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Defined in header <nlohmann/json.hpp>

    template<typename U, typename V, typename... Args> class ObjectType = std::map,
    template<typename U, typename... Args> class ArrayType = std::vector,
    class StringType = std::string,
    class BooleanType = bool,
    class NumberIntegerType = std::int64_t,
    class NumberUnsignedType = std::uint64_t,
    class NumberFloatType = double,
    template<typename U> class AllocatorType = std::allocator,
    template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,
    class BinaryType = std::vector<std::uint8_t>,
    class CustomBaseClass = void
class basic_json;

Template parameters

Template parameter Description Derived type
ObjectType type for JSON objects object_t
ArrayType type for JSON arrays array_t
StringType type for JSON strings and object keys string_t
BooleanType type for JSON booleans boolean_t
NumberIntegerType type for JSON integer numbers number_integer_t
NumberUnsignedType type for JSON unsigned integer numbers number_unsigned_t
NumberFloatType type for JSON floating-point numbers number_float_t
AllocatorType type of the allocator to use
JSONSerializer the serializer to resolve internal calls to to_json() and from_json() json_serializer
BinaryType type for binary arrays binary_t
CustomBaseClass extension point for user code json_base_class_t


  • json - default specialization
  • ordered_json - specialization that maintains the insertion order of object keys

Iterator invalidation



The class satisfies the following concept requirements:



  • StandardLayoutType: JSON values have standard layout: All non-static data members are private and standard layout types, the class has no virtual functions or (virtual) base classes.


  • EqualityComparable: JSON values can be compared with ==, see operator==.
  • LessThanComparable: JSON values can be compared with <, see operator<.
  • Swappable: Any JSON lvalue or rvalue of can be swapped with any lvalue or rvalue of other compatible types, using unqualified function swap.
  • NullablePointer: JSON values can be compared against std::nullptr_t objects which are used to model the null value.


  • Container: JSON values can be used like STL containers and provide iterator access.
  • ReversibleContainer: JSON values can be used like STL containers and provide reverse iterator access.

Member types


  • exception - general exception of the basic_json class
    • parse_error - exception indicating a parse error
    • invalid_iterator - exception indicating errors with iterators
    • type_error - exception indicating executing a member function with a wrong type
    • out_of_range - exception indicating access out of the defined range
    • other_error - exception indicating other library errors

Container types

Type Definition
value_type basic_json
reference value_type&
const_reference const value_type&
difference_type std::ptrdiff_t
size_type std::size_t
allocator_type AllocatorType<basic_json>
pointer std::allocator_traits<allocator_type>::pointer
const_pointer std::allocator_traits<allocator_type>::const_pointer
iterator LegacyBidirectionalIterator
const_iterator constant LegacyBidirectionalIterator
reverse_iterator reverse iterator, derived from iterator
const_reverse_iterator reverse iterator, derived from const_iterator
iteration_proxy helper type for items function

JSON value data types

Parser callback

Member functions

Object inspection

Functions to inspect the type of a JSON value.

Value access

Direct access to the stored value of a JSON value.

Element access

Access to the JSON value

  • at - access specified element with bounds checking
  • operator[] - access specified element
  • value - access specified object element with default value
  • front - access the first element
  • back - access the last element


  • find - find an element in a JSON object
  • count - returns the number of occurrences of a key in a JSON object
  • contains - check the existence of an element in a JSON object


  • begin - returns an iterator to the first element
  • cbegin - returns a const iterator to the first element
  • end - returns an iterator to one past the last element
  • cend - returns a const iterator to one past the last element
  • rbegin - returns an iterator to the reverse-beginning
  • rend - returns an iterator to the reverse-end
  • crbegin - returns a const iterator to the reverse-beginning
  • crend - returns a const iterator to the reverse-end
  • items - wrapper to access iterator member functions in range-based for


  • empty - checks whether the container is empty
  • size - returns the number of elements
  • max_size - returns the maximum possible number of elements


  • clear - clears the contents
  • push_back - add a value to an array/object
  • operator+= - add a value to an array/object
  • emplace_back - add a value to an array
  • emplace - add a value to an object if key does not exist
  • erase - remove elements
  • insert - inserts elements
  • update - updates a JSON object from another object, overwriting existing keys
  • swap - exchanges the values

Lexicographical comparison operators

Serialization / Dumping

  • dump - serialization

Deserialization / Parsing

  • parse (static) - deserialize from a compatible input
  • accept (static) - check if the input is valid JSON
  • sax_parse (static) - generate SAX events

JSON Pointer functions

  • flatten - return flattened JSON value
  • unflatten - unflatten a previously flattened JSON value

JSON Patch functions

  • patch - applies a JSON patch
  • patch_inplace - applies a JSON patch in place
  • diff (static) - creates a diff as a JSON patch

JSON Merge Patch functions

Static functions

  • meta - returns version information on the library
  • get_allocator - returns the allocator associated with the container

Binary formats

  • from_bjdata (static) - create a JSON value from an input in BJData format
  • from_bson (static) - create a JSON value from an input in BSON format
  • from_cbor (static) - create a JSON value from an input in CBOR format
  • from_msgpack (static) - create a JSON value from an input in MessagePack format
  • from_ubjson (static) - create a JSON value from an input in UBJSON format
  • to_bjdata (static) - create a BJData serialization of a given JSON value
  • to_bson (static) - create a BSON serialization of a given JSON value
  • to_cbor (static) - create a CBOR serialization of a given JSON value
  • to_msgpack (static) - create a MessagePack serialization of a given JSON value
  • to_ubjson (static) - create a UBJSON serialization of a given JSON value

Non-member functions


Helper classes



The example shows how the library is used.

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

using json = nlohmann::json;

int main()
    // create a JSON object
    json j =
        {"pi", 3.141},
        {"happy", true},
        {"name", "Niels"},
        {"nothing", nullptr},
            "answer", {
                {"everything", 42}
        {"list", {1, 0, 2}},
            "object", {
                {"currency", "USD"},
                {"value", 42.99}

    // add new values
    j["new"]["key"]["value"] = {"another", "list"};

    // count elements
    auto s = j.size();
    j["size"] = s;

    // pretty print with indent of 4 spaces
    std::cout << std::setw(4) << j << '\n';


    "answer": {
        "everything": 42
    "happy": true,
    "list": [
    "name": "Niels",
    "new": {
        "key": {
            "value": [
    "nothing": null,
    "object": {
        "currency": "USD",
        "value": 42.99
    "pi": 3.141,
    "size": 8

See also

Version history

  • Added in version 1.0.0.

Last update: November 1, 2023