An updated discussion on C++ type design, especially the overlap between Regular and reference types.

Revisiting Regular Types

By Titus Winters

From the article:

With 20 years of experience, we know that Regular type design is a good pattern - we should model user-defined types based on the syntax and semantics of built-in types where possible. However, common formulations of Regular type semantics only apply to values, and for performance reasons we commonly pass by reference in C++. In order to use such a reference as if it were its underlying Regular type we need some structural knowledge of the program to guarantee that the type isn’t being concurrently accessed. Using similar structural knowledge, we can treat some non-Regular types as if they were Regular, including reference types which don’t own their data. Under such an analysis, string_view indeed behaves as if it were Regular when applied to common usage (as a parameter). However, span does not, and further it is (currently) impossible to have shallow copy, deep compare, and Regular const semantics in the same type in C++.

This analysis provides us some basis to evaluate non-owning reference parameters types (like string_view and span) in a practical fashion, without discarding Regular design.