You cannot eat your cake and have it.

You cannot consume something and still possess it in its original state. The proverb expresses the impossibility of satisfying two desires when one rules out the other.

More broadly, it asks someone to recognize a trade-off. Gaining one benefit may require giving up another, however much the person would prefer to keep both.

Use it when someone wants the benefits of a choice without accepting what that choice costs. It fits situations where the two desired outcomes genuinely conflict, rather than cases where both are possible.