When using the list class of Example 10.17, the typical C++ programmer will use a pointer type for generic parameter V, so that list_nodes point to the elements of the list. An alternative implementation would include next and prev pointers for the list within the elements themselves— typically by arranging for the element type to inherit from something like the gp_list_node class of Example 10.14. The result is sometimes called an intrusive list.

(a) Explain how you might build intrusive lists in C++ without requiring users to pepper their code with explicit type casts. Given multiple inheritance, you will probably need to determine, for each concrete element type, the offset within the representation of the type at which the next and prev pointers appear. For further ideas, search for information on the boost::intrusive::list class of the popular Boost library.

(b) Discuss the relative advantages and disadvantages of intrusive and non-intrusive lists.