Regular Types and Why Do I Care?

By Victor Ciura

'Regular' is not exactly a new concept (pun intended). If we reflect back on STL and its design principles, as best described by Alexander Stepanov in his 1998 'Fundamentals of Generic Programming' paper or his lecture on this topic, from 2002, we see that regular types naturally appear as necessary foundational concepts in programming.

Why do we need to bother with such taxonomies? Well, the STL now informally assumes such properties about the types it deals with and imposes such conceptual requirements for its data structures and algorithms to work properly. The new Concepts Lite proposal (hopefully part of C++20) is based on precisely defined foundational concepts such as Semiregular, Regular, EqualityComparable, DefaultConstructible, LessThanComparable (strict weak ordering), etc. Formal specification of concepts is an ongoing effort in the ISO C++ Committee and these STL library concepts requirements are being refined as part of Ranges TS proposal (<experimental/ranges/concepts>).

Recent STL additions such as string_view, tuple, reference_wrapper, as well as new incoming types for C++20 like std::span raise new questions regarding values types, reference types and non-owning 'borrow' types.

Designing and implementing regular types is crucial in everyday programing, not just library design. Properly constraining types and function prototypes will result in intuitive usage; conversely, breaking subtle contracts for functions and algorithms will result in unexpected behavior for the caller.

This talk will explore the relation between Regular types (and other concepts) and STL containers & algorithms with examples, common pitfalls and guidance.