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1.6 KiB
1.6 KiB
Object Order
The JSON standard 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
.
??? 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
(integration) or nlohmann::fifo_map
(integration).