http://www.everfall.com/paste/id.php?m46jrb36o7qu

This is a short example. It has all the comments inside. Since "in" keyword tells the compiler to make a copy of an class object - a new object is created. But constructor for this object is not called(you can check it). In my opinion it is a big issue and needs to be improved.

On Tue, 30 Sep 2008 23:37:50 +0400, Eldar Insafutdinov <e.insafutdinov@gmail.com> wrote:

> http://www.everfall.com/paste/id.php?m46jrb36o7qu
>
> This is a short example. It has all the comments inside. Since "in" keyword tells the compiler to make a copy of an class object - a new object is created.

No, that's not true. No heap activity is done under the hood, a copy of /pointer/ is passed (as opposed to a copy of /instance/).
In means that changes to the variable won't be visible to the caller:

void foo(in TreeIter it)
{
    it = null; // makes local change
}

An important thing to note is that `in` is not recursive (as const or invariant).

> But constructor for this object is not called(you can check it). In my opinion it is a big issue and needs to be improved.

No, it shouldn't generate heap activity unless really needed. In this case do the following:

void foo(const(TreeIter) it)
{
    auto itCopy = it.clone();
    // do whatever you wish with a copy.
}

But in general, a const reference should be enough to you.