JSON for Modern C++  2.0.6

§ parse() [3/6]

template<template< typename U, typename V, typename... Args > class ObjectType = std::map, template< typename U, typename... Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = std::int64_t, class NumberUnsignedType = std::uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator>
static basic_json nlohmann::basic_json::parse ( std::istream &  i,
const parser_callback_t  cb = nullptr 
)
inlinestatic
Parameters
[in,out]istream to read a serialized JSON value from
[in]cba parser callback function of type parser_callback_t which is used to control the deserialization by filtering unwanted values (optional)
Returns
result of the deserialization
Complexity
Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the parser callback function cb has a super-linear complexity.
Note
A UTF-8 byte order mark is silently ignored.
Example
The example below demonstrates the parse() function with and without callback function.
1 #include <json.hpp>
2 
3 using json = nlohmann::json;
4 
5 int main()
6 {
7  // a JSON text
8  auto text = R"(
9  {
10  "Image": {
11  "Width": 800,
12  "Height": 600,
13  "Title": "View from 15th Floor",
14  "Thumbnail": {
15  "Url": "http://www.example.com/image/481989943",
16  "Height": 125,
17  "Width": 100
18  },
19  "Animated" : false,
20  "IDs": [116, 943, 234, 38793]
21  }
22  }
23  )";
24 
25  // fill a stream with JSON text
26  std::stringstream ss;
27  ss << text;
28 
29  // parse and serialize JSON
30  json j_complete = json::parse(ss);
31  std::cout << std::setw(4) << j_complete << "\n\n";
32 
33 
34  // define parser callback
35  json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
36  {
37  // skip object elements with key "Thumbnail"
38  if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
39  {
40  return false;
41  }
42  else
43  {
44  return true;
45  }
46  };
47 
48  // fill a stream with JSON text
49  ss.clear();
50  ss << text;
51 
52  // parse (with callback) and serialize JSON
53  json j_filtered = json::parse(ss, cb);
54  std::cout << std::setw(4) << j_filtered << '\n';
55 }
basic_json<> json
default JSON class
Definition: json.hpp:10155
static basic_json parse(T(&array)[N], const parser_callback_t cb=nullptr)
deserialize from an array
Definition: json.hpp:5897
std::function< bool(int depth, parse_event_t event, basic_json &parsed)> parser_callback_t
per-element parser callback type
Definition: json.hpp:1016
the parser read a key of a value in an object
parse_event_t
JSON callback events.
Definition: json.hpp:946
Output (play with this example online):
{
    "Image": {
        "Animated": false,
        "Height": 600,
        "IDs": [
            116,
            943,
            234,
            38793
        ],
        "Thumbnail": {
            "Height": 125,
            "Url": "http://www.example.com/image/481989943",
            "Width": 100
        },
        "Title": "View from 15th Floor",
        "Width": 800
    }
}

{
    "Image": {
        "Animated": false,
        "Height": 600,
        "IDs": [
            116,
            943,
            234,
            38793
        ],
        "Title": "View from 15th Floor",
        "Width": 800
    }
}
The example code above can be translated with
g++ -std=c++11 -Isrc doc/examples/parse__istream__parser_callback_t.cpp -o parse__istream__parser_callback_t 
See also
parse(const char*, const parser_callback_t) for a version that reads from a string
Since
version 1.0.0

Definition at line 5965 of file json.hpp.