C++ Primer Plus

Concepts, Refinements, and Models

The STL has several features, such as kinds of iterators, that aren’t expressible in the C++ language. That is, although you can design, say, a class that has the properties of a forward iterator, you can’t have the compiler restrict an algorithm to using only that class. The reason is that the forward iterator is a set of requirements, not a type. The requirements could be satisfied by an iterator class you’ve designed, but they could also be satisfied by an ordinary pointer. An STL algorithm works with any iterator implementation that meets its requirements. STL literature uses the word concept to describe a set of requirements. Thus, there is an input iterator concept, a forward iterator concept, and so on. ...