Move semantics for D

[Jonathan M Davis]

On Friday, July 13, 2012 12:24:45 monarch_dodra wrote:
> On Friday, 13 July 2012 at 09:47:26 UTC, Jonathan M Davis wrote:
> > On Friday, July 13, 2012 11:23:41 monarch_dodra wrote:
> >> I think that in the case of your example, if "range" fulfills
> >> the
> >> requirements for (at least) an input range, then "save"
> >> *should*
> >> be called instead of "opSlice". I'm *think* this is what the
> >> compiler does, but I'm not 100% sure.
> > 
> > The compiler never calls save. It will call opSlice on
> > non-ranges, but it'll
> > never call save on anything. It doesn't even know that save
> > exists. It's
> > unnecessary for ranges which aren't reference types, and you
> > already have to
> > worry about calling save on ranges that could be reference
> > types any time that
> > you pass them to a function that you don't want to consume it,
> > so it's not
> > really a big deal to have to call save with foreach for such
> > ranges. Most
> > range-based functions don't even use foreach much anyway.
> > 
> > - Jonathan M Davis
> 
> What exactly are the semantics of save? The reference in
> std.range isn't very clear. It would appear it is only useful it
> its *existence* that promotes a range from input to forward.
> However, how is it different from a simple assignment?
> 
> Also, if you are supposed to call save before a foreach
> "consumes" your range, then why does foreach even bother making a
> copy of the range before iterating on it? Isn't this behavior
> promoting dangerous usage for ranges where save is simply
> "{return this;}", but bites you in the ass the day you use a
> range with a specific save?
> 
> I'm confused...

save exists so that classes can be forward ranges. Arrays and most structs are 
copied when you assign them to another variable or pass them to a function, 
but classes aren't. So, save was introduced to make it so that there is a way 
to explicitly copy a range. It becomes useful even for structs and arrays in 
that it documents that you're copying it, but it's outright necessary for 
classes. Unfortunately, since arrays and structs are by far the most common 
range types, save doesn't get used anywhere near as much as it should be. 
Basically, you use save if you want to guarantee that the range is copied, and 
you don't use save if you don't care.

You can also look at this:

http://stackoverflow.com/questions/11190204/how-do-you-use-ranges-in-
d/11194827#11194827

- Jonathan M Davis