basic_json_cursor.md

jsoncons::basic_json_cursor

#include <jsoncons/json_cursor.hpp>

template<
    class CharT,
    class Source=jsoncons::stream_source<CharT>,
    class Allocator=std::allocator<char>> basic_json_cursor;

A pull parser for reporting JSON parse events. A typical application will repeatedly process the current() event and call the next() function to advance to the next event, until done() returns true. In addition, when positioned on a begin_object event, the read_to function can pull a complete object representing the events from begin_object to end_object, and when positioned on a begin_array event, a complete array representing the events from begin_array to end_array.

basic_json_cursor is noncopyable and nonmoveable.

Aliases for common character types are provided:

Type	Definition
json_stream_cursor (since 0.167.0)	basic_json_cursor<char,jsoncons::stream_source<char>>
json_string_cursor (since 0.167.0)	basic_json_cursor<char,jsoncons::string_source<char>>
wjson_stream_cursor (since 0.167.0)	basic_json_cursor<wchar_t,jsoncons::stream_source<wchar_t>>
wjson_string_cursor (since 0.167.0)	basic_json_cursor<wchar_t,jsoncons::string_source<wchar_t>>
json_cursor (until 0.167.0)	basic_json_cursor<char>
wjson_cursor (until 0.167.0)	basic_json_cursor<wchar_t>

Implemented interfaces

basic_staj_cursor

Constructors

template <class Sourceable>
basic_json_cursor(Sourceable&& source, 
                  const basic_json_decode_options<CharT>& options = basic_json_decode_options<CharT>(),
                  std::function<bool(json_errc,const ser_context&)> err_handler = default_json_parsing(),
                  const Allocator& alloc = Allocator()); (1)

template <class Sourceable>
basic_json_cursor(Sourceable&& source, 
                  std::error_code& ec); (2)

template <class Sourceable>
basic_json_cursor(Sourceable&& source, 
                  const basic_json_decode_options<CharT>& options,
                  std::error_code& ec); (3)

template <class Sourceable>
basic_json_cursor(Sourceable&& source, 
                  const basic_json_decode_options<CharT>& options,
                  std::function<bool(json_errc,const ser_context&)> err_handler,
                  std::error_code& ec); (4)

template <class Sourceable>
basic_json_cursor(std::allocator_arg_t, const Allocator& alloc, 
                  Sourceable&& source, 
                  const basic_json_decode_options<CharT>& options,
                  std::function<bool(json_errc,const ser_context&)> err_handler,
                  std::error_code& ec); (5)

Constructor (1) reads from a character sequence or stream and throws a ser_error if a parsing error is encountered while processing the initial event. Constructors (2)-(5) read from a character sequence or stream and set ec if a parsing error is encountered while processing the initial event.

Note: It is the programmer's responsibility to ensure that basic_json_cursor does not outlive the source, as basic_json_cursor holds a pointer to but does not own this resource.

Parameters

source - a value from which a jsoncons::basic_string_view<char_type> is constructible, or a value from which a source_type is constructible. In the case that a jsoncons::basic_string_view<char_type> is constructible from source, source is dispatched immediately to the parser. Otherwise, the json_cursor reads from a source_type in chunks.

Member functions

bool done() const override;

Checks if there are no more events.

const basic_staj_event& current() const override;

Returns the current basic_staj_event.

void read_to(json_visitor& visitor) override

Feeds the current and succeeding staj events through the provided visitor, until the visitor indicates to stop. If a parsing error is encountered, throws a ser_error.

void read_to(basic_json_visitor<char_type>& visitor,
            std::error_code& ec) override

Feeds the current and succeeding staj events through the provided visitor, until the visitor indicates to stop. If a parsing error is encountered, sets ec.

void next() override;

Advances to the next event. If a parsing error is encountered, throws a ser_error.

void next(std::error_code& ec) override;

Advances to the next event. If a parsing error is encountered, sets ec.

