If a list is empty, which statement about front() and back() is true?

• A. They require that the list is not empty
• B. They can be called safely and return default values
• C. They throw a runtime exception
• D. They return null pointers

Answer: (A)

The key idea is that front() and back() return references to the first and last elements, respectively, and there must be elements to reference. If the list is empty, there is no element to refer to, so calling either function has undefined behavior. That’s why they cannot be called safely on an empty list, and they do not guarantee returning a default value or a null pointer. It’s also not guaranteed that an exception will be thrown in all environments. The correct stance is that you must ensure the list is non-empty before using these functions.