nlohmann::json orders Json objects alphabetically

[AdamSlay]

When trying to create a function map for the EntityManager::addComponent<component>() functions, I was running into an issue where the component objects would be added alphabetically which would result in segfaults as the AI component would always be added first. The AI component depends on the Transform component already being attached to the entity, thus the segfaults.

I couldn't figure out why this was happening as the function map itself is not alphabetical, nor are the entity template json files. Eventually I started digging into the nlohmann::json library which I am using in this project and I discovered that the default behavior for ordering json objects in the library is to sort them alphabetically:

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
}

So, the EventManager.cpp file uses the nlohmann::unordered_json type.