C++ Templates: The Complete Guide, 2nd Edition

Appendix E

Concepts

For many years now, C++ language designers have explored how to constrain the parameters of templates. For example, in our prototypical max() template, we’d like to state up front that it shouldn’t be called for types that aren’t comparable using the less-than operator. Other templates may want to require that they be instantiated with types that are valid “iterator” types (for some formal definition of that term) or valid “arithmetic” type (which could be a broader notion than the set of built-in arithmetic types).

A concept is a named set of constraints on one or more template parameters. While C++11 was being a developed, a very rich concept system was designed for it, but integrating the feature into the language specification ...