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65 lines
1.6 KiB
Markdown
65 lines
1.6 KiB
Markdown
# <small>nlohmann::</small>ordered_json
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```cpp
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using ordered_json = basic_json<ordered_map>;
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```
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This type preserves the insertion order of object keys.
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## Iterator invalidation
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The type is based on [`ordered_map`](ordered_map.md) which in turn uses a `std::vector` to store object elements.
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Therefore, adding object elements can yield a reallocation in which case all iterators (including the
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[`end()`](basic_json/end.md) iterator) and all references to the elements are invalidated. Also, any iterator or
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reference after the insertion point will point to the same index, which is now a different value.
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## Complexity
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[`ordered_map`](ordered_map.md) has no lookup index: every key-based object operation is a linear scan, so building or
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parsing an object of `n` keys costs O(n²) rather than O(n log n). See
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[`ordered_map` complexity](ordered_map.md#complexity) for the per-operation table and for measured numbers.
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## Examples
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??? example
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The example below demonstrates how `ordered_json` preserves the insertion order of object keys.
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```cpp
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#include <iostream>
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#include <nlohmann/json.hpp>
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using ordered_json = nlohmann::ordered_json;
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int main()
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{
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ordered_json j;
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j["one"] = 1;
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j["two"] = 2;
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j["three"] = 3;
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std::cout << j.dump(2) << '\n';
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}
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```
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Output:
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```json
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{
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"one": 1,
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"two": 2,
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"three": 3
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}
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```
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## See also
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- [ordered_map](ordered_map.md)
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- [Object Order](../features/object_order.md)
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## Version history
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Since version 3.9.0.
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