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February 11, 2011
Inheritance problem
Hello,

I've a problem with my class inheritance. I have class called Texture
which implements the interface IDrawable and the abstract class
APickable.
The Texture-class contains 3 members which looks like this:

GLuint pTextureID;

Size pSize1, pSize2;

Finally... my Texture-class looks like:

class Texture : APickable, IDrawable {
  protected {
     GLuint pTextureID;
     Size pSize, pAnoutherSize;
  }
}

now... I have a second class called Animation which looks like this:

class Animation : Texture {
  private {
     Texture[] pFrames;
  }
  public {
     this(string file, string[] paths...) {
        super(file);
        pFrames ~= this;
        foreach(string cur; paths) {
           pFrames ~= new Texture(cur);
        }
     }

     Size getSize() {
        return pFrames[0].pSize;
     }
  }
}

As I know, pFrames[0].pSize can be called... pSize in the Texture-
class is marked as protected, but I get the following error:

Error: class Texture member pSize is not accessible.

When I mark the protected members of the Texture-class as public, it
works (should be clear), but why do I get this error when mark them
as protected?

I hope anyone can help to solve the problem.
February 11, 2011
Re: Inheritance problem
On Fri, 11 Feb 2011 15:40:18 -0500, %u <unknown@unknown.com> wrote:

> Hello,
>
> I've a problem with my class inheritance. I have class called Texture
> which implements the interface IDrawable and the abstract class
> APickable.
> The Texture-class contains 3 members which looks like this:
>
> GLuint pTextureID;
>
> Size pSize1, pSize2;
>
> Finally... my Texture-class looks like:
>
> class Texture : APickable, IDrawable {
>    protected {
>       GLuint pTextureID;
>       Size pSize, pAnoutherSize;
>    }
> }
>
> now... I have a second class called Animation which looks like this:
>
> class Animation : Texture {
>    private {
>       Texture[] pFrames;
>    }
>    public {
>       this(string file, string[] paths...) {
>          super(file);
>          pFrames ~= this;
>          foreach(string cur; paths) {
>             pFrames ~= new Texture(cur);
>          }
>       }
>
>       Size getSize() {
>          return pFrames[0].pSize;
>       }
>    }
> }
>
> As I know, pFrames[0].pSize can be called... pSize in the Texture-
> class is marked as protected, but I get the following error:
>
> Error: class Texture member pSize is not accessible.
>
> When I mark the protected members of the Texture-class as public, it
> works (should be clear), but why do I get this error when mark them
> as protected?
>
> I hope anyone can help to solve the problem.

protected means you cannot access it outside the *instance*.  The pFrames  
array references *other instances* of Texture, so they are not accessible.

http://www.digitalmars.com/d/2.0/attribute.html#ProtectionAttribute

"If accessing a protected instance member through a derived class member  
function, that member can only be accessed for the object instance which  
is the ‘this’ object for the member function call."

-Steve
February 11, 2011
Re: Inheritance problem
== Auszug aus Steven Schveighoffer (schveiguy@yahoo.com)'s Artikel
> On Fri, 11 Feb 2011 15:40:18 -0500, %u <unknown@unknown.com> wrote:
> > Hello,
> >
> > I've a problem with my class inheritance. I have class called
Texture
> > which implements the interface IDrawable and the abstract class
> > APickable.
> > The Texture-class contains 3 members which looks like this:
> >
> > GLuint pTextureID;
> >
> > Size pSize1, pSize2;
> >
> > Finally... my Texture-class looks like:
> >
> > class Texture : APickable, IDrawable {
> >    protected {
> >       GLuint pTextureID;
> >       Size pSize, pAnoutherSize;
> >    }
> > }
> >
> > now... I have a second class called Animation which looks like
this:
> >
> > class Animation : Texture {
> >    private {
> >       Texture[] pFrames;
> >    }
> >    public {
> >       this(string file, string[] paths...) {
> >          super(file);
> >          pFrames ~= this;
> >          foreach(string cur; paths) {
> >             pFrames ~= new Texture(cur);
> >          }
> >       }
> >
> >       Size getSize() {
> >          return pFrames[0].pSize;
> >       }
> >    }
> > }
> >
> > As I know, pFrames[0].pSize can be called... pSize in the Texture-
> > class is marked as protected, but I get the following error:
> >
> > Error: class Texture member pSize is not accessible.
> >
> > When I mark the protected members of the Texture-class as public,
it
> > works (should be clear), but why do I get this error when mark
them
> > as protected?
> >
> > I hope anyone can help to solve the problem.
> protected means you cannot access it outside the *instance*.  The
pFrames
> array references *other instances* of Texture, so they are not
accessible.
> http://www.digitalmars.com/d/2.0/attribute.html#ProtectionAttribute
> "If accessing a protected instance member through a derived class
member
> function, that member can only be accessed for the object instance
which
> is the ‘this’ object for the member function call."
> -Steve

Thanks, but what about the following:

import std.stdio : writeln;

class a  {

	public this(int v) {
		myVar = v;
	}

	protected int myVar;

