I was able to make it work.

It's not exactly a singleton. I just need my input object, sound object, gui object etc to access each other.

I think passing things around all the time is more like spaghetti code than to have 1 single place to store a reference to something. Less typing as well.

There is a lot of hidden dangers with using globally accessible variables. For one thing, it's hard to keep track of all the points of access. Because the objects can be accessed at any time, there is little options for the code to tell you that you need to initialize some global variable before the code can work correctly. At some point you'll forget that one of the classes in your code depends on a global variable and you'll forget to initialize it. Now you're wasting time fixing something that could have been prevented had you declared it as an input argument in your constructor. Sure, you can declare everything global, but you should keep in mind that scoping and proper class interfaces are there to tell you what information you need and hide information that should not be accessed elsewhere. It limits how many things you'll have to focus on and will keep you from creating functions that have hidden side effects.