
j[L"field1"] = L"test";
j[L"field2"] = 3.9;
j[L"field3"] = true;

std::wcout << j << L"\n";

which prints

{"field1":"test","field2":3.9,"field3":true}

ojson and wojson

The ojson (wojson) class is an instantiation of the basic_json class template that uses char (wchar_t) as the character type and keeps object members in their original order.

ojson o = ojson::parse(R"(
{
    "street_number" : "100",
    "street_name" : "Queen St W",
    "city" : "Toronto",
    "country" : "Canada"
}
)");

std::cout << pretty_print(o) << std::endl;

Output:

{
    "street_number": "100",
    "street_name": "Queen St W",
    "city": "Toronto",
    "country": "Canada"
}

Insert “postal_code” at end

o.insert_or_assign("postal_code", "M5H 2N2");

std::cout << pretty_print(o) << std::endl;

Output:

{
    "street_number": "100",
    "street_name": "Queen St W",
    "city": "Toronto",
    "country": "Canada",
    "postal_code": "M5H 2N2"
}

Insert “province” before “country”

auto it = o.find("country");
o.insert_or_assign(it,"province","Ontario");

std::cout << pretty_print(o) << std::endl;

Output:

{
    "street_number": "100",
    "street_name": "Queen St W",
    "city": "Toronto",
    "province": "Ontario",
    "country": "Canada",
    "postal_code": "M5H 2N2"
}

For more information, consult the latest examples, documentation and roadmap.