What is the difference between an intrusive and a non-intrusive linked list?

• A. Intrusive lists store the next pointer inside the stored data; non-intrusive lists use a separate node wrapper.
• B. Intrusive lists require an extra allocation per node; non-intrusive lists do not.
• C. Intrusive lists do not allow in-place data modification; non-intrusive lists do.
• D. Intrusive lists and non-intrusive lists are the same concept.

Answer: (D)

The key idea here is where the linking pointers live relative to the data being linked. In an intrusive list, the pointers that connect elements (like the next and possibly previous links) are stored inside the elements themselves. In a non-intrusive list, the pointers live in separate node objects that wrap around the actual data.

This distinction matters because intrusive lists require the data type to include linkage fields, so you can only put that data into such a list and you’re effectively embedding the list’s structure into the data. That can save memory and allocations and can improve cache performance, but it reduces flexibility since the data type must be designed for the list. Non-intrusive lists decouple the linking from the data, so you can link data types you don’t want to modify and can have multiple lists pointing to the same data, but you incur extra node allocations and an extra level of indirection when traversing.

Therefore, the best description is that intrusive lists store the next pointer inside the stored data; non-intrusive lists use a separate node wrapper. The idea that they are the same concept is not accurate because they differ in where the linkage pointers live and how memory is managed.