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nlohmannjson/doc/mkdocs/docs/features/object_order.md

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2020-07-24 12:04:58 +00:00
# Object Order
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.
The default type `nlohmann::json` uses a `std::map` to store JSON objects, and thus stores object keys **sorted alphabetically**.
??? example
```cpp
#include <iostream>
#include "json.hpp"
using json = nlohmann::json;
int main()
{
json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;
std::cout << j.dump(2) << '\n';
}
```
Output:
```json
{
"one": 1,
"three": 3,
"two": 2
}
```
If you do want to preserve the **insertion order**, you can try the type [`nlohmann::ordered_json`](https://github.com/nlohmann/json/issues/2179).
??? example
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using ordered_json = nlohmann::ordered_json;
int main()
{
ordered_json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;
std::cout << j.dump(2) << '\n';
}
```
Output:
```json
{
"one": 1,
"two": 2,
"three": 3
}
```
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)).