const ser_context& context() const override;

Returns the current context

void reset();

Reset cursor to read another value from the same source

template <class Sourceable>
reset(Sourceable&& source)

Reset cursor to read new value from a new source

Non-member functions

template <class CharT, class Source, class Allocator> basic_staj_filter_view operator|(basic_json_cursor<CharT,Source,Allocator>& cursor, std::function<bool(const basic_staj_event&, const ser_context&)> pred);

Examples

Input JSON file book_catalog.json:

[ 
  { 
      "author" : "Haruki Murakami",
      "title" : "Hard-Boiled Wonderland and the End of the World",
      "isbn" : "0679743464",
      "publisher" : "Vintage",
      "date" : "1993-03-02",
      "price": 18.90
  },
  { 
      "author" : "Graham Greene",
      "title" : "The Comedians",
      "isbn" : "0099478374",
      "publisher" : "Vintage Classics",
      "date" : "2005-09-21",
      "price": 15.74
  }
]

Read JSON parse events

#include <jsoncons/json_cursor.hpp>
#include <string>
#include <fstream>

using namespace jsoncons;

int main()
{
    std::ifstream is("book_catalog.json");

    json_stream_cursor cursor(is);

    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\n";
                break;
        }
    }
}

Output:

begin_array
begin_object
key: author
string_value: Haruki Murakami
key: title
string_value: Hard-Boiled Wonderland and the End of the World
key: isbn
string_value: 0679743464
key: publisher
string_value: Vintage
key: date
string_value: 1993-03-02
key: price
double_value: 19
end_object
begin_object
key: author
string_value: Graham Greene
key: title
string_value: The Comedians
key: isbn
string_value: 0099478374
key: publisher
string_value: Vintage Classics
key: date
string_value: 2005-09-21
key: price
double_value: 16
end_object
end_array

Filter the event stream

#include <jsoncons/json_cursor.hpp>
#include <string>
#include <fstream>

using namespace jsoncons;

int main()
{
    bool author_next = false;
    auto filter = [&](const staj_event& event, const ser_context&) -> bool
    {
        if (event.event_type() == staj_event_type::key &&
            event.get<jsoncons::string_view>() == "author")
        {
            author_next = true;
            return false;
        }
        if (author_next)
        {
            author_next = false;
            return true;
        }
        return false;
    };

    std::ifstream is("book_catalog.json");

    json_stream_cursor cursor(is);
    auto filtered_c = cursor | filter;

    for (; !filtered_c.done(); filtered_c.next())
    {
        const auto& event = filtered_c.current();
        switch (event.event_type())
        {
            case staj_event_type::string_value:
                std::cout << event.get<jsoncons::string_view>() << "\n";
                break;
        }
    }
}

Output:

Haruki Murakami
Graham Greene

Pull nested objects into a basic_json

#include <jsoncons/json_cursor.hpp>
#include <jsoncons/json.hpp> // json_decoder and json
#include <fstream>

int main()
{
    std::ifstream is("book_catalog.json");

    json_stream_cursor cursor(is);

    json_decoder<json> decoder;
    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";
                cursor.read_to(decoder);
                json j = decoder.get_result();
                std::cout << pretty_print(j) << "\n";
                break;
            }
            default:
            {
                std::cout << "Unhandled event type: " << event.event_type() << " " << "\n";
                break;
            }
        }
    }
}

Output:

begin_array
begin_object
{
    "author": "Haruki Murakami",
    "date": "1993-03-02",
    "isbn": "0679743464",
    "price": 18.9,
    "publisher": "Vintage",
    "title": "Hard-Boiled Wonderland and the End of the World"
}
begin_object
{
    "author": "Graham Greene",
    "date": "2005-09-21",
    "isbn": "0099478374",
    "price": 15.74,
    "publisher": "Vintage Classics",
    "title": "The Comedians"
}
end_array

See also

basic_staj_event

staj_array_iterator

staj_object_iterator