68 lines
1.6 KiB
Markdown
68 lines
1.6 KiB
Markdown
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# Object Order
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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.
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The default type `nlohmann::json` uses a `std::map` to store JSON objects, and thus stores object keys **sorted alphabetically**.
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??? example
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```cpp
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#include <iostream>
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#include "json.hpp"
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using json = nlohmann::json;
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int main()
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{
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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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"three": 3,
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"two": 2
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}
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```
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If you do want to preserve the **insertion order**, you can try the type [`nlohmann::ordered_json`](https://github.com/nlohmann/json/issues/2179).
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??? example
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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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Alternatively, you can use a more sophisticated ordered map like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
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