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].
}

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.

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";
}

Am 19.04.2016 um 12:41 schrieb Jeff Thompson:
> 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].
> }
>

Due to an omission in the type system, this requires the use of the helper type std.typecons.Rebindable:

    auto array = new Rebindable!(immutable(C))[1];
    array[0] = new immutable C(10);

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.