Architecture

!!! info

This page is still under construction. Its goal is to provide a high-level overview of the library's architecture.
This should help new contributors to get an idea of the used concepts and where to make changes.

Overview

The main structure is class nlohmann::basic_json.

• public API
• container interface
• iterators

Template specializations

• describe template parameters of basic_json
• json
• ordered_json via ordered_map

Value storage

Values are stored as a tagged union of value_t and json_value.

/// the type of the current element
value_t m_type = value_t::null;

/// the value of the current element
json_value m_value = {};

with

enum class value_t : std::uint8_t
{
    null,             ///< null value
    object,           ///< object (unordered set of name/value pairs)
    array,            ///< array (ordered collection of values)
    string,           ///< string value
    boolean,          ///< boolean value
    number_integer,   ///< number value (signed integer)
    number_unsigned,  ///< number value (unsigned integer)
    number_float,     ///< number value (floating-point)
    binary,           ///< binary array (ordered collection of bytes)
    discarded         ///< discarded by the parser callback function
};

union json_value {
  /// object (stored with pointer to save storage)
  object_t *object;
  /// array (stored with pointer to save storage)
  array_t *array;
  /// string (stored with pointer to save storage)
  string_t *string;
  /// binary (stored with pointer to save storage)
  binary_t *binary;
  /// boolean
  boolean_t boolean;
  /// number (integer)
  number_integer_t number_integer;
  /// number (unsigned integer)
  number_unsigned_t number_unsigned;
  /// number (floating-point)
  number_float_t number_float;
};

Parsing inputs (deserialization)

Input is read via input adapters that abstract a source with a common interface:

/// read a single character
std::char_traits<char>::int_type get_character() noexcept;

/// read multiple characters to a destination buffer and
/// returns the number of characters successfully read
template<class T>
std::size_t get_elements(T* dest, std::size_t count = 1);

List examples of input adapters.

SAX Interface

TODO

Writing outputs (serialization)

Output is written via output adapters:

template<typename T>
void write_character(CharType c);

template<typename CharType>
void write_characters(const CharType* s, std::size_t length);

List examples of output adapters.

Value conversion

template<class T>
void to_json(basic_json& j, const T& t);

template<class T>
void from_json(const basic_json& j, T& t);

Additional features

• JSON Pointers
• Binary formats
• Custom base class
• Conversion macros

Details namespace

• C++ feature backports