You can't have your cake and eat it too.
If you have the cake, it is yours and you should be able to do with it what you want, such as eat it. If you don't want to eat the cake, why do you have it? Give it to me and I'll eat it...assuming it's a cake that I like.
If you don't have the cake, how can you eat it? It's not there. You'll have to eat something else. If someone else has the cake, they may or may not choose to share it with you. If they do, then that part of the cake is yours and you can eat it. That's why they gave it to you. If not, then once again you'll have to eat something else...or jealously stand by and watch them eat the cake. I suppose that is an option too.
I guess the moral is that if you have cake, you better eat it before someone else comes along and finishes it off for you.