mutable array of immutable objects

[Jeff Thompson via Digitalmars-d]

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. I can modify 
the array but not the elements they point to. What's so special 
about a string? Why can't I do that with my own class?

void main(string[] args)
{
   auto array = new string[1];
   array[0] = "a";
}