Skip to content

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.

Default behavior: sort keys

The default type nlohmann::json uses a std::map to store JSON objects, and thus stores object keys sorted alphabetically.

Example
#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:

{
  "one": 1,
  "three": 3,
  "two": 2
}

Alternative behavior: preserve insertion order

If you do want to preserve the insertion order, you can try the type nlohmann::ordered_json.

Example
#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:

{
  "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).

Notes on parsing

Note that you also need to call the right parse function when reading from a file. Assume file input.json contains the JSON object above:

{
  "one": 1,
  "two": 2,
  "three": 3
}

Right way

The following code correctly calls the parse function from nlohmann::ordered_json:

std::ifstream i("input.json");
auto j = nlohmann::ordered_json::parse(i);
std::cout << j.dump(2) << std::endl;

The output will be:

{
  "one": 1,
  "two": 2,
  "three": 3
}
Wrong way

The following code incorrectly calls the parse function from nlohmann::json which does not preserve the insertion order, but sorts object keys. Assigning the result to nlohmann::ordered_json compiles, but does not restore the order from the input file.

std::ifstream i("input.json");
nlohmann::ordered_json j = nlohmann::json::parse(i);
std::cout << j.dump(2) << std::endl;

The output will be:

{
  "one": 1,
  "three": 3
  "two": 2,
}

Last update: May 17, 2022
Back to top