Have your cake and eat it
Definition 1 of 1
Pro Tip 1/3Impossibility Emphasis
This phrase stresses the impossibility of having two conflicting desires met at the same time. 
He wants a high-paying job without working hard; he's trying to have his cake and eat it.
Pro Tip 2/3Unrealistic Expectations
Using this phrase suggests that someone's expectations are unrealistic or greedy. 
You can't expect to party all night and still perform well on the test; you can't have your cake and eat it too.
Pro Tip 3/3Common Uses
It's often used to highlight someone's awareness of the need to make a choice. 
If you buy a house, you can't travel as much—you can't have your cake and eat it.