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.
