json/docs/mkdocs/docs/home/architecture.md

Architecture

This page gives a high-level overview of the library's architecture. It should help new contributors to get an idea of the used concepts and where to make changes.

Overview

The library is built around a single class template, nlohmann::basic_json. A basic_json value is a node in a tree of JSON values. All other components either create such a tree from an input (parsing), write a tree to an output (serialization), or give access to it (iterators, JSON Pointer, conversions).

flowchart LR
    input[/"input<br>(string, stream,<br>iterator range, file)"/]
    ia["input adapter"]
    lexer["lexer"]
    parser["parser"]
    breader["binary_reader"]
    sax["SAX interface"]
    value[("basic_json<br>value tree")]
    serializer["serializer"]
    bwriter["binary_writer"]
    oa["output adapter"]
    output[/"output<br>(string, stream,<br>vector)"/]

    input --> ia
    ia --> lexer --> parser --> sax
    ia --> breader --> sax
    sax --> value
    value --> serializer --> oa
    value --> bwriter --> oa
    oa --> output

• JSON text is read by an input adapter, tokenized by the lexer, and turned into SAX events by the parser.
• Binary formats (BJData, BSON, CBOR, MessagePack, UBJSON) are read by an input adapter and turned into the same SAX events by the binary_reader.
• A SAX consumer receives the events. The one used by parse builds a basic_json value tree.
• The serializer (JSON text) or the binary_writer (binary formats) writes a value tree to an output adapter.

Source layout

The public headers are in include/nlohmann:

• json.hpp defines class basic_json.
• json_fwd.hpp contains forward declarations.
• adl_serializer.hpp, byte_container_with_subtype.hpp, and ordered_map.hpp define adl_serializer, byte_container_with_subtype, and ordered_map.
• json_view.hpp is a separate, optional header that defines basic_json_document and basic_json_view, a flat-index, read-only, non-owning way to look at a parsed JSON text; see Zero-copy JSON views. It builds on json.hpp internals (it requires the same library version) and has its own detail/view/ subdirectory.

Everything else lives in detail/ and namespace nlohmann::detail, which is not part of the public API. Paths below are relative to include/nlohmann.

Component	Location
Value type enumeration	detail/value_t.hpp
Input adapters	detail/input/input_adapters.hpp
Lexer	detail/input/lexer.hpp, detail/input/number_parse.hpp, detail/input/string_scan.hpp
Parser	detail/input/parser.hpp
SAX interface and DOM builders	detail/input/json_sax.hpp
Binary format readers	detail/input/binary_reader.hpp
JSON serializer	detail/output/serializer.hpp, detail/conversions/to_chars.hpp
Binary format writers	detail/output/binary_writer.hpp
Output adapters	detail/output/output_adapters.hpp
Iterators	detail/iterators/
Conversions from/to arbitrary types	detail/conversions/from_json.hpp, detail/conversions/to_json.hpp
JSON Pointer	detail/json_pointer.hpp
Exceptions	detail/exceptions.hpp
Type traits and C++ feature backports	detail/meta/
Macros	detail/macro_scope.hpp, detail/macro_unscope.hpp, detail/abi_macros.hpp

The single-header version single_include/nlohmann/json.hpp is generated from these files with make amalgamate and must not be edited by hand. The same command also generates single_include/nlohmann/json_view.hpp from json_view.hpp and detail/view/.

Template parameters

basic_json is parameterized by the types it uses to store values and to convert from and to other types:

Template parameter	Default	Used for
ObjectType	std::map	objects, see object_t
ArrayType	std::vector	arrays, see array_t
StringType	std::string	strings and object keys, see string_t
BooleanType	bool	Booleans, see boolean_t
NumberIntegerType	std::int64_t	signed integers, see number_integer_t
NumberUnsignedType	std::uint64_t	unsigned integers, see number_unsigned_t
NumberFloatType	double	floating-point numbers, see number_float_t
AllocatorType	std::allocator	allocating objects, arrays, strings, and binary values
JSONSerializer	adl_serializer	conversions from/to other types, see adl_serializer
BinaryType	std::vector<std::uint8_t>	binary values, see binary_t
CustomBaseClass	void	an optional base class, see json_base_class_t

The library provides two specializations:

• json uses all default template arguments.
• ordered_json uses ordered_map as ObjectType to keep the insertion order of object keys.

The requirements on the template arguments are listed in Template Parameter Requirements.

Value storage

Each basic_json value stores its content as a tagged union: an enumeration value_t names the type of the value, and a union json_value holds the value itself. Both are members of the nested struct data, which is the only data member m_data of basic_json:

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

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

data m_data = {};

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;
};

Objects, arrays, strings, and binary values are allocated on the heap with AllocatorType, and the union only stores a pointer to them. This keeps a basic_json value small: one pointer-sized union and one byte for the type. The class maintains the invariant that the pointer matching m_type is never null; assert_invariant() checks it with runtime assertions.

