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basic_json::parse

// (1)
template<typename InputType>
static basic_json parse(InputType&& i,
                        const parser_callback_t cb = nullptr,
                        const bool allow_exceptions = true,
                        const bool ignore_comments = false);

// (2)
template<typename IteratorType>
static basic_json parse(IteratorType first, IteratorType last,
                        const parser_callback_t cb = nullptr,
                        const bool allow_exceptions = true,
                        const bool ignore_comments = false);
  1. Deserialize from a compatible input.
  2. Deserialize from a pair of character iterators

    The value_type of the iterator must be a integral type with size of 1, 2 or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16 and UTF-32.

Template parameters

InputType

A compatible input, for instance:

  • 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.
IteratorType
a compatible iterator type

Parameters

i (in)
Input to parse from.
cb (in)
a parser callback function of type parser_callback_t which is used to control the deserialization by filtering unwanted values (optional)
allow_exceptions (in)
whether to throw exceptions in case of a parse error (optional, true by default)
ignore_comments (in)
whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)
first (in)
iterator to start of character range
last (in)
iterator to end of character range

Return value

Deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

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 or reading from (1) the input i or (2) the iterator range [first, last] has a super-linear complexity.

Notes

(1) A UTF-8 byte order mark is silently ignored.

Examples

Parsing from a charater array

The example below demonstrates the parse() function reading from an array.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a JSON text
    char text[] = R"(
    {
        "Image": {
            "Width":  800,
            "Height": 600,
            "Title":  "View from 15th Floor",
            "Thumbnail": {
                "Url":    "http://www.example.com/image/481989943",
                "Height": 125,
                "Width":  100
            },
            "Animated" : false,
            "IDs": [116, 943, 234, 38793]
        }
    }
    )";

    // parse and serialize JSON
    json j_complete = json::parse(text);
    std::cout << std::setw(4) << j_complete << "\n\n";
}

Output:

{
    "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
    }
}
Parsing from a string

The example below demonstrates the parse() function with and without callback function.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a JSON text
    auto text = R"(
    {
        "Image": {
            "Width":  800,
            "Height": 600,
            "Title":  "View from 15th Floor",
            "Thumbnail": {
                "Url":    "http://www.example.com/image/481989943",
                "Height": 125,
                "Width":  100
            },
            "Animated" : false,
            "IDs": [116, 943, 234, 38793]
        }
    }
    )";

    // parse and serialize JSON
    json j_complete = json::parse(text);
    std::cout << std::setw(4) << j_complete << "\n\n";


    // define parser callback
    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
    {
        // skip object elements with key "Thumbnail"
        if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
        {
            return false;
        }
        else
        {
            return true;
        }
    };

    // parse (with callback) and serialize JSON
    json j_filtered = json::parse(text, cb);
    std::cout << std::setw(4) << j_filtered << '\n';
}

Output:

{
    "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
    }
}
Parsing from an input stream

The example below demonstrates the parse() function with and without callback function.

#include <iostream>
#include <iomanip>
#include <sstream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a JSON text
    auto text = R"(
    {
        "Image": {
            "Width":  800,
            "Height": 600,
            "Title":  "View from 15th Floor",
            "Thumbnail": {
                "Url":    "http://www.example.com/image/481989943",
                "Height": 125,
                "Width":  100
            },
            "Animated" : false,
            "IDs": [116, 943, 234, 38793]
        }
    }
    )";

    // fill a stream with JSON text
    std::stringstream ss;
    ss << text;

    // parse and serialize JSON
    json j_complete = json::parse(ss);
    std::cout << std::setw(4) << j_complete << "\n\n";


    // define parser callback
    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
    {
        // skip object elements with key "Thumbnail"
        if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
        {
            return false;
        }
        else
        {
            return true;
        }
    };

    // fill a stream with JSON text
    ss.clear();
    ss << text;

    // parse (with callback) and serialize JSON
    json j_filtered = json::parse(ss, cb);
    std::cout << std::setw(4) << j_filtered << '\n';
}

Output:

{
    "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
    }
}
Parsing from a contiguous container

The example below demonstrates the parse() function reading from a contiguous container.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a JSON text given as std::vector
    std::vector<uint8_t> text = {'[', '1', ',', '2', ',', '3', ']', '\0'};

    // parse and serialize JSON
    json j_complete = json::parse(text);
    std::cout << std::setw(4) << j_complete << "\n\n";
}

Output:

[
    1,
    2,
    3
]
Effect of allow_exceptions parameter

The example below demonstrates the effect of the allow_exceptions parameter in the ┬┤parse()` function.

#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // an invalid JSON text
    std::string text = R"(
    {
        "key": "value without closing quotes
    }
    )";

    // parse with exceptions
    try
    {
        json j = json::parse(text);
    }
    catch (json::parse_error& e)
    {
        std::cout << e.what() << std::endl;
    }

    // parse without exceptions
    json j = json::parse(text, nullptr, false);

    if (j.is_discarded())
    {
        std::cout << "the input is invalid JSON" << std::endl;
    }
    else
    {
        std::cout << "the input is valid JSON: " << j << std::endl;
    }
}

Output:

[json.exception.parse_error.101] parse error at line 4, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \u000A or \n; last read: '"value without closing quotes<U+000A>'
the input is invalid JSON

Version history

  • Added in version 1.0.0.
  • Overload for contiguous containers (1) added in version 2.0.3.
  • Ignoring comments via ignore_comments added in version 3.9.0.