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Primer for C++ Concepts
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A C++ Concepts Primer

C++ Concepts Primer

Introduction

Right now C++ is experiencing a renaissance, fueled by new language features and an extended standard library introduced with C++11, C++14, and C++17. It is a different beast than C++98 (want to feel really old? that's ~two decades ago!), while still keeping to its core philosophies: the zero-overhead principle, having a simple and direct mapping to hardware, and to be completely multi-paradigm. The language has become more powerful, but also simpler, since many of the "rough edges" in C++98 have been removed and replaced with more modern variants, that are safer and more general to use. Interest in the C++ programming language has surged again, and with C++20 coming soon™ doubly so. Concepts will be in C++20 folks!

In this primer we'll be looking at concepts, a way to constrain templates, which lead to: less verbose template errors, a way to overload functions based on constraints, and more explicitly defined function template interfaces by using requirements. This primer will teach you how to apply constraints to template parameters by using the requires clause, and how to define your own set of requirements by using the many forms of requires expression. You'll then see that we can compose requirements together to form useful concepts, which can be used together with terse syntax to write safe and less verbose generic code.

Documents

  • Primer (pdf): the primer itself, which consists of a short overview of generic programming in a pre-concepts world, the problems with using unconstrained template parameters, what methods we currently have (e.g. type traits and SFINAE) for solving these or similar problems, and why they are insufficient. We then present Concepts, which are a method to constrain template parameters by using requires clauses and requires expressions. The primer also covers standard library concepts, their use in the ranges proposal, and their different terse syntax proposed for C++20 (which is more a historical curiosity nowadays, since the syntax has already been selected now). Warning! This is still work in progress!
  • Slides (pdf): essentially the same information as the above, but condensed, and not as good. Good for a quick read :).

Typesetting

Both the primer and the slides are typeset with LaTeX, and are available as PDFs in the section above, but if if you still want to typeset the latest version for some reason, here is how to do it (in TeXLive): go into primer or slides, and then call make.

Examples

Almost all examples in the primer should be runnable after doing the hack in concepts.h. In fact, I would recommend you to just import concepts.h if you want to play around with concepts. The only examples that aren't runnable, are those that use the more novel terse syntaxes (i.e. concepts in-place syntax and the adjective syntax). The natural syntax as in the TS:es should work fine. You can compile all examples in the primer by going to primer/listings and calling make. Since snippets in the primer is taken directly from these listings, everything you see in the primer is syntactically correct as they can be built / run.

Acknowledgements

  • Roger Orr: for his awesome Concepts Lite in Practice talk and for the article from ACCU 2016. Some motivating examples (e.g bad errors, and type traits) in this primer are borrowed (with consent) from there.
  • Andrew Sutton: for another great talk: Generic Programming with Concepts at C++Now 2015. Many of the early examples in this primer are based from that talk (with some modifications).
  • Peter Sommerlad: and the rest of the committee, for allowing me to participate in the discussion on Concepts, Modules and Contracts in the Rapperswil 2018 EWG meeting. It was great fun, and it was very interesting to get a perspective on how the commitee operates; while the work was tiring, it was very rewarding!
  • Tobias Lasser: and the rest of the teachers/participants in the course seminar Discovering and Teaching Modern C++, for the very interesting talks and for providing a platform where like-minded people can discuss and teach modern C++ to each other. It forced me (being really lazy), to write something that might (?) be useful (I hope?) to others.

References

  • Bjarne Stroustrup. Concepts: The Future of Generic Programming. Document P0557 R1, 31/01/2017.
  • Casey Carter and Eric Niebler. Standard Library Concepts (SLC). Document P0898 R3, 08/06/2018.
  • Eric Niebler, Casey Carter, C. Di Bella. The One Ranges Proposal. Document P0896 R2, 25/06/2018.
  • Köppe et al. Yet Another Approach For Constrained Declarations. Document P1141 R0, 23/06/2018.
  • Stroustrup B. A Minimal Solution to the Concepts Syntax Problems. Document P1079 R0, 06/05/2018.
  • Sutter H. Concepts In-Place Syntax (the post-Jacksonville variant!). Document P0745 R1, 29/04/2018.
  • Working Draft, C++ Extension for Concepts (Concepts TS Draft). Technical Spec. D4553, 02/10/2015.
  • Wording Paper, C++ Extension for Concepts (C++20WD Syntax). Document P0734 R0, 14/07/2017.
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