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digitalmars.D.learn - Accessing outer class attribute from inner struct

reply Andre Pany <andre s-e-a-p.de> writes:
Hi,

I build some framework to access Delphi components from D.
Delphi supports property array access "StringGrid1.Columns[2]" 
which is translated in Delphi to a private method call 
"GetColumn(2)".

I need to imitate this behavior in my D code.
Therefore my TCustomGrid class has a inner struct ColumnsArray 
with an opIndex.
While accessing opIndex I need to call a DLL method. Therefore I 
need the "reference" attribute which is available in my 
TCustomGrid via inheritance.

To make my question short:) If ColumnsArray is a class I can 
access the attribute "reference" but not if it is a struct. I 
would rather prefer a struct, but with a struct
it seems I cannot access "reference".

How can I access "reference" from my inner struct?

class TCustomGrid: TCustomPresentedScrollBox
{	
	struct ColumnsArray
	{
		TColumn opIndex(int index)
		{
			int r = getIntegerIndexedPropertyReference(reference, 
"Columns", index);
			return new TColumn(r);
		}
	}
	
	ColumnsArray Columns;
...
}

Kind regards
André
Aug 28 2017
parent reply Moritz Maxeiner <moritz ucworks.org> writes:
On Monday, 28 August 2017 at 21:52:58 UTC, Andre Pany wrote:

 [...]

 To make my question short:) If ColumnsArray is a class I can 
 access the attribute "reference" but not if it is a struct. I 
 would rather prefer a struct, but with a struct
 it seems I cannot access "reference".

 How can I access "reference" from my inner struct?

 [...]


Add an explicit class reference member to to it:
---
class TCustomGrid: TCustomPresentedScrollBox
{
	struct ColumnsArray
	{
		TCustomGrid parent;

		TColumn opIndex(int index)
		{
			int r = getIntegerIndexedPropertyReference(reference, 
"Columns", index);
			return new TColumn(r);
		}
	}
	
	ColumnsArray Columns;

	this()
	{
		Columns = ColumnsArray(this);
	}
...
}
---

Nesting structs inside anything other than functions[1] is for 
visibility/protection encapsulation and namespacing only.

[1] non-static structs in functions are special as they have 
access to the surrounding stack frame
Aug 28 2017
parent reply Andre Pany <andre s-e-a-p.de> writes:
On Monday, 28 August 2017 at 22:28:18 UTC, Moritz Maxeiner wrote:

 On Monday, 28 August 2017 at 21:52:58 UTC, Andre Pany wrote:

 [...]

 To make my question short:) If ColumnsArray is a class I can 
 access the attribute "reference" but not if it is a struct. I 
 would rather prefer a struct, but with a struct
 it seems I cannot access "reference".

 How can I access "reference" from my inner struct?

 [...]


 Add an explicit class reference member to to it:
 ---
 class TCustomGrid: TCustomPresentedScrollBox
 {
 	struct ColumnsArray
 	{
 		TCustomGrid parent;

 		TColumn opIndex(int index)
 		{
 			int r = getIntegerIndexedPropertyReference(reference, 
 "Columns", index);
 			return new TColumn(r);
 		}
 	}
 	
 	ColumnsArray Columns;

 	this()
 	{
 		Columns = ColumnsArray(this);
 	}
 ...
 }
 ---

 Nesting structs inside anything other than functions[1] is for 
 visibility/protection encapsulation and namespacing only.

 [1] non-static structs in functions are special as they have 
 access to the surrounding stack frame


Unfortunately thats not possible. ColumnsArray and the attribute 
will become a string mixin to avoid boilerplate.

It would be error prone if I have to initialize them in the 
constructor too. I want just 1 single coding line for this 
property. That is also the reason I do not want to use a class, 
as I would have to initialize them in the constructor.

Kind regards
André
Aug 28 2017
parent reply Moritz Maxeiner <moritz ucworks.org> writes:
On Monday, 28 August 2017 at 22:47:12 UTC, Andre Pany wrote:

 On Monday, 28 August 2017 at 22:28:18 UTC, Moritz Maxeiner 
 wrote:

 On Monday, 28 August 2017 at 21:52:58 UTC, Andre Pany wrote:

 [...]

 To make my question short:) If ColumnsArray is a class I can 
 access the attribute "reference" but not if it is a struct. I 
 would rather prefer a struct, but with a struct
 it seems I cannot access "reference".

 How can I access "reference" from my inner struct?

 [...]


 Add an explicit class reference member to to it:
 ---
 class TCustomGrid: TCustomPresentedScrollBox
 {
 	struct ColumnsArray
 	{
 		TCustomGrid parent;

 		TColumn opIndex(int index)
 		{
 			int r = getIntegerIndexedPropertyReference(reference, 
 "Columns", index);
 			return new TColumn(r);
 		}
 	}
 	
 	ColumnsArray Columns;

 	this()
 	{
 		Columns = ColumnsArray(this);
 	}
 ...
 }
 ---

 Nesting structs inside anything other than functions[1] is for 
 visibility/protection encapsulation and namespacing only.

