json/docs/mkdocs/docs/api/ordered_map.md

nlohmann::ordered_map

template<class Key, class T, class IgnoredLess = std::less<Key>,
         class Allocator = std::allocator<std::pair<const Key, T>>>
struct ordered_map : std::vector<std::pair<const Key, T>, Allocator>;

A minimal map-like container that preserves insertion order for use within nlohmann::ordered_json (nlohmann::basic_json<ordered_map>).

Template parameters

Key

key type

T

mapped type

IgnoredLess

comparison function (ignored and only added to ensure compatibility with #!cpp std::map)

Allocator

allocator type

Iterator invalidation

The type uses a std::vector to store object elements. Therefore, adding elements can yield a reallocation in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

When the storage grows, the keys are copied and the mapped values are moved to the new storage. A plain std::vector would copy the whole elements instead, because their #!cpp const keys make them not nothrow move constructible; for ordered_json, this would be a deep copy of every nested value. The values are only copied if T is not default constructible or not nothrow move assignable.

Member types

• key_type - key type (Key)
• mapped_type - mapped type (T)
• Container - base container type (#!cpp std::vector<std::pair<const Key, T>, Allocator>)
• iterator
• const_iterator
• size_type
• value_type
• key_compare - key comparison function

std::equal_to<Key>  // until C++14

std::equal_to<>     // since C++14

Member functions

• (constructor)
• (destructor)
• emplace
• operator[]
• at
• erase
• count
• find
• insert

Exception safety

emplace, operator[], and insert(value) have the strong exception guarantee: if an exception is thrown (for instance, because copying a key or allocating memory fails), the contents of the container are unchanged.

Complexity

Because the elements are stored in a std::vector in insertion order, there is no index to look a key up by. Every key-based operation performs a linear scan over the stored elements. With n denoting the number of elements in the container:

Operation	Complexity	Note
emplace	O(n)	scans for an existing key, then appends (amortized O(1))
operator[]	O(n)	delegates to emplace (non-const) or at (const)
at	O(n)	throws #!cpp std::out_of_range if the key is not found
find	O(n)
count	O(n)	the result is always 0 or 1
erase(key)	O(n)	scan, then move the remaining elements one position down
erase(pos), erase(first, last)	O(n)	moves all elements after the erased range
insert(value)	O(n)	equivalent to emplace
insert(first, last)	O((n + m) * m)	for m inserted elements

This differs from #!cpp std::map, where the same operations are O(log n).

!!! warning "Quadratic cost of building large objects"

Because every insertion scans all elements inserted so far, building an object of `n` distinct keys costs
**O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key as well as to
parsing one, since the parser inserts each key as it is read.

The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows
steeply for machine-generated objects with many thousands of keys. Measured with `-O2 -DNDEBUG` for parsing a flat
object of `n` keys, relative to `#!cpp nlohmann::json` (which uses `#!cpp std::map`):

| `n`    | `json` | `ordered_json` | factor |
|--------|--------|----------------|--------|
| 2000   | 0.7 ms | 3.6 ms         | 5×     |
| 4000   | 0.8 ms | 14.0 ms        | 19×    |
| 8000   | 1.6 ms | 67.8 ms        | 43×    |
| 16 000 | 3.3 ms | 181.6 ms       | 54×    |

If key order matters for objects of that size, consider a container with a lookup index, such as
[`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map)
([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)), as the object type -- see
[object order](../features/object_order.md).

Examples

??? example

The example shows the different behavior of `std::map` and `nlohmann::ordered_map`.
 
```cpp
--8<-- "examples/ordered_map.cpp"
```

Output:

```json
--8<-- "examples/ordered_map.output"
```

See also

• ordered_json

Version history

• Added in version 3.9.0 to implement nlohmann::ordered_json.
• Added key_compare member in version 3.11.0.
• Changed in version 3.13.0: growing the storage moves the mapped values instead of copying them.