Joystick events now refer to an SDL_JoystickID. This is because SDL 2.0 can handle joysticks coming and going, as devices are plugged in and pulled out during your game's lifetime, so the index into the device list that 1.2 uses would be meaningless as the available device list changes.
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You should also check out the new Game Controller API too, because it's cool, and maybe you did a lot of tap dancing with the 1.2 API that this new code would solve more cleanly. You can find it in SDL_gamecontroller.h. The Game Controller API integrates really nicely with Steam Big Picture Mode: you get automatic configuration of most controllers, and a nice UI if you have to manually configure it. In either case, Steam passes this configuration on to your SDL application