Node index of JSON views

A basic_json_document (see Zero-copy JSON views) does not build a tree of values. Its parser (detail/view/builder.hpp) writes a flat array of 16-byte nodes, one per value and one per object key, in document order. A basic_json_view is a pointer to the document and a pointer to one node. The layout is struct node in detail/view/node.hpp (the numbers are bit offsets, 32 bits per row):

packet-beta
  0-7: "kind"
  8-15: "flags"
  16-31: "extra"
  32-63: "off"
  64-95: "len"
  96-127: "next"

Bytes	Field	Type	Contents
0	kind	uint8_t	the type, numbered as value_t: 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float
1	flags	uint8_t	bits 0-1: where a string's bytes are (0: the source text, 1: the buffer of decoded strings, for strings with escapes); bit 2: the value of a boolean
2-3	extra	uint16_t	numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their hash index (1-based), or 0; otherwise 0
4-7	off	uint32_t	where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object
8-11	len	uint32_t	strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements
12-15	next	uint32_t	arrays and objects: the number of nodes of the subtree, including the node itself

• Integers keep their converted value in bytes 8-15 instead of len and next; the length of their token follows from the number of digits in extra (and the sign). Non-negative integers are unsigned integers, as with parse.
• Floats keep only their token. The digit layout in extra lets the conversion read the digits without scanning the token again, and only when the value is read.
• Object members are the node of the key (a string) followed by the nodes of the value.
• Navigation needs no pointers: the elements of an array or object follow its node, and the node after a value's subtree is next nodes further for an array or object, and the next node otherwise (document_data::after). Views step from element to element this way and skip whole subtrees in constant time.
• Offsets are 32 bits wide, so a document is limited to 4 GiB (out_of_range.416).
• Large objects (128 members or more) get a hash index after parsing (detail/view/object_index.hpp): an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does not compare every key. The object's extra holds the number of its table. Only 65,535 tables fit into extra; objects beyond them are searched linearly.

For example, #!json {"a": [1, 2.5]} becomes five nodes. Each node's elements follow it, and next leads from an array or object past its subtree:

flowchart LR
    n0["0: object<br>len 1, next 5"]
    n1["1: key a"]
    n2["2: array<br>len 2, next 3"]
    n3["3: unsigned integer 1"]
    n4["4: float 2.5"]
    e(["end"])
    n0 --> n1 --> n2 --> n3 --> n4 --> e
    n0 -. next .-> e
    n2 -. next .-> e

Node	kind	extra	off	len	next
0	1 (object)	0	0	1	5
1	3 (string)	0	2	1	0
2	2 (array)	0	6	2	3
3	6 (unsigned integer)	0x0001	7	-	-
4	7 (float)	0x0101	10	3	0

All flags are 0. The integer's bytes 8-15 hold its value, 1; its extra says it has one digit. The float's extra says it has one integer and one fraction digit, and its len is that of the token 2.5.

Input adapters

Input is read via input adapters that abstract a source. Every input adapter provides this interface:

/// the type of the characters in the input
using char_type = ...;

/// read a single character; returns std::char_traits<char_type>::eof() at the end of the input
typename std::char_traits<char_type>::int_type get_character();

/// read up to count * sizeof(T) bytes into dest and return the number of bytes read
/// (used by the binary readers)
template<class T>
std::size_t get_elements(T* dest, std::size_t count = 1);

The lexer detects two optional extensions at compile time. Only iterator_input_adapter provides them, and only for random-access input of single-byte characters:

• supports_seek, get_consumed_count(), and copy_consumed_range() let the lexer reconstruct already consumed input for error messages instead of copying every character it reads.
• supports_bulk_scan, bulk_data(), bulk_remaining(), and bulk_skip() let the lexer scan strings directly in contiguous memory, several bytes at a time.

The function input_adapter picks the right adapter for the argument passed to parse, accept, sax_parse, or the from_* functions:

• iterator_input_adapter reads from an iterator range, which also covers strings, containers, and pointers.
• wide_string_input_adapter reads from ranges of wchar_t, char16_t, or char32_t and converts them to UTF-8. It cannot be used for binary formats; its get_elements() throws.
• input_stream_adapter reads from a std::istream.
• file_input_adapter reads from a std::FILE*.

SAX interface

The parser does not build values itself. It reports what it reads as events to a SAX consumer, which implements the interface json_sax: null, boolean, number_integer, number_unsigned, number_float, string, binary, start_object, key, end_object, start_array, end_array, and parse_error.

The library comes with two consumers in detail/input/json_sax.hpp:

• json_sax_dom_parser builds a basic_json value tree. parse uses it.
• json_sax_dom_callback_parser does the same, but calls a parser callback for each event, which can skip values. parse uses it when a callback is given.

The binary_reader emits the same events for binary formats, so sax_parse works with a user-defined consumer for JSON and for all binary formats alike.

Output adapters

Output is written via output adapters:

void write_character(CharType c);

void write_characters(const CharType* s, std::size_t length);

The serializer (used by dump and operator<<) and the binary_writer (used by the to_* functions) write to one of these adapters:

• output_vector_adapter appends to a std::vector.
• output_stream_adapter writes to a std::ostream.
• output_string_adapter appends to a string.

Value conversion

Values are converted from and to other types with the JSONSerializer template parameter. The default, adl_serializer, calls the free functions

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

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

found by argument-dependent lookup. The library defines them for standard types in detail/conversions; users add them for their own types, see Arbitrary Type Conversions. The serialization macros generate these functions.

Additional features

• JSON Pointer (class json_pointer) addresses values inside a tree. It is also the basis of JSON Patch.
• Binary formats are read by binary_reader and written by binary_writer.
• A custom base class can add members to every basic_json value.
• Serialization macros generate to_json and from_json functions for user-defined types.

Details namespace

Namespace nlohmann::detail contains all implementation details. It is not part of the public API and may change in any release. Besides the components above, it contains:

• type traits to detect the capabilities of user-defined types (detail/meta/type_traits.hpp),
• backports of C++14/17 features to C++11 (detail/meta/cpp_future.hpp), and
• helpers such as string_concat and string_escape.