api/basic_json/object_t/index.md

# nlohmann::basic_json::object_t

```
using object_t = ObjectType<StringType,
                            basic_json,
                            default_object_comparator_t,
                            AllocatorType<std::pair<const StringType, basic_json>>>;
```

The type used to store JSON objects.

[RFC 8259](https://tools.ietf.org/html/rfc8259) 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 described below.

## Template parameters

`ObjectType` : the container to store objects (e.g., `std::map` or `std::unordered_map`)

`StringType` : the type of the keys or names (e.g., `std::string`). The comparison function `std::less<StringType>` is used to order elements inside the container.

`AllocatorType` : the allocator to use for objects (e.g., `std::allocator`)

## Notes

#### Default type

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

```
// until C++14
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
>

// since C++14
std::map<
  std::string, // key_type
  basic_json, // value_type
  std::less<>, // key_compare
  std::allocator<std::pair<const std::string, basic_json>> // allocator_type
>
```

See [`default_object_comparator_t`](https://json.nlohmann.me/api/basic_json/default_object_comparator_t/index.md) for more information.

#### 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, it is unspecified which one of the values for a given key will be chosen. For instance, `{"key": 2, "key": 1}` could be equal to either `{"key": 1}` or `{"key": 2}`. To reject duplicate keys instead of silently resolving them one way or another, see [this parsing recipe](https://json.nlohmann.me/features/parsing/parser_callbacks/#recipe-rejecting-duplicate-object-keys).
- Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md)) 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 8259](https://tools.ietf.org/html/rfc8259) specifies:

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

In this class, the object's limit of nesting is not explicitly constrained. 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`](https://json.nlohmann.me/api/basic_json/max_size/index.md) 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.

#### Object key order

The order name/value pairs are added to the object are *not* preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as `std::map` with `std::less` is used by default. Please note this behavior conforms to [RFC 8259](https://tools.ietf.org/html/rfc8259), because any order implements the specified "unordered" nature of JSON objects.

#### Cross-`basic_json` conversion requirements

When converting an object from one `basic_json` specialization to another via the [converting constructor](https://json.nlohmann.me/api/basic_json/basic_json/#overload-4), the target `object_t`'s `key_type` must be directly constructible from the source `basic_json`'s `string_t` type (or more generally, from the source object's key type). If this requirement is not met, the conversion does not fail; instead, the object is silently converted as an array of key-value pairs, which is incorrect. See [issue #3425](https://github.com/nlohmann/json/issues/3425) for details and an example.

## Examples

Example

The following code shows that `object_t` is by default, a typedef to `std::map<json::string_t, json>`.

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

using json = nlohmann::json;

int main()
{
    std::cout << std::boolalpha << std::is_same<std::map<json::string_t, json>, json::object_t>::value << std::endl;
}
```

Output:

```
true
```

## Version history

- Added in version 1.0.0.