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digitalmars.D.learn - Error: 'this' is only defined in non-static member functions

reply A Guy With a Question <aguywithaquestion gmail.com> writes:
I have an interface where I have a classes embedded in it's scope 
(trying to create something for the classes that implement the 
interface can use for unittesting).

     interface IExample
     {
         // stuff ...
         class Tester
         {

         }
     }

I'm trying to make an instance of it like:

     auto tester = new IExample.Tester();

And I get the above error.
Nov 22 2017
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 11/22/17 5:19 PM, A Guy With a Question wrote:

 I have an interface where I have a classes embedded in it's scope 
 (trying to create something for the classes that implement the interface 
 can use for unittesting).
 
      interface IExample
      {
          // stuff ...
          class Tester
          {
 
          }
      }
 
 I'm trying to make an instance of it like:
 
      auto tester = new IExample.Tester();
 
 And I get the above error.


Hm... not sure how this works for inner classes of Interfaces, but if 
it's anything like inner classes of classes, then they have a hidden 
'outer' pointer pointing at the owning instance of the class.

This allows access to the outer class's members. So you need an instance 
to instantiate.

I bet it's the same for interfaces.

What you probably want is a static class, which will remove that connection:

static class Tester
...

-Steve
Nov 22 2017
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 11/22/17 5:36 PM, Steven Schveighoffer wrote:

 This allows access to the outer class's members. So you need an instance 
 to instantiate.
 
 I bet it's the same for interfaces.


All that being said, the error message is quite lousy.

-Steve
Nov 22 2017
parent reply A Guy With a Question <aguywithaquestion gmail.com> writes:
On Wednesday, 22 November 2017 at 22:37:46 UTC, Steven 
Schveighoffer wrote:

 On 11/22/17 5:36 PM, Steven Schveighoffer wrote:

 This allows access to the outer class's members. So you need 
 an instance to instantiate.
 
 I bet it's the same for interfaces.


 All that being said, the error message is quite lousy.

 -Steve


Yup that worked. Thanks!

Out of curiosity, what does static mean in that context? When I 
think of a static class I think of them in the context of Java or 

namespaces that you can't directly import the contents of.
Nov 22 2017
next sibling parent A Guy With a Question <aguywithaquestion gmail.com> writes:
On Wednesday, 22 November 2017 at 22:45:53 UTC, A Guy With a 
Question wrote:

 On Wednesday, 22 November 2017 at 22:37:46 UTC, Steven 
 Schveighoffer wrote:

 On 11/22/17 5:36 PM, Steven Schveighoffer wrote:

 This allows access to the outer class's members. So you need 
 an instance to instantiate.
 
 I bet it's the same for interfaces.


 All that being said, the error message is quite lousy.

 -Steve


 Yup that worked. Thanks!

 Out of curiosity, what does static mean in that context? When I 
 think of a static class I think of them in the context of Java 

 like namespaces that you can't directly import the contents of.


Clearly static has a slightly different meaning in D.
Nov 22 2017
prev sibling next sibling parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Wednesday, November 22, 2017 22:45:53 A Guy With a Question via 
Digitalmars-d-learn wrote:

 On Wednesday, 22 November 2017 at 22:37:46 UTC, Steven

 Schveighoffer wrote:

 On 11/22/17 5:36 PM, Steven Schveighoffer wrote:

 This allows access to the outer class's members. So you need
 an instance to instantiate.

 I bet it's the same for interfaces.


 All that being said, the error message is quite lousy.

 -Steve


 Yup that worked. Thanks!

 Out of curiosity, what does static mean in that context? When I
 think of a static class I think of them in the context of Java or

 namespaces that you can't directly import the contents of.


A static, nested class does not have access to the class that it's in. It's
basically a normal class that's namespaced within another class, whereas
non-static, nested class is associated with a specific instance of the class
and has access to that class instance via its outer member.

https://dlang.org/spec/class.html#nested
http://ddili.org/ders/d.en/nested.html

Similarly, if a struct, class, or function is nested within a function, and
it is not marked with static, then it has access to the function that it's
in, whereas if it's marked with static, then it acts more like a normal
struct/class/function and does not have access to the function that it's in
and is just namespaced within that function.

- Jonathan M Davis
Nov 22 2017
parent reply A Guy With a Question <aguywithaquestion gmail.com> writes:
 here as non-static, nested class is associated with a specific 
 instance of the class and has access to that class instance via 
 its outer member.

 - Jonathan M Davis


Hmmm...now you have me very intrigued. What is a use-case where 
you'd want to use a non-static embedded class? Sorry if I'm 
asking too many questions. But there's a lot to dig into with 
this language.
Nov 22 2017
parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Thursday, November 23, 2017 00:17:46 A Guy With a Question via 
Digitalmars-d-learn wrote:

 here as non-static, nested class is associated with a specific
 instance of the class and has access to that class instance via
 its outer member.

