JSON for Modern C++  3.0
template<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 = int64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator>
using nlohmann::basic_json::object_t = ObjectType<StringType, basic_json, std::less<StringType>, AllocatorType<std::pair<const StringType, basic_json>>>

RFC 7159 describes JSON objects as follows:

An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

To store objects in C++, a type is defined by the template parameters ObjectType which chooses the container (e.g., std::map or std::unordered_map), StringType which chooses the type of the keys or names, and AllocatorType which chooses the allocator to use.

Default type

With the default values for ObjectType (std::map), StringType (std::string), and AllocatorType (std::allocator), the default value for object_t is:

std::map<
std::string, // key_type
basic_json, // value_type
std::less<std::string>, // key_compare
std::allocator<std::pair<const std::string, basic_json>> // allocator_type
>

Behavior

The choice of object_t influences the behavior of the JSON class. With the default type, objects have the following behavior:

  • When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.
  • When the names within an object are not unique, later stored name/value pairs overwrite previously stored name/value pairs, leaving the used names unique. For instance, {"key": 1} and {"key": 2, "key": 1} will be treated as equal and both stored as {"key": 1}.
  • Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see dump) in this order. For instance, {"b": 1, "a": 2} and {"a": 2, "b": 1} will be stored and serialized as {"a": 2, "b": 1}.
  • When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, {"b": 1, "a": 2} and {"a": 2, "b": 1} will be treated as equal.

Limits

RFC 7159 specifies:

An implementation may set limits on the maximum depth of nesting.

In this class, the object's limit of nesting is not constraint explicitly. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON object.

Storage

Objects are stored as pointers in a basic_json type. That is, for any access to object values, a pointer of type object_t* must be dereferenced.

See also
array_t