json_decoder.md

jsoncons::json_decoder

#include <jsoncons/json_decoder.hpp>

template <typename Json,class TempAllocator>
json_decoder

Member types

Member type	Definition
result_allocator_type	Json::allocator_type (until 0.171.0)
allocator_type	Json::allocator_type (since 0.171.0)
temp_allocator_type	TempAllocator

Constructors

json_decoder(result_allocator_arg_t, 
    const result_allocator_type& alloc);                            (1)   (until 0.171.0)

json_decoder(result_allocator_arg_t, 
    const result_allocator_type& alloc, 
    const temp_allocator_type& temp_alloc);                         (1)   (until 0.171.0)

json_decoder(const allocator_type& alloc = allocator_type(),         
    const temp_allocator_type& temp_alloc = temp_allocator_type()); (1)   (since 0.171.0)

json_decoder(const temp_allocator_type& temp_alloc = 
    temp_allocator_type());                                         (2)   (until 0.171.0)

json_decoder(temp_allocator_arg_t, 
    const temp_allocator_type& temp_alloc);                         (2)   (since 0.171.0)

Member functions

allocator_type get_allocator() const

Returns the allocator associated with the json value.

bool is_valid() const

Checks if the decoder contains a valid basic_json value. Initially is_valid() is false, becomes true when a basic_json value has been received, and becomes false when get_result() is called.

Json get_result()

Returns the json value v stored in the decoder as std::move(v). If before calling this function is_valid() is false, an assertion_error is thrown. After get_result() is called, 'is_valid()' becomes false. Once the result has been retrieved, get_result cannot be called again until another basic_json value has been received.

Examples

Decode a JSON text using stateful result and work allocators

Input JSON file book_catalog.json:

[ 
  { 
      "author" : "Haruki Murakami",
      "title" : "Hard-Boiled Wonderland and the End of the World",
      "isbn" : "0679743464",
      "publisher" : "Vintage",
      "date" : "1993-03-02",
      "price": 18.90
  },
  { 
      "author" : "Graham Greene",
      "title" : "The Comedians",
      "isbn" : "0099478374",
      "publisher" : "Vintage Classics",
      "date" : "2005-09-21",
      "price": 15.74
  }
]

int main()
{
    // Given allocator my_alloc with a single-argument constructor my_alloc(int),
    // use my_alloc(1) to allocate basic_json memory, my_alloc(2) to allocate
    // working memory used by json_decoder, and my_alloc(3) to allocate
    // working memory used by basic_json_reader. 

    using my_json = basic_json<char,sorted_policy,my_alloc>;

    std::ifstream is("book_catalog.json");
    json_decoder<my_json,my_alloc> decoder(my_alloc(1),my_alloc(2));
    basic_json_reader<char,stream_source<char>,my_alloc> reader(is, decoder, my_alloc(3));
    reader.read();

    my_json j = decoder.get_result();
    std::cout << pretty_print(j) << "\n";
}

Output:

[
    {
        "author": "Haruki Murakami",
        "date": "1993-03-02",
        "isbn": "0679743464",
        "price": 18.9,
        "publisher": "Vintage",
        "title": "Hard-Boiled Wonderland and the End of the World"
    },
    {
        "author": "Graham Greene",
        "date": "2005-09-21",
        "isbn": "0099478374",
        "price": 15.74,
        "publisher": "Vintage Classics",
        "title": "The Comedians"
    }
]

Decode a CSV text using stateful result and work allocators

Input CSV file bond_yields.csv:

Date,1Y,2Y,3Y,5Y
2017-01-09,0.0062,0.0075,0.0083,0.011
2017-01-08,0.0063,0.0076,0.0084,0.0112
2017-01-08,0.0063,0.0076,0.0084,0.0112

int main()
{
    // Given allocator my_alloc with a single-argument constructor my_alloc(int),
    // use my_alloc(1) to allocate basic_json memory, my_alloc(2) to allocate
    // working memory used by json_decoder, and my_alloc(3) to allocate
    // working memory used by basic_csv_reader. 

    using my_json = basic_json<char,sorted_policy,my_alloc>;

    auto options = csv::csv_options{}
        .assume_header(true);

    std::ifstream is("bond_yields.csv");
    json_decoder<my_json,my_alloc> decoder(my_alloc(1),my_alloc(2));
    csv::basic_csv_reader<char,stream_source<char>,free_list_allocator<char>> reader(is, decoder, options, my_alloc(3));
    reader.read();

    my_json j = decoder.get_result();
    std::cout << pretty_print(j) << "\n";
}

Output:

[
    {
        "1Y": 0.0062,
        "2Y": 0.0075,
        "3Y": 0.0083,
        "5Y": 0.011,
        "Date": "2017-01-09"
    },
    {
        "1Y": 0.0063,
        "2Y": 0.0076,
        "3Y": 0.0084,
        "5Y": 0.0112,
        "Date": "2017-01-08"
    },
    {
        "1Y": 0.0063,
        "2Y": 0.0076,
        "3Y": 0.0084,
        "5Y": 0.0112,
        "Date": "2017-01-08"
    }
]

See also

basic_json_visitor