mutable array of immutable objects

[Timon Gehr via Digitalmars-d]

On 19.04.2016 14:07, Jeff Thompson wrote:
> On Tuesday, 19 April 2016 at 11:43:22 UTC, Anonymouse wrote:
>> On Tuesday, 19 April 2016 at 10:41:05 UTC, Jeff Thompson wrote:
>>> I want to create a mutable array of immutable objects, but the code
>>> below gives the error shown below. It seems that "new
>>> immutable(C)[1]" makes the entire array immutable, but it seems I
>>> should be able to change the elements of an array of pointers to an
>>> object even though the objects are immutable. How to do that?
>>>
>>> class C {
>>>   this(int x) immutable { this.x = x; }
>>>   int x;
>>> }
>>>
>>> void main(string[] args)
>>> {
>>>   auto array = new immutable(C)[1];
>>>   array[0] = new immutable C(10); // Error: Cannot modify immutable
>>> expression array[0].
>>> }
>>
>> Mind that this is akin to declaring a string (immutable(char)[]) and
>> trying to modify an element.
>>
>> You can append, though. Or rather, make a new array/slice with the new
>> elements concatenated into it.
>
> Thanks but then I'm confused as to why the following code is allowed. It
> is a mutable array of immutable strings.

mutable array of mutable strings, actually. immutable(string)[] would be 
a mutable array of immutable strings.

> I can modify the array but not
> the elements they point to. What's so special about a string?

string is not a class. It is an alias for immutable(char)[].

> Why can't I do that with my own class?
> ...

D has no built-in way to express it.
There is https://dlang.org/phobos/std_typecons.html#.Rebindable.