Copy constructors for lazy initialization

[Bruno Medeiros]

On 03/06/2010 14:48, Andrei Alexandrescu wrote:
> On 06/03/2010 06:30 AM, Bruno Medeiros wrote:
>> On 29/05/2010 03:02, Jonathan M Davis wrote:
>>> Andrei Alexandrescu wrote:
>>>
>>> Not knowing what other implications there are, I'm fine with the
>>> change, but
>>> the fact that D creates the array when you try and insert into it (or
>>> append
>>> to it in the case of normal arrays IIRC) rather than blowing up on null
>>> seems like a source of subtle bugs and that perhaps it's not the best
>>> design
>>> choice. But maybe there was a good reason for that that I'm not aware
>>> of, so
>>> it could be that it really should stay as-is. It's just that it seems
>>> danger-prone and that the situation that you're trying to fix wouldn't
>>> be an
>>> issue if the array stayed null until the programmer made it otherwise.
>>>
>>> - Jonathan M Davis
>>
>> Yeah, I agree. I mean, for me the problem here is that the
>> map/associative-array is not really a value type, nor a reference type,
>> but something in between. I think it might be better for it to be just a
>> proper reference type.
>
> An associative array is a reference type. The problem discussed above
> applies to all reference types.
>
>> I'm not saying that Andrei suggestion has no merit though. Rather, I
>> have to admit I am not familiar with these C++ idioms and techniques.
>> Can someone explain me why we need a copy constructor in the first
>> place, instead of just using a reference object, aka a class, and an
>> optional clone method?
>
> The problem is that null objects don't obey reference semantics. The
> example discussed in this group is:
>
> void fun(int[int] a) { a[5] = 10; }
>
> void main(string[] args) {
> int[int] x;
> if (args.length & 1) x[0] = 42;
> fun(x);
> assert(x[5] == 10);
> }
>
> The program will fail or not depending on the number of arguments passed.
>
>
> Andrei

In my notion of reference type (which you may argue is not the correct 
one, but that's a different issue) a null object cannot be used at all 
(other than checking its identity against other objects). Thus that 
program would always fail, either on 'a[5] = 10;' or 'x[0] = 42;', 
according to these semantics


-- 
Bruno Medeiros - Software Engineer