 [1] non-static structs in functions are special as they have 
 access to the surrounding stack frame


 Unfortunately thats not possible. ColumnsArray and the 
 attribute will become a string mixin to avoid boilerplate.

 It would be error prone if I have to initialize them in the 
 constructor too. I want just 1 single coding line for this 
 property. That is also the reason I do not want to use a class, 
 as I would have to initialize them in the constructor.


---
class C
{
     struct S
     {
     }
     S s;
}
---

is semantically equivalent to

---
struct S
{
}

class C
{
     S s;
}
---

with the two differences being
- namespacing (outside of C one has to use C.S to access S)
- you can protect the visibility of the S from outside the module 
C resides in via private,public, etc.

In both cases S doesn't inherently how about C, which means a 
solution using default initialization is not feasible, as S.init 
can't know about any particular instance of C.
I don't think there's any way for you to avoid using a class 
constructor.
Aug 28 2017
parent reply Andre Pany <andre s-e-a-p.de> writes:
On Monday, 28 August 2017 at 23:12:40 UTC, Moritz Maxeiner wrote:

 In both cases S doesn't inherently how about C, which means a 
 solution using default initialization is not feasible, as 
 S.init can't know about any particular instance of C.
 I don't think there's any way for you to avoid using a class 
 constructor.


Thanks for the explanation. I now tried to use a class and use a 
static opIndex. But it seems from a static method you also cannot 
access the attributes of a outer class :)

class TCustomGrid: TCustomPresentedScrollBox
{	
	class ColumnsArray
	{
		static TColumn opIndex(int index)
		{
			// Reference is defined in TCustomGrid via inheritene
			int r = getIntegerIndexedPropertyReference(reference, 
"Columns", index);
			return new TColumn(r);
		}
	}
	
	alias Columns = ColumnsArray;
...
}

This seems like an unnecessary limitation...

Kind regards
André
Aug 29 2017
parent reply Moritz Maxeiner <moritz ucworks.org> writes:
On Tuesday, 29 August 2017 at 07:59:40 UTC, Andre Pany wrote:

 On Monday, 28 August 2017 at 23:12:40 UTC, Moritz Maxeiner 
 wrote:

 In both cases S doesn't inherently how about C, which means a 
 solution using default initialization is not feasible, as 
 S.init can't know about any particular instance of C.
 I don't think there's any way for you to avoid using a class 
 constructor.


 Thanks for the explanation. I now tried to use a class and use 
 a static opIndex. But it seems from a static method you also 
 cannot access the attributes of a outer class :)


A nested class' outer property (when nested inside another class) 
is a class reference, which means we not only require a class 
instance of the outer class to reference, but also a class 
instance of the nested class to store said class reference to the 
other class in.
A static class method (by definition) is invoked without a class 
instance.
The two are inherently incompatible.


 [...]

 This seems like an unnecessary limitation...


I can only recommend reading the language specification w.r.t, 
nested classes [1] if it seems that way to you, because it is not.

[1] https://dlang.org/spec/class.html#nested
Aug 29 2017
parent Andre Pany <andre s-e-a-p.de> writes:
On Tuesday, 29 August 2017 at 08:30:24 UTC, Moritz Maxeiner wrote:

 On Tuesday, 29 August 2017 at 07:59:40 UTC, Andre Pany wrote:

 On Monday, 28 August 2017 at 23:12:40 UTC, Moritz Maxeiner 
 wrote:

 In both cases S doesn't inherently how about C, which means a 
 solution using default initialization is not feasible, as 
 S.init can't know about any particular instance of C.
 I don't think there's any way for you to avoid using a class 
 constructor.


 Thanks for the explanation. I now tried to use a class and use 
 a static opIndex. But it seems from a static method you also 
 cannot access the attributes of a outer class :)


 A nested class' outer property (when nested inside another 
 class) is a class reference, which means we not only require a 
 class instance of the outer class to reference, but also a 
 class instance of the nested class to store said class 
 reference to the other class in.
 A static class method (by definition) is invoked without a 
 class instance.
 The two are inherently incompatible.


 [...]

 This seems like an unnecessary limitation...


 I can only recommend reading the language specification w.r.t, 
 nested classes [1] if it seems that way to you, because it is 
 not.

 [1] https://dlang.org/spec/class.html#nested


I think I found a solution which fulfills all goals but I have to 
try it out. A property method "Columns" will return an 
initialized struct "ColumnsArray" as proposed by you.
This should nicely work as string mixin. Thanks for your help.

Kind regards
André
Aug 29 2017