An enumerator is an iterators that provides operator bool() const
which returns true when it is valid to dereference the iterator.
This is similar to MoveNext in the IEnumerator C# interface,
but fits in better with C++ notions of iteration.
If the iterator has an end it behaves like i != end.
The basic idea is that enumerator<I,T> is an iterator
that uses I for iteration and returns value_type T,
perhaps different from I::value_type.
The canonical example is applying a function from T → U on an input iterator.
The resulting iterator should be of type enumerator<I,U>.
All classes deriving from enumerator are simple: drag in the
iterator with using enumerator::i then add
the necessary accoutrements in the constructor and implement
operator bool() const, operator*, and operator++ pre and post increment.
These hide the implementation in the base class instead of overriding, but enumerator
algorithms pass everything by value so the correct implementation
will be supplied at compile time.
The convention is to use class_ for class names and class instead of make_class.
Users never need to use the class directly, just the corresponding template function.
The purpose is to make C++ more expressive for implementing numerical algorithms. The bias is to assume all iterators are input iterators over constant data. The inspiration comes from Fortran and APL, but incorporates recent functional programming notions.
Specialize enumerator for all iterator categories, not just input iterator.
operator, => concatenate
operator^ => monotonic cycle