basic_json_reader.md

jsoncons::basic_json_reader

#include <jsoncons/json_reader.hpp>

template<
    typename CharT,
    typename Source=jsoncons::stream_source<CharT>,
    typename TempAllocator=std::allocator<char>
>
class basic_json_reader 

basic_json_reader uses the incremental parser basic_json_parser to read arbitrarily large files in chunks. A basic_json_reader can read a sequence of JSON texts from a stream, using read_next(), which omits the check for unconsumed non-whitespace characters.

basic_json_reader is noncopyable and nonmoveable.

A number of specializations for common character types are defined:

Type	Definition
json_string_reader	basic_json_reader<char,string_source<char>> (since 0.164.0)
wjson_string_reader	basic_json_reader<wchar_t,string_source<wchar_t>> (since 0.164.0)
json_stream_reader	basic_json_reader<char,stream_source<char>> (since 0.164.0)
wjson_stream_reader	basic_json_reader<wchar_t,stream_source<wchar_t>> (since 0.164.0)
json_reader	Constructible from either a string or stream (deprecated since 0.164.0)
wjson_reader	Constructible from either a wide character string or stream (deprecated since 0.164.0)

Member types

Type	Definition
char_type	CharT
source_type	Source
string_view_type

Constructors

template <typename Sourceable>
explicit basic_json_reader(Sourceable&& source,                      (1)
    const TempAllocator& alloc = TempAllocator());         

template <typename Sourceable>
basic_json_reader(Sourceable&& source, 
    const basic_json_options<CharT>& options, 
    const TempAllocator& alloc = TempAllocator());                   (2)

template <typename Sourceable>
basic_json_reader(Sourceable&& source,
    std::function<bool(json_errc,const ser_context&)> err_handler,   (3)   (deprecated in 0.171.0) 
    const TempAllocator& alloc = TempAllocator());                    

template <typename Sourceable>
basic_json_reader(Sourceable&& source, 
    const basic_json_options<CharT>& options,                        (4)   (deprecated in 0.171.0) 
    std::function<bool(json_errc,const ser_context&)> err_handler, 
    const TempAllocator& alloc = TempAllocator()); 

template <typename Sourceable>
basic_json_reader(Sourceable&& source,                               (5)
    basic_json_visitor<CharT>& visitor, 
    const TempAllocator& alloc = TempAllocator()); 

template <typename Sourceable>
basic_json_reader(Sourceable&& source, 
    basic_json_visitor<CharT>& visitor,                              (6)
    const basic_json_options<CharT>& options, 
    const TempAllocator& alloc = TempAllocator()); 

template <typename Sourceable>
basic_json_reader(Sourceable&& source,
    basic_json_visitor<CharT>& visitor,                              (7)   (deprecated in 0.171.0) 
    std::function<bool(json_errc,const ser_context&)> err_handler, 
    const TempAllocator& alloc = TempAllocator()); 

template <typename Sourceable>
basic_json_reader(Sourceable&& source,
    basic_json_visitor<CharT>& visitor,                              (8)   (deprecated in 0.171.0)
    const basic_json_options<CharT>& options,
    std::function<bool(json_errc,const ser_context&)> err_handler, 
    const TempAllocator& alloc = TempAllocator()); 

Constructors (1)-(4) use a default basic_json_visitor that discards the JSON parse events, and are for validation only.

(1) Constructs a basic_json_reader that reads from a character sequence or stream source, uses default options and a default JSON parsing error handling.

(2) Constructs a basic_json_reader that reads from a character sequence or stream source, uses the specified options and a default JSON parsing error handling.

(3) Constructs a basic_json_reader that reads from a character sequence or stream source, uses default options and a specified JSON parsing error handling.

(4) Constructs a basic_json_reader that reads from a character sequence or stream source, uses the specified options and a specified JSON parsing error handling.

Constructors (5)-(8) take a user supplied basic_json_visitor that receives JSON parse events, such as a json_decoder.

(5) Constructs a basic_json_reader that reads from a character sequence or stream source, emits JSON parse events to the specified basic_json_visitor, and uses default options and a default JSON parsing error handling.

(6) Constructs a basic_json_reader that reads from a character sequence or stream source, emits JSON parse events to the specified basic_json_visitor and uses the specified options and a default JSON parsing error handling.

(7) Constructs a basic_json_reader that reads from a character sequence or stream source, emits JSON parse events to the specified basic_json_visitor and uses default options and a specified JSON parsing error handling.

(8) Constructs a basic_json_reader that reads from a character sequence or stream source, emits JSON parse events to the specified basic_json_visitor and uses the specified options and a specified JSON parsing error handling.

Note: It is the programmer's responsibility to ensure that basic_json_reader does not outlive any source or visitor passed in the constuctor, as basic_json_reader holds pointers to but does not own these resources.

Parameters

source - a value from which a source_type is constructible.

Member functions

bool eof() const

Returns true when there are no more JSON texts to be read from the stream, false otherwise

void read(); (1)
void read(std::error_code& ec); (2)

Reads the next JSON text from the stream and reports JSON events to a basic_json_visitor, such as a json_decoder. Override (1) throws if parsing fails, or there are any unconsumed non-whitespace characters left in the input. Override (2) sets ec to a json_errc if parsing fails or if there are any unconsumed non-whitespace characters left in the input.

void read_next()
void read_next(std::error_code& ec)

Reads the next JSON text from the stream and reports JSON events to a basic_json_visitor, such as a json_decoder. Override (1) throws ser_error if parsing fails. Override (2) sets ec to a json_errc if parsing fails.

void check_done(); (1)
void check_done(std::error_code& ec); (2)

Override (1) throws if there are any unconsumed non-whitespace characters in the input. Override (2) sets ec to a json_errc if there are any unconsumed non-whitespace characters left in the input.

std::size_t line() const

std::size_t column() const

Examples

Parsing JSON text with exceptions

std::string input = R"({"field1"{}})";    

json_decoder<json> decoder;
json_string_reader reader(input,decoder);

try
{
    reader.read();
    json j = decoder.get_result();
}
catch (const ser_error& e)
{
    std::cout << e.what() << '\n';
}

Output:

Expected name separator ':' at line 1 and column 10

Parsing JSON text with error codes

std::string input = R"({"field1":ru})";    
std::istringstream is(input);

json_decoder<json> decoder;
json_stream_reader reader(is,decoder);

std::error_code ec;
reader.read(ec);

if (!ec)
{
    json j = decoder.get_result();   
}
else
{
    std::cerr << ec.message() 
              << " at line " << reader.line() 
              << " and column " << reader.column() << '\n';
}

Output:

Expected value at line 1 and column 11

Reading a sequence of JSON texts from a stream

jsoncons supports reading a sequence of JSON texts, such as shown below (json-texts.json):

{"a":1,"b":2,"c":3}
{"a":4,"b":5,"c":6}
{"a":7,"b":8,"c":9}

This is the code that reads them:

std::ifstream is("json-texts.json");
if (!is.is_open())
{
    throw std::runtime_error("Cannot open file");
}

json_decoder<json> decoder;
json_stream_reader reader(is,decoder);

while (!reader.eof())
{
    reader.read_next();

    // until 1.0.0
    //if (!reader.eof())
    //{
    //    json j = decoder.get_result();
    //    std::cout << j << '\n';
    //}
    // since 1.0.0
    json j = decoder.get_result();
    std::cout << j << '\n';
}

Output:

{"a":1,"b":2,"c":3}
{"a":4,"b":5,"c":6}
{"a":7,"b":8,"c":9}