Difference between input range and forward range

[Jonathan M Davis via Digitalmars-d]

On Tuesday, 10 November 2015 at 14:28:15 UTC, Shachar Shemesh 
wrote:
> It seems that "foreach" requires that the range be copyable, in 
> direct contradiction to the fact that Range is not a forward 
> range.

To add to what I said, whether a range is "copyable" has nothing 
to do with whether it is a forward range or not. What's supposed 
to be guaranteed with any and all ranges is that if you copy it, 
it has the exact same state as the original had - but _not_ that 
that state is separate from the original. So, if you have

auto copy = orig;

then you can pop elements off of copy like you would have popped 
them off of orig, but you must on longer pop elements off of 
orig. _copy_ is now the range. This is critical when you consider 
that most ranges will be copied when they're passed to a 
function. If they weren't copyable, they'd be almost useless.

> If you couple that with the extremely amorphous definition of 
> "save" for forward ranges, there seems to be quite a confusion 
> between the two.

There is nothing amorphous about the definition of save. save 
duplicates a range such that you have two ranges of the same type 
which are independent of one another and which contain exactly 
the same elements. save provides a way to get a copy of the 
range's state regardless of whether it's value type, reference 
type, or pseudo-reference type.

Consider, for instance, a range which is implemented as a class. 
It's a full-on reference type, and copying it around will never 
result in its state being copied. Without save, there would be no 
way to duplicate it. And save provides the _only_ generic means 
of duplicating a range. It's a bug any time that generic code 
depends on the state of a range being duplicated when it's 
copied. Unfortunately, because most ranges _do_ duplicate their 
state when they're copied, there's plenty of code out there which 
is buggy and relies on ranges being duplicated when they're 
copied. That's why it's so critical to test algorithms with a 
variety of range types.

- Jonathan M Davis