# sax_parse - JSON for Modern C++
Author: Niels Lohmann

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# 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,
                          const bool ignore_trailing_commas = false);

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

Read from input and generate SAX events

1. Read from a compatible input.

2. Read from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

   The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If `SentinelType` differs from `IteratorType`, it must be comparable to the iterator type with `operator!=`.

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
  - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
`IteratorType`
  a compatible iterator type for overload (2); a pair of character iterators whose `value_type` is an integral type with a size of 1, 2, or 4 bytes (interpreted respectively as UTF-8, UTF-16, and UTF-32)
`SentinelType`

  defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for overload (2), for instance.

  - a custom sentinel type for C++20 ranges
  - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
`SAX`
  a class fulfilling the SAX event listener interface; see `json_sax`

## Parameters

`i` (in)
  Input to parse from
`sax` (in)
  SAX event listener (must not be null)
`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)
`ignore_trailing_commas` (in)
  whether trailing commas in arrays or objects should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
`first` (in)
  iterator to the start of a character range
`last` (in)
  iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with `operator!=`

## Return value

return value of the last processed SAX event

## Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

## Exceptions

- Throws `parse_error.101` in case of an unexpected token, or empty input like a null `FILE*` or `char*` pointer.

## 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; note 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("null()");
            return true;
        }

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

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

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

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

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

        bool start_object(std::size_t elements) override
        {
            events.push_back("start_object(elements=" + std::to_string(elements) + ")");
            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(elements=" + std::to_string(elements) + ")");
            return true;
        }

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

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

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

        bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override
        {
            events.push_back("parse_error(position=" + std::to_string(position) + ", last_token=" + last_token + ",\n            ex=" + 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],
                "DeletionDate": null,
                "Distance": 12.723374634
            }
        }]
        )";

        // create a SAX event consumer object
        sax_event_consumer sec;

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

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

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

Output:

    start_object(elements=18446744073709551615)
    key(val=Image)
    start_object(elements=18446744073709551615)
    key(val=Width)
    number_unsigned(val=800)
    key(val=Height)
    number_unsigned(val=600)
    key(val=Title)
    string(val=View from 15th Floor)
    key(val=Thumbnail)
    start_object(elements=18446744073709551615)
    key(val=Url)
    string(val=http://www.example.com/image/481989943)
    key(val=Height)
    number_unsigned(val=125)
    key(val=Width)
    number_unsigned(val=100)
    end_object()
    key(val=Animated)
    boolean(val=false)
    key(val=IDs)
    start_array(elements=18446744073709551615)
    number_unsigned(val=116)
    number_unsigned(val=943)
    number_unsigned(val=234)
    number_integer(val=-38793)
    end_array()
    key(val=DeletionDate)
    null()
    key(val=Distance)
    number_float(val=12.723375, s=12.723374634)
    end_object()
    end_object()
    parse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],
                ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)

    result: false

## See also

- parse - deserialize from a compatible input
- accept - check if the input is valid JSON

## Version history

- Added in version 3.2.0.
- Ignoring comments via `ignore_comments` added in version 3.9.0.
- Added `ignore_trailing_commas` in version 3.13.0.
- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

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);`.

July 23, 2026