Bug 12109 – foreach with class alias this range iteration inconsistency

Sorry for the title, hard to explain in a few words: class A { int[] a = [1, 2, 3]; alias a this; } void main() { import std.stdio, std.array; auto a = new A(); while (!a.empty) a.popFront(); writeln(a.a); // [] auto b = new A(); foreach (_; b) {} writeln(b.a); // [1, 2, 3] } The problem: iterating using popFront() manually gives a different result from using foreach. The foreach iteration doesn't consume the range, while the popFront/empty does. Presumably the foreach is making a copy of the aliased array and iterating that instead of the A object directly. Expected: foreach should be consistent with using popFront/empty manually and should consume the range, i.e. the final writeln(b.a) should give [] It is very important that these are consistent otherwise algorithms that change from using popFront/empty to foreach will change behaviour when the user is using a class with alias this range. This may explain the variations seen in Issue 9506 between releases.

(In reply to comment #0) > Sorry for the title, hard to explain in a few words: This has nothing to do with AliasThis. All forward ranges behave this way with foreach (i.e. foreach operates on a `save`'d copy). When `save` is not necessary (such as in algorithms that have already called `save`), a for-loop should be used, as seen in many places in Phobos.

Sorry, yes, you're right. I'm not thinking.