Skip to content

# nlohmann::basic_json::sax_parse

    // (1)
    template <typename InputType, typename SAX>
    static bool sax_parse(InputType&& i,
                          SAX* sax,
                          input_format_t format = input_format_t::json,
                          const bool strict = true,
                          const bool ignore_comments = false);

    // (2)
    template<class IteratorType, class SAX>
    static bool sax_parse(IteratorType first, IteratorType last,
                          SAX* sax,
                          input_format_t format = input_format_t::json,
                          const bool strict = true,
                          const bool ignore_comments = false);

Read from input and generate SAX events

1. Read from a compatible input.

2. Read from a pair of character iterators

   The value_type of the iterator must be an integral type with size of 1, 2 or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16 and UTF-32.

The SAX event lister must follow the interface of `json_sax`.

## Template parameters

`InputType`

  A compatible input, for instance:

  - an `std::istream` object
  - a `FILE` pointer
  - a C-style array of characters
  - a pointer to a null-terminated string of single byte characters
  - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
`IteratorType`
  Description
`SAX`
  Description

## Parameters

`i` (in)
  Input to parse from.
`sax` (in)
  SAX event listener
`format` (in)
  the format to parse (JSON, CBOR, MessagePack, or UBJSON) (optional, `input_format_t::json` by default), see `input_format_t` for more information
`strict` (in)
  whether the input has to be consumed completely (optional, `true` by default)
`ignore_comments` (in)
  whether comments should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
`first` (in)
  iterator to start of character range
`last` (in)
  iterator to end of character range

## Return value

return value of the last processed SAX event

## Exception safety

## Complexity

Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the SAX consumer `sax` has a super-linear complexity.

## Notes

A UTF-8 byte order mark is silently ignored.

## Examples

Example

The example below demonstrates the `sax_parse()` function reading from string and processing the events with a user-defined SAX event consumer.

    #include <iostream>
    #include <iomanip>
    #include <sstream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    // a simple event consumer that collects string representations of the passed
    // values; not inheriting from json::json_sax_t is not required, but can
    // help not to forget a required function
    class sax_event_consumer : public json::json_sax_t
    {
      public:
        std::vector<std::string> events;

        bool null() override
        {
            events.push_back("value: null");
            return true;
        }

        bool boolean(bool val) override
        {
            events.push_back("value: " + std::string(val ? "true" : "false"));
            return true;
        }

        bool number_integer(number_integer_t val) override
        {
            events.push_back("value: " + std::to_string(val));
            return true;
        }

        bool number_unsigned(number_unsigned_t val) override
        {
            events.push_back("value: " + std::to_string(val));
            return true;
        }

        bool number_float(number_float_t val, const string_t& s) override
        {
            events.push_back("value: " + s);
            return true;
        }

        bool string(string_t& val) override
        {
            events.push_back("value: " + val);
            return true;
        }

        bool start_object(std::size_t elements) override
        {
            events.push_back("start: object");
            return true;
        }

        bool end_object() override
        {
            events.push_back("end: object");
            return true;
        }

        bool start_array(std::size_t elements) override
        {
            events.push_back("start: array");
            return true;
        }

        bool end_array() override
        {
            events.push_back("end: array");
            return true;
        }

        bool key(string_t& val) override
        {
            events.push_back("key: " + val);
            return true;
        }

        bool binary(json::binary_t& val) override
        {
            events.push_back("binary");
            return true;
        }

        bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override
        {
            events.push_back("error: " + std::string(ex.what()));
            return false;
        }
    };

    int main()
    {
        // a JSON text
        auto text = R"(
        {
            "Image": {
                "Width":  800,
                "Height": 600,
                "Title":  "View from 15th Floor",
                "Thumbnail": {
                    "Url":    "http://www.example.com/image/481989943",
                    "Height": 125,
                    "Width":  100
                },
                "Animated" : false,
                "IDs": [116, 943, 234, 38793],
                "Distance": 12.723374634
            }
        }
        )";

        // create a SAX event consumer object
        sax_event_consumer sec;

        // parse and serialize JSON
        bool result = json::sax_parse(text, &sec);

        // output the recorded events
        for (auto& event : sec.events)
        {
            std::cout << "(" << event << ") ";
        }

        // output the result of sax_parse
        std::cout << "\nresult: " << std::boolalpha << result << std::endl;
    }

Output:

    (start: object) (key: Image) (start: object) (key: Width) (value: 800) (key: Height) (value: 600) (key: Title) (value: View from 15th Floor) (key: Thumbnail) (start: object) (key: Url) (value: http://www.example.com/image/481989943) (key: Height) (value: 125) (key: Width) (value: 100) (end: object) (key: Animated) (value: false) (key: IDs) (start: array) (value: 116) (value: 943) (value: 234) (value: 38793) (end: array) (key: Distance) (value: 12.723374634) (end: object) (end: object)
    result: true

## Version history

- Added in version 3.2.0.
- Ignoring comments via `ignore_comments` added in version 3.9.0.

Deprecation

Overload (2) replaces calls to `sax_parse` with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like `sax_parse({ptr, ptr+len});` with `sax_parse(ptr, ptr+len);`.

Back to top