 - Jonathan M Davis


 Hmmm...now you have me very intrigued. What is a use-case where
 you'd want to use a non-static embedded class? Sorry if I'm
 asking too many questions. But there's a lot to dig into with
 this language.


It would make sense with something like the nodes of a linked list if they
needed access to the container for some reason. Pretty much any case where a
an instance of a nested class is going to be associated with a specific
instance of its parent class and needs access to it would be a canditate.
It's not that uncommon to see cases in C++ or Java where you'd pass a
pointer to the "parent" to an instance of a nested class when it's created,
and having outer built-in is kind of like that.

Personally, I've never had a use for it. I don't even use classes much in D,
since I rarely need inheritance. And as I understand it, most D programs
don't use classes very heavily for that very reason. So, I have no idea how
common it is to use nested classes in this manner, but I expect that someone
has found it useful at some point.

I thought that this meaning of static for nested classes came from Java, but
it's been a while since I've done much with Java, so I don't know.

- Jonathan M Davis
Nov 22 2017
next sibling parent Jacob Carlborg <doob me.com> writes:
On 2017-11-23 01:35, Jonathan M Davis wrote:


 It would make sense with something like the nodes of a linked list if they
 needed access to the container for some reason. Pretty much any case where a
 an instance of a nested class is going to be associated with a specific
 instance of its parent class and needs access to it would be a canditate.
 It's not that uncommon to see cases in C++ or Java where you'd pass a
 pointer to the "parent" to an instance of a nested class when it's created,
 and having outer built-in is kind of like that.
 
 Personally, I've never had a use for it. I don't even use classes much in D,
 since I rarely need inheritance. And as I understand it, most D programs
 don't use classes very heavily for that very reason. So, I have no idea how
 common it is to use nested classes in this manner, but I expect that someone
 has found it useful at some point.
 
 I thought that this meaning of static for nested classes came from Java, but
 it's been a while since I've done much with Java, so I don't know.


Yeah, it's very similar in D and Java.

Another example that comes from Java (before version 8) is to have a 
class with a nested anonymous class that implements an interface. It's 
very common in Java for event handling or similar actions. Other 
languages like D or Java 8 would use a delegate/lambda for the same 
thing. Something like:

class WindowController
{
     Button button;
     Window window;

     this()
     {
         button = new Button;
         window = new Window;
         button.onClick = new class() Clickable {
             window.close();
         };
     }
}

The anonymous class could have been a named class as well and to be able 
to access the "window" instance variable of the controller it needs to 
have access to the outer context and cannot be static.

-- 
/Jacob Carlborg
Nov 23 2017
prev sibling parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 11/22/17 7:35 PM, Jonathan M Davis wrote:

 On Thursday, November 23, 2017 00:17:46 A Guy With a Question via
 Digitalmars-d-learn wrote:

 here as non-static, nested class is associated with a specific
 instance of the class and has access to that class instance via
 its outer member.

 - Jonathan M Davis


 Hmmm...now you have me very intrigued. What is a use-case where
 you'd want to use a non-static embedded class? Sorry if I'm
 asking too many questions. But there's a lot to dig into with
 this language.


 
 It would make sense with something like the nodes of a linked list if they
 needed access to the container for some reason. Pretty much any case where a
 an instance of a nested class is going to be associated with a specific
 instance of its parent class and needs access to it would be a canditate.
 It's not that uncommon to see cases in C++ or Java where you'd pass a
 pointer to the "parent" to an instance of a nested class when it's created,
 and having outer built-in is kind of like that.


The intent is exactly for porting Java code that uses either anonymous 
classes or inner classes.

-Steve
Nov 24 2017
prev sibling parent reply Mike Parker <aldacron gmail.com> writes:
On Wednesday, 22 November 2017 at 22:45:53 UTC, A Guy With a 
Question wrote:


 Out of curiosity, what does static mean in that context? When I 
 think of a static class I think of them in the context of Java 

 like namespaces that you can't directly import the contents of.


Actually, they work exactly like Java's static nested classes in 
this context.


class OuterClass {
     ...
     static class StaticNestedClass {
         ...
     }
}

OuterClass.StaticNestedClass nestedObject =
      new OuterClass.StaticNestedClass();

https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html
Nov 22 2017
parent Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Thursday, November 23, 2017 00:49:33 Mike Parker via Digitalmars-d-learn 
wrote:

 On Wednesday, 22 November 2017 at 22:45:53 UTC, A Guy With a

 Question wrote:

 Out of curiosity, what does static mean in that context? When I
 think of a static class I think of them in the context of Java

 like namespaces that you can't directly import the contents of.


 Actually, they work exactly like Java's static nested classes in
 this context.


 class OuterClass {
      ...
      static class StaticNestedClass {
          ...
      }
 }

 OuterClass.StaticNestedClass nestedObject =
       new OuterClass.StaticNestedClass();

 https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html


I thought that that was the case, but my Java is so rusty that I wasn't
sure.

- Jonathan M Davis
Nov 22 2017