Tag dispatching
Tag dispatching is a technique that enables us to select one or another function overload at compile time. It is an alternative to std::enable_if and SFINAE and is simple to understand and use. The term tag describes an empty class that has no members (data), or functions (behavior). Such a class is only used to define a parameter (usually the last) of a function to decide whether to select it at compile-time, depending on the supplied arguments. To better understand this, let’s consider an example.
The standard library contains a utility function called std::advance that looks as follows:
template<typename InputIt, typename Distance> void advance(InputIt& it, Distance n);
Notice that in C++17, this is also constexpr (more about this, shortly). This function increments the given iterator by n elements. However, there are several categories of iterators (input, output, forward, bidirectional, and random access). That means such an operation...