Concepts in Pressio

Concept-driven design has been and still is critical in the development of pressio. Here we describe some fundamental notions that are at the core of our design approach.

concept = syntactic constraints + semantics

In pressio, we follow the approach in “Design of Concept Libraries for C++” (2011) in defining a concept to be a meaningful combination of syntactic requirements + axioms/semantics. Pure syntax can be fully specified and checked by compilers supporting C++20. On the contrary, axioms/semantics properties cannot be spelled out and statically checked. Axioms are, in fact, invariants assumed to hold. In the C++ standard, axioms and semantics are specified/listed as comments. Here, we adopt a different approach to present axioms (similar to “Design of Concept Libraries for C++”, 2011)), that we believe benefits readability: axioms are explicitly listed using the keyword “axiom”. Note, however, that if you want to compile the concepts below using a C++20 compiler, you have to comment out all axioms.

• concepts are hard and it will take a few iterations to get meaninngful concepts done

• not all components have the same maturity, so this will be reflected in the corresponding concepts too

• the documentation of each concept follows the standard: we have syntax plus semantics

“Satisfying” vs. “modeling” a concept

Quoting the C++ standard: “It is said that a standard concept is satisfied if its syntactic requirements are met, and is modeled if it is satisfied and its semantic requirements (if any) are also met.”