I am using this code to detect controllers connected to my system.

int numJoysticksEnumerated = SDL_NumJoysticks();
std::cout << "Number of joysticks to check: " << numJoysticksEnumerated << std::endl;
for (int i = 0; i < numJoysticksEnumerated; ++i) {
    std::cout << "Checking joystick ID " << i << std::endl;
    if (SDL_IsGameController(i)) {
        SDL_GameController* gamePad = SDL_GameControllerOpen(i);
        if (gamePad) {
            std::cout << "Controller detected." << std::endl;
        } else {
            logSDLError(std::cout, "Controller detection failure.");
        }
    }
}

When I call

SDL_ControllerButtonEvent::which

afterwards, the lowest index is the one of the controller I connected most recently and the highest index is the one of the controller I connected first. This seems counter-intuitive since the Xbox-controller lights show the controller that was connected first as the first controller and so on. If I connect an additional controller after the first detection and run the detection again, the new controller will get a higher ID.

If for example I do this in order:

Controller 1 connected
Controller 2 connected
Controller 3 connected
Start game
Run controller detection
Controller 4 connected
Run controller detection

The result is:

Controller    SDL_JoystickID
1             2
2             1
3             0
4             3