Constrained Templates and Concepts

Templates define parameterized families of C++ entities such as functions (Chapter 10 ) or class types (Chapter 17 ). In practice, templates are mostly parameterized on seemingly arbitrary types. Yet most templates cannot be instantiated for just any type; at least not without error. The binary std::max<>() function template, for instance, can only be instantiated for types that support the < operator. And the std::vector<> and std::optional<> class templates may not be instantiated for reference types, void, or array types. Most of your own templates have similar restrictions as well, whether you realized it or not.