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>).

Key : key type

T : mapped type

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

Allocator : allocator type

• 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

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

??? 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"
```

• ordered_json

• Added in version 3.9.0 to implement nlohmann::ordered_json.
• Added key_compare member in version 3.11.0.