User defined type and foreach

[Tony]

On Friday, 17 November 2017 at 07:40:35 UTC, Mike Parker wrote:

>
> You might also find use in this article (poorly adapted from 
> Chapter 6 of Learning D by the publisher, but still readable):
>
> https://www.packtpub.com/books/content/understanding-ranges
>
>> makes a distinction about "range consumption" with regard to a 
>> "reference type" or a "value type" and it isn't clear to me 
>> why there would be a difference.
>
> With a value type, you're consuming a copy of the original 
> range, so you can reuse it after. With a reference type, you're 
> consuming the original range and therefore can't reuse it.
>
>
> ========
> struct ValRange {
>     int[] items;
>     bool empty() @property { return items.length == 0; }
>     int front() @property { return items[0]; }
>     void popFront() { items = items[1 .. $]; }
> }
>
> class RefRange {
>     int[] items;
>     this(int[] src) { items = src; }
>     bool empty() @property { return items.length == 0; }
>     int front() @property { return items[0]; }
>     void popFront() { items = items[1 .. $]; }
> }
>
> void main() {
>     import std.stdio;
>
>     int[] ints = [1, 2, 3];
>     auto valRange = ValRange(ints);
>
>     writeln("Val 1st Run:");
>     foreach(i; valRange) writeln(i);
>     assert(!valRange.empty);
>
>     writeln("Val 2nd Run:");
>     foreach(i; valRange) writeln(i);
>     assert(!valRange.empty);
>
>     auto refRange = new RefRange(ints);
>
>     writeln("Ref 1st Run:");
>     foreach(i; refRange) writeln(i);
>     assert(refRange.empty);
>
>     writeln("Ref 2nd Run:");
>     foreach(i; refRange) writeln(i); // prints nothing
> }

Thanks for the reference and the code. I will have to iterate 
over the packpub text a while consulting the docs. I see that the 
code runs as you say, but I don't understand what's going on. You 
say with regard to a "value type" : "you're consuming a copy of 
the original range" but I don't see anything different between 
the processing in the struct versus in the class. They both have 
a dynamic array variable that they re-assign a "slice" to (or 
maybe that is - that they modify to be the sliced version). 
Anyway, I can't see why the one in the struct shrinks and then 
goes back to what it was originally. It's like calls were made by 
the compiler that aren't shown.