[Ed.: We're calling particular attention to this paper as of broad interest to the community, although still undergoing standardization. This is one of the major papers being considered at the upcoming Bristol standards meeting in April for near-term standardization work.]

A new WG21 paper is available. A copy is linked below, and the paper will also appear in the next normal WG21 mailing. If you are not a committee member, please use the comments section below or the std-proposals forum for public discussion.

Document number: N3580

Date: 2013-03-17

Concepts Lite: Constraining Templates with Predicates

by Andrew Sutton, Bjarne Stroustrup, Gabriel Dos Reis

Excerpt:

In this paper, we introduce template constraints (a.k.a., “concepts lite”), an extension of C++ that allows the use of predicates to constrain template arguments. The proposed feature is minimal, principled, and uncomplicated. Template constraints are applied to enforce the correctness of template use, not the correctness of template definitions. The design of these features is intended to support easy and incremental adoption by users. More precisely, constraints:

• allow programmers to directly state the requirements of a set of template arguments as part of a template’s interface,
• support function overloading and class template specialization based on constraints,
• fundamentally improve diagnostics by checking template arguments in terms of stated intent at the point of use, and
• do all of this without any runtime overhead or longer compilation times.

This work is implemented as a branch of GCC-4.8 and is available for download at http://concepts.axiomatics.org/˜ans/. The implementation includes a compiler and a modified standard library that includes constraints. Note that, as of the time of writing, all major features described in this report have been implemented.

This paper is organized like this:

• Tutorial: introduces the basic notions of constraints, shows examples of their use, and gives examples of how to define constraints.
• Discussion: explains what constrains are not. In particular, we try to outline constraints’s relation to concepts and to dispel some common misconceptions about concepts.
• User’s guide: provides many more tutorial examples and demonstrate the completeness of the constraints mechanism.
• Implementation: gives an overview of our GCC compiler support for constraints.
• Extensions: we discuss how constraints might be extended to interact with other proposed features.
• Language definition: presents a semi-formal definition of constraints.