have cake and eat it too

To have or do two things that one desires that are normally contradictory or impossible to have or do simultaneously. Because "have" can also mean "eat," this expression may seem redundant. However, it is based on the meaning of "have" as "to possess," i.e., to maintain possession of one's cake while still eating it, an obvious impossibility. You're never going to save enough money to buy a house if you keep buying expensive appliances and cars. You can't have your cake and eat it, too. Too many people want to have their cake and eat it, demanding all sorts of social benefits from the government but being unwilling to pay any taxes to fund them.

Cliché to have in one's possession something and be able to use or exploit it; to have it both ways. (Usually stated in the negative.) Tom wants to have his cake and eat it too. It can't be done. Don't buy a car if you want to walk and stay healthy. You can't eat your cake and have it too.