json/docs/mkdocs/docs/features/object_order.md

# Object Order

The [JSON standard](https://tools.ietf.org/html/rfc8259.html) defines objects as "an unordered collection of zero or more name/value pairs". As such, an implementation does not need to preserve any specific order of object keys.

## Default behavior: sort keys

The default type `nlohmann::json` uses a `std::map` to store JSON objects, and thus stores object keys **sorted alphabetically**.

??? example "Example: `json` sorts object keys"

    ```cpp
    #include <iostream>
    #include <nlohmann/json.hpp>
    
    using json = nlohmann::json;
    
    int main()
    {
        json j;
        j["one"] = 1;
        j["two"] = 2;
        j["three"] = 3;
        
        std::cout << j.dump(2) << '\n';
    }
    ```
    
    Output:

    ```json
    {
      "one": 1,
      "three": 3,
      "two": 2
    }
    ```

## Alternative behavior: preserve insertion order

If you do want to preserve the **insertion order**, you can use the type [`nlohmann::ordered_json`](../api/ordered_json.md).

??? example "Example: `ordered_json` preserves insertion order"

    ```cpp
    --8<-- "examples/ordered_json.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/ordered_json.output"
    ```

Alternatively, [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) also preserves the insertion order and, unlike [`ordered_map`](../api/ordered_map.md), keeps a lookup index, so it does not have the quadratic cost described below. It is used through a small adapter ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).

If the order does not matter and you only want faster lookup, `boost::unordered_flat_map`, `absl::flat_hash_map`, `absl::node_hash_map`, and several other hash maps work through an adapter that restores the template argument order `basic_json` expects; see [Template Parameter Requirements](types/template_parameters.md#objecttype). Note these are *unordered*, not insertion-ordered.

[`tsl::ordered_map`](https://github.com/Tessil/ordered-map) cannot be used: its iterators expose the mapped value as `const`, while `basic_json` needs to modify it in place.

The [`ordered_map`](../api/ordered_map.md) behind `nlohmann::ordered_json` is deliberately minimal and has no lookup
index, so every key access is a linear scan and building an object of `n` keys costs O(n²). This is unnoticeable at
typical object sizes but becomes significant for objects with many thousands of keys; see
[`ordered_map` complexity](../api/ordered_map.md#complexity). The alternatives above keep a lookup index and do not
have this cost.

### Notes on parsing

Note that you also need to call the right [`parse`](../api/basic_json/parse.md) function when reading from a file.
Assume file `input.json` contains the JSON object above:

```json
{
  "one": 1,
  "two": 2,
  "three": 3
}
```

!!! success "Right way"

    The following code correctly calls the `parse` function from `nlohmann::ordered_json`:

    ```cpp
    std::ifstream i("input.json");
    auto j = nlohmann::ordered_json::parse(i);
    std::cout << j.dump(2) << std::endl;
    ```

    The output will be:

    ```json
    {
      "one": 1,
      "two": 2,
      "three": 3
    }
    ```

??? failure "Wrong way"

    The following code incorrectly calls the `parse` function from `nlohmann::json` which does not preserve the
    insertion order, but sorts object keys. Assigning the result to `nlohmann::ordered_json` compiles, but does not
    restore the order from the input file.

    ```cpp
    std::ifstream i("input.json");
    nlohmann::ordered_json j = nlohmann::json::parse(i);
    std::cout << j.dump(2) << std::endl;
    ```

    The output will be:

    ```json
    {
      "one": 1,
      "three": 3,
      "two": 2
    }
    ```