JSON for Modern C++  3.9.0

◆ sax_parse() [2/3]

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

The SAX event lister must follow the interface of json_sax.

This function reads from a compatible input. Examples are:

  • 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.
Parameters
[in]iinput to read from
[in,out]saxSAX event listener
[in]formatthe format to parse (JSON, CBOR, MessagePack, or UBJSON)
[in]strictwhether the input has to be consumed completely
[in]ignore_commentswhether comments should be ignored and treated like whitespace (true) or yield a parse error (true); (optional, false by default); only applies to the JSON file format.
Returns
return value of the last processed SAX event
Exceptions
parse_error.101if a parse error occurs; example: ""unexpected end of input; expected string literal""
parse_error.102if to_unicode fails or surrogate error
parse_error.103if to_unicode fails
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.
Note
A UTF-8 byte order mark is silently ignored.
Example
The example below demonstrates the sax_parse() function reading from string and processing the events with a user-defined SAX event consumer.
1 #include <iostream>
2 #include <iomanip>
3 #include <sstream>
4 #include <nlohmann/json.hpp>
5 
6 using json = nlohmann::json;
7 
8 // a simple event consumer that collects string representations of the passed
9 // values; not inheriting from json::json_sax_t is not required, but can
10 // help not to forget a required function
11 class sax_event_consumer : public json::json_sax_t
12 {
13  public:
14  std::vector<std::string> events;
15 
16  bool null() override
17  {
18  events.push_back("value: null");
19  return true;
20  }
21 
22  bool boolean(bool val) override
23  {
24  events.push_back("value: " + std::string(val ? "true" : "false"));
25  return true;
26  }
27 
28  bool number_integer(number_integer_t val) override
29  {
30  events.push_back("value: " + std::to_string(val));
31  return true;
32  }
33 
34  bool number_unsigned(number_unsigned_t val) override
35  {
36  events.push_back("value: " + std::to_string(val));
37  return true;
38  }
39 
40  bool number_float(number_float_t val, const string_t& s) override
41  {
42  events.push_back("value: " + s);
43  return true;
44  }
45 
46  bool string(string_t& val) override
47  {
48  events.push_back("value: " + val);
49  return true;
50  }
51 
52  bool start_object(std::size_t elements) override
53  {
54  events.push_back("start: object");
55  return true;
56  }
57 
58  bool end_object() override
59  {
60  events.push_back("end: object");
61  return true;
62  }
63 
64  bool start_array(std::size_t elements) override
65  {
66  events.push_back("start: array");
67  return true;
68  }
69 
70  bool end_array() override
71  {
72  events.push_back("end: array");
73  return true;
74  }
75 
76  bool key(string_t& val) override
77  {
78  events.push_back("key: " + val);
79  return true;
80  }
81 
82  bool binary(json::binary_t& val) override
83  {
84  events.push_back("binary");
85  return true;
86  }
87 
88  bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override
89  {
90  events.push_back("error: " + std::string(ex.what()));
91  return false;
92  }
93 };
94 
95 int main()
96 {
97  // a JSON text
98  auto text = R"(
99  {
100  "Image": {
101  "Width": 800,
102  "Height": 600,
103  "Title": "View from 15th Floor",
104  "Thumbnail": {
105  "Url": "http://www.example.com/image/481989943",
106  "Height": 125,
107  "Width": 100
108  },
109  "Animated" : false,
110  "IDs": [116, 943, 234, 38793],
111  "Distance": 12.723374634
112  }
113  }
114  )";
115 
116  // create a SAX event consumer object
117  sax_event_consumer sec;
118 
119  // parse and serialize JSON
120  bool result = json::sax_parse(text, &sec);
121 
122  // output the recorded events
123  for (auto& event : sec.events)
124  {
125  std::cout << "(" << event << ") ";
126  }
127 
128  // output the result of sax_parse
129  std::cout << "\nresult: " << std::boolalpha << result << std::endl;
130 }

Output (play with this example online):
(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
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/sax_parse.cpp -o sax_parse 
Since
version 3.2.0

Definition at line 23201 of file json.hpp.

nlohmann::basic_json::number_unsigned_t
NumberUnsignedType number_unsigned_t
a type for a number (unsigned)
Definition: json.hpp:17159
nlohmann::basic_json::number_integer_t
NumberIntegerType number_integer_t
a type for a number (integer)
Definition: json.hpp:17088
nlohmann::basic_json::binary_t
nlohmann::byte_container_with_subtype< BinaryType > binary_t
a type for a packed binary type
Definition: json.hpp:17298
nlohmann::basic_json::json_sax_t
json_sax< basic_json > json_sax_t
SAX interface type, see nlohmann::json_sax.
Definition: json.hpp:16628
nlohmann::basic_json::string_t
StringType string_t
a type for a string
Definition: json.hpp:16990
nlohmann::basic_json::binary
static basic_json binary(const typename binary_t::container_type &init)
explicitly create a binary array (without subtype)
Definition: json.hpp:18067
nlohmann::basic_json::number_float_t
NumberFloatType number_float_t
a type for a number (floating-point)
Definition: json.hpp:17227
nlohmann::basic_json::parse_error
detail::parse_error parse_error
exception indicating a parse error
Definition: json.hpp:16641
nlohmann::basic_json::exception
detail::exception exception
general exception of the basic_json class
Definition: json.hpp:16639
nlohmann::basic_json::sax_parse
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)
generate SAX events
Definition: json.hpp:23201
nlohmann::json
basic_json<> json
default JSON class
Definition: json.hpp:2933