json/doc/mkdocs/docs/features/parsing/sax_interface.md

SAX Interface

The library uses a SAX-like interface with the following functions:

interface json::sax_t {
    + {abstract} bool null()

    + {abstract} bool boolean(bool val)

    + {abstract} bool number_integer(number_integer_t val)
    + {abstract} bool number_unsigned(number_unsigned_t val)

    + {abstract} bool number_float(number_float_t val, const string_t& s)

    + {abstract} bool string(string_t& val)
    + {abstract} bool binary(binary_t& val)

    + {abstract} bool start_object(std::size_t elements)
    + {abstract} bool end_object()
    + {abstract} bool start_array(std::size_t elements)
    + {abstract} bool end_array()
    + {abstract} bool key(string_t& val)

    + {abstract} bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex)
}

// called when null is parsed
bool null();

// called when a boolean is parsed; value is passed
bool boolean(bool val);

// called when a signed or unsigned integer number is parsed; value is passed
bool number_integer(number_integer_t val);
bool number_unsigned(number_unsigned_t val);

// called when a floating-point number is parsed; value and original string is passed
bool number_float(number_float_t val, const string_t& s);

// called when a string is parsed; value is passed and can be safely moved away
bool string(string_t& val);
// called when a binary value is parsed; value is passed and can be safely moved away
bool binary(binary& val);

// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)
bool start_object(std::size_t elements);
bool end_object();
bool start_array(std::size_t elements);
bool end_array();
// called when an object key is parsed; value is passed and can be safely moved away
bool key(string_t& val);

// called when a parse error occurs; byte position, the last token, and an exception is passed
bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);

The return value of each function determines whether parsing should proceed.

To implement your own SAX handler, proceed as follows:

1. Implement the SAX interface in a class. You can use class nlohmann::json_sax<json> as base class, but you can also use any class where the functions described above are implemented and public.
2. Create an object of your SAX interface class, e.g. my_sax.
3. Call #!cpp bool json::sax_parse(input, &my_sax); where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.

Note the sax_parse function only returns a #!cpp bool indicating the result of the last executed SAX event. It does not return json value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your parse_error implementation. Internally, the SAX interface is used for the DOM parser (class json_sax_dom_parser) as well as the acceptor (json_sax_acceptor), see file json_sax.hpp.