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125 lines
4.7 KiB
Markdown
125 lines
4.7 KiB
Markdown
# <small>nlohmann::</small>ordered_map
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```cpp
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template<class Key, class T, class IgnoredLess = std::less<Key>,
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class Allocator = std::allocator<std::pair<const Key, T>>>
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struct ordered_map : std::vector<std::pair<const Key, T>, Allocator>;
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```
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A minimal map-like container that preserves insertion order for use within [`nlohmann::ordered_json`](ordered_json.md)
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(`nlohmann::basic_json<ordered_map>`).
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## Template parameters
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`Key`
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: key type
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`T`
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: mapped type
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`IgnoredLess`
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: comparison function (ignored and only added to ensure compatibility with `#!cpp std::map`)
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`Allocator`
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: allocator type
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## Iterator invalidation
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The type uses a `std::vector` to store object elements. Therefore, adding elements can yield a reallocation in which
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case all iterators (including the `end()` iterator) and all references to the elements are invalidated.
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## Member types
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- **key_type** - key type (`Key`)
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- **mapped_type** - mapped type (`T`)
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- **Container** - base container type (`#!cpp std::vector<std::pair<const Key, T>, Allocator>`)
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- **iterator**
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- **const_iterator**
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- **size_type**
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- **value_type**
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- **key_compare** - key comparison function
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```cpp
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std::equal_to<Key> // until C++14
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std::equal_to<> // since C++14
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```
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## Member functions
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- (constructor)
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- (destructor)
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- **emplace**
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- **operator\[\]**
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- **at**
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- **erase**
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- **count**
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- **find**
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- **insert**
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## Complexity
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Because the elements are stored in a `std::vector` in insertion order, there is no index to look a key up by. Every
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key-based operation performs a **linear scan** over the stored elements. With `n` denoting the number of elements in the
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container:
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| Operation | Complexity | Note |
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|----------------------------------------|----------------|----------------------------------------------------------|
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| **emplace** | O(n) | scans for an existing key, then appends (amortized O(1)) |
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| **operator\[\]** | O(n) | delegates to **emplace** (non-const) or **at** (const) |
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| **at** | O(n) | throws `#!cpp std::out_of_range` if the key is not found |
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| **find** | O(n) | |
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| **count** | O(n) | the result is always 0 or 1 |
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| **erase(key)** | O(n) | scan, then move the remaining elements one position down |
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| **erase(pos)**, **erase(first, last)** | O(n) | moves all elements after the erased range |
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| **insert(value)** | O(n) | equivalent to **emplace** |
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| **insert(first, last)** | O((n + m) * m) | for `m` inserted elements |
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This differs from `#!cpp std::map`, where the same operations are O(log n).
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!!! warning "Quadratic cost of building large objects"
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Because every insertion scans all elements inserted so far, building an object of `n` distinct keys costs
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**O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key as well as to
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parsing one, since the parser inserts each key as it is read.
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The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows
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steeply for machine-generated objects with many thousands of keys. Measured with `-O2 -DNDEBUG` for parsing a flat
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object of `n` keys, relative to `#!cpp nlohmann::json` (which uses `#!cpp std::map`):
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| `n` | `json` | `ordered_json` | factor |
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|--------|--------|----------------|--------|
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| 2000 | 0.7 ms | 3.6 ms | 5× |
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| 4000 | 0.8 ms | 14.0 ms | 19× |
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| 8000 | 1.6 ms | 67.8 ms | 43× |
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| 16 000 | 3.3 ms | 181.6 ms | 54× |
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If key order matters for objects of that size, consider a container with a lookup index, such as
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[`tsl::ordered_map`](https://github.com/Tessil/ordered-map)
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([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)), as the object type -- see
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[object order](../features/object_order.md).
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## Examples
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??? example
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The example shows the different behavior of `std::map` and `nlohmann::ordered_map`.
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```cpp
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--8<-- "examples/ordered_map.cpp"
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```
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Output:
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```json
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--8<-- "examples/ordered_map.output"
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```
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## See also
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- [ordered_json](ordered_json.md)
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## Version history
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- Added in version 3.9.0 to implement [`nlohmann::ordered_json`](ordered_json.md).
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- Added **key_compare** member in version 3.11.0.
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