pass-by-ref semantics for structs (was Deque impl.)

[Steven Schveighoffer]

On Thu, 31 Jan 2013 07:27:08 -0500, d coder <dlang.coder at gmail.com> wrote:

> On Thu, Jan 31, 2013 at 5:30 PM, monarch_dodra <monarchdodra at gmail.com>  
> wrote:
>> The pull is kind of stuck in limbo, specifically because of the problems
>> associated with implementing reference semantics with structs :/
>
>
> Thanks for the enlightening email.
>
> I am of the considered view that reference semantics with structs in D
> is tough (if not impossible) with default constructor and postblit
> constructor (used when passing objects). This is because you can not
> initialize any object (wrapped in the struct) in the default
> constructor and if you are passing the struct as a function parameter,
> it is not possible to initialize these internal objects (before
> passing them) in the postblit constructor. I faced this in some of my
> code. Do not know if you are facing the same scenario in the DList
> implementation.

It's pretty simple.  Containers are reference types, and it makes most  
sense to pass them by reference.

In order to be sane, containers must be implemented as classes.   
Otherwise, you have behavior like this:

void foo(int[int] aa, int x, int y)
{
    aa[x] = y;
}

void main()
{
    int[int] aa;
    foo(aa, 1, 2);
    aa[3] = 4;
    foo(aa, 5, 6);
    assert(!(1 in aa));
    assert(aa[3] == 4);
    assert(aa[5] == 6);
}

In other words, the only way to turn a default-instantiated struct into a  
valid struct is to use it once.  Not the behavior you want.

Classes don't have this problem, because you must instantiate them to use  
them.

-Steve