• d-bugmail puremagic.com (53/53) Apr 12 2014 https://issues.dlang.org/show_bug.cgi?id=12566

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=12566

          Issue ID: 12566
           Summary: Give DList true reference semantics
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: Phobos
          Assignee: nobody puremagic.com
          Reporter: monarchdodra gmail.com

DList currently uses some weird semantics. It is neither value nor reference
semantics. The behavior is documented, but the documentation is the result of
what I think was a buggy behavior to begin with.

Issues with the current behavior include:
1. It's surprising. I think *everyone* simply expects reference semantics (or
wants value semantics). But they know what to *expect*. Currently, they don't
get what they expect:
//----
    auto a = DList!size_t(1, 2, 3);
    auto b = a; //Deep copy? Shallow copy?
    b.removeBack();
    assert(!equal(a[], b[])); //different
    assert(!(a is b)); //different
    assert(equal(a[], [1, 2, 3])); //a was unchanged.

    b.insertBack(3); //Put 3 back in.
    writeln(a[]); //prints [1, 2, 3, 3]
    assert(equal(a[], [1, 2, 3])); //fails. a was changed.
//----
Unless you actually *wrote* DList, I don't think you could really understand
*why* this is happening.

2. Data is *never* released: No matter how many time you call "removeBack" or
"removeFront", the nodes are never actually excised from the chain. This can
lead to a "leak" in the sense that dead data is actually still referenced by
live pointers. This program will crawl to a stall and halt:
    auto d = DList!size_t(1, 1);
    foreach (k; 0 .. size_t.max - 1)
    {
        if (k%100_000 == 0) writeln(k);
        d.removeBack();
        d.insertBack(k);
    }

3. It's inefficient and buggy. *trivial* use cases are *still* failing:
//----
  auto dl2 = DList!int([2,7]);
  dl2.removeFront();
  assert(dl2[].walkLength == 1);
  dl2.removeBack();
  assert(dl2.empty, "not empty?!"); //Fails
//----

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