Re: Nested sibling classes

[Steven Schveighoffer via Digitalmars-d-learn]

On 1/12/23 12:05 PM, seany wrote:

How can I make it, that classes b and c can access each other, and 
create instances of each other freely? Thank you.




So to just point out something that wasn't discussed by Salih:

When you declare a field of a class with an initializer, *that 
initializer is run at compile-time*. Which means, that even if it did 
work, every instance of every b would start out with the same exact `C` 
object (not a copy, the same one).


This is different than many other languages which treat initializers as 
part of the constructor (and run when you initialize a class). In D, the 
bits are simply copied into the new memory as the default state.


For this reason you should almost *never* initialize a class reference 
in a non-static field. Consider that if you ever modified that instance 
named `C`, all new instances of `b` would have a reference to that 
modified instance!


The reason the compiler doesn't like it is because it doesn't know how 
to initialize a `c` at compile time, since it needs the context pointer 
to the outer class.


Just moving initialization into the constructor should fix the problem, 
you don't need to make them static. Now, maybe you didn't intend to have 
a nested class with a reference to the outer class, and in that case, 
you should make it static.


-Steve

Re: Nested sibling classes

[Salih Dincer via Digitalmars-d-learn]

On Thursday, 12 January 2023 at 17:46:45 UTC, seany wrote:


Please, can you explain what role "static" plays here? Thank 
you again


Of course, there are actually 2 paragraphs of information and 
examples [here](

https://dlang.org/spec/class.html#nested-context):

Non-static nested classes work by containing an extra hidden 
member (called the context pointer) that is the frame pointer 
of the enclosing function if it is nested inside a function, or 
the this reference of the enclosing class's instance if it is 
nested inside a class.


When a non-static nested class is instantiated, the context 
pointer is assigned before the class's constructor is called, 
therefore the constructor has full access to the enclosing 
variables. A non-static nested class can only be instantiated 
when the necessary context pointer information is available.


SDB@79

Re: Nested sibling classes

[seany via Digitalmars-d-learn]

On Thursday, 12 January 2023 at 17:41:39 UTC, Salih Dincer wrote:

On Thursday, 12 January 2023 at 17:05:04 UTC, seany wrote:

How can I make it, that classes b and c can access each other, 
and create instances of each other freely? Thank you.


Ignoring the typos you could try auto and static:

```d
class a
{ //outer

  static class b
  { // inner 1

c C;
this()
{
  this.C = new c;
  //writeln(this.C.i);
}
  }

  static class c
  { // inner 2
int i = 10;
  }
}

int main ()
{
  int[21][1] test;
  test[0][20] = 19;

  assert(test[0][20] == 19);

  auto B = new a.b;
  auto C = new a.c;

  assert(B.C.i == 10);
  assert(C.i == 10);

  return 0;
 }
```
SDB@79


Hi

Moving the "new c" within the this() function indeed solved it. 
Thank you for pointing out the typos.


Please, can you explain what role "static" plays here? Thank you 
again

Re: Nested sibling classes

[Salih Dincer via Digitalmars-d-learn]

On Thursday, 12 January 2023 at 17:05:04 UTC, seany wrote:

How can I make it, that classes b and c can access each other, 
and create instances of each other freely? Thank you.


Ignoring the typos you could try auto and static:

```d
class a
{ //outer

  static class b
  { // inner 1

c C;
this()
{
  this.C = new c;
  //writeln(this.C.i);
}
  }

  static class c
  { // inner 2
int i = 10;
  }
}

int main ()
{
  int[21][1] test;
  test[0][20] = 19;

  assert(test[0][20] == 19);

  auto B = new a.b;
  auto C = new a.c;

  assert(B.C.i == 10);
  assert(C.i == 10);

  return 0;
 }
```
SDB@79