1# SAX Interface
2
3The library uses a SAX-like interface with the following functions:
4
5```plantuml
6interface json::sax_t {
7    + {abstract} bool null()
8
9    + {abstract} bool boolean(bool val)
10
11    + {abstract} bool number_integer(number_integer_t val)
12    + {abstract} bool number_unsigned(number_unsigned_t val)
13
14    + {abstract} bool number_float(number_float_t val, const string_t& s)
15
16    + {abstract} bool string(string_t& val)
17    + {abstract} bool binary(binary_t& val)
18
19    + {abstract} bool start_object(std::size_t elements)
20    + {abstract} bool end_object()
21    + {abstract} bool start_array(std::size_t elements)
22    + {abstract} bool end_array()
23    + {abstract} bool key(string_t& val)
24
25    + {abstract} bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex)
26}
27```
28
29```cpp
30// called when null is parsed
31bool null();
32
33// called when a boolean is parsed; value is passed
34bool boolean(bool val);
35
36// called when a signed or unsigned integer number is parsed; value is passed
37bool number_integer(number_integer_t val);
38bool number_unsigned(number_unsigned_t val);
39
40// called when a floating-point number is parsed; value and original string is passed
41bool number_float(number_float_t val, const string_t& s);
42
43// called when a string is parsed; value is passed and can be safely moved away
44bool string(string_t& val);
45// called when a binary value is parsed; value is passed and can be safely moved away
46bool binary(binary& val);
47
48// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)
49bool start_object(std::size_t elements);
50bool end_object();
51bool start_array(std::size_t elements);
52bool end_array();
53// called when an object key is parsed; value is passed and can be safely moved away
54bool key(string_t& val);
55
56// called when a parse error occurs; byte position, the last token, and an exception is passed
57bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);
58```
59
60The return value of each function determines whether parsing should proceed.
61
62To implement your own SAX handler, proceed as follows:
63
641. 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.
652. Create an object of your SAX interface class, e.g. `my_sax`.
663. 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.
67
68Note 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`.
69