Re: 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.

Re: mutable array of immutable objects

[Sönke Ludwig via Digitalmars-d]

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);

Re: 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";
}

Re: mutable array of immutable objects

[Anonymouse via Digitalmars-d]

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.