The iterator based approach of STL works very elegantly for 1 dimensional data structures but fails to generalize cleanly for higher dimensional structures. For example, there is no easily defined way of iterating over a 2d array or a graph. Also, the notion of regular types, discussed in your book Elements of Programming, also fails to generalize for 2 or higher dimensional types, like complex numbers and matrices. They lack the total ordering property.

Of course, you can artificially define an ordering, say force a row-by-row iteration over a 2D-Array or a breadth-first iterator over a tree or an artificial ordering on complex numbers, but such constructs feel artificial. Do you think this limitation is fundamental to the iterator based design approach?

As we all know, C++ is far from perfect. There are several features which you discuss in your books and papers, like concepts and UNDERLYING_TYPE, which C++ is currently missing but proposed for C++17 (e.g. destructive move). However there are things you have criticized before, like the memory allocation interface, which are still as they were 25 years back.

What do you dislike the most about C++? What would you change or add to the language to make it better?

In you book "Elements of Programming", you spend a lot of time on concepts. The paper "A Concept Design for the STL", the basis of the latest concept design for C++, references your book extensively. You of course co-authored that paper. I am therefore quite keen to hear your views on C++ Concepts.

Do you think that language support for concepts (or equivalent constructs like Haskell typeclasses) is important for writing generic code? How deeply are you involved in the effort to get concepts into the C++ standard?