If we want to allow people to create ranges that are classes (as opposed
to structs) the requirement for a save() function is a must. This is
because copying class ranges with
Range copy = original;
only creates a new alias for original; the two share the same state.
So this solves our forward vs. input ranges dilemma quite nicely: the
save() function at the same time allows distinction between the two
(input ranges don't define a save() function) and also allows ranges to
be implemented as classes as well as structs.
To summarize:
Input ranges: empty front popFront
Forward ranges: + save
Bidir ranges: + back popBack
Random-access ranges: + opIndex opIndexAssign
The disadvantage is that all non-input ranges must define save(), which
is a trivial functions in most cases:
Range save() { return this; }
Andrei