}

class b : a {

	private a[] moreInstances;

	this(int v, int[] vars...) {
		super(v);
		moreInstances ~= this;

		foreach(int cur; vars) {
			moreInstances ~= new a(cur);
		}
	}

	int getVar() {
		return moreInstances[1].myVar;
	}

}

void main(string[] args) {
	b exp = new b(0, 1, 2);
	writeln(exp.getVar());
}

This compiles fine and prints the number 1. myVar is also protected
in class a, I also call myVar in the getVar()-method of class b.
February 11, 2011
Re: Inheritance problem
On Fri, 11 Feb 2011 16:14:31 -0500, %u <unknown@unknown.com> wrote:

> == Auszug aus Steven Schveighoffer (schveiguy@yahoo.com)'s Artikel

>
> Thanks, but what about the following:
>
> import std.stdio : writeln;
>
> class a  {
>
> 	public this(int v) {
> 		myVar = v;
> 	}
>
> 	protected int myVar;
>
> }
>
> class b : a {
>
> 	private a[] moreInstances;
>
> 	this(int v, int[] vars...) {
> 		super(v);
> 		moreInstances ~= this;
>
> 		foreach(int cur; vars) {
> 			moreInstances ~= new a(cur);
> 		}
> 	}
>
> 	int getVar() {
> 		return moreInstances[1].myVar;
> 	}
>
> }
>
> void main(string[] args) {
> 	b exp = new b(0, 1, 2);
> 	writeln(exp.getVar());
> }
>
> This compiles fine and prints the number 1. myVar is also protected
> in class a, I also call myVar in the getVar()-method of class b.

Any code can access any members defined in the current module, regardless  
of access attributes (that rule is outlined in the link I sent, I just  
didn't quote that part).  You have to split this into multiple modules to  
see the other rules take effect.

-Steve
February 11, 2011
Re: Inheritance problem
== Auszug aus Steven Schveighoffer (schveiguy@yahoo.com)'s Artikel
> On Fri, 11 Feb 2011 16:14:31 -0500, %u <unknown@unknown.com> wrote:
> > == Auszug aus Steven Schveighoffer (schveiguy@yahoo.com)'s Artikel
> >
> > Thanks, but what about the following:
> >
> > import std.stdio : writeln;
> >
> > class a  {
> >
> > 	public this(int v) {
> > 		myVar = v;
> > 	}
> >
> > 	protected int myVar;
> >
> > }
> >
> > class b : a {
> >
> > 	private a[] moreInstances;
> >
> > 	this(int v, int[] vars...) {
> > 		super(v);
> > 		moreInstances ~= this;
> >
> > 		foreach(int cur; vars) {
> > 			moreInstances ~= new a(cur);
> > 		}
> > 	}
> >
> > 	int getVar() {
> > 		return moreInstances[1].myVar;
> > 	}
> >
> > }
> >
> > void main(string[] args) {
> > 	b exp = new b(0, 1, 2);
> > 	writeln(exp.getVar());
> > }
> >
> > This compiles fine and prints the number 1. myVar is also
protected
> > in class a, I also call myVar in the getVar()-method of class b.
> Any code can access any members defined in the current module,
regardless
> of access attributes (that rule is outlined in the link I sent, I
just
> didn't quote that part).  You have to split this into multiple
modules to
> see the other rules take effect.
> -Steve

Ah, okay... this means I have to Texture and Animation into one
module. Thanks a lot!
February 11, 2011
Re: Inheritance problem
Steven Schveighoffer:

> Any code can access any members defined in the current module, regardless  
> of access attributes

I am not sure if Walter understands how much this rule makes it hard for people not already used to protected/private attributes to understand and learn to use those attributes correctly. The C# compiler doesn't have that rule, and it's much more strict. I think this makes learning the usage of those attributes faster.

Bye,
bearophile
February 11, 2011
Re: Inheritance problem
== Auszug aus bearophile (bearophileHUGS@lycos.com)'s Artikel
> Steven Schveighoffer:
> > Any code can access any members defined in the current module,
regardless
> > of access attributes
> I am not sure if Walter understands how much this rule makes it
hard for people not already used to protected/private attributes to
understand and learn to use those attributes correctly. The C#
compiler doesn't have that rule, and it's much more strict. I think
this makes learning the usage of those attributes faster.
> Bye,
> bearophile

I already used protected/private (and other languages like Java
and... D too), but it was a bit unintelligible because protected
means (for me) that every instance of an inheritanced class can
access the protected members.
An alternative to protected like modprotected or something like this
should be added that people can declare it as modprotected which
means that inheritanced classes of another module can also access
these attributes.
February 11, 2011
Re: Inheritance problem
On 2/11/11, bearophile <bearophileHUGS@lycos.com> wrote:
> Steven Schveighoffer:
>
>> Any code can access any members defined in the current module, regardless
>>
>> of access attributes
>
> I am not sure if Walter understands how much this rule makes it hard for
> people not already used to protected/private attributes to understand and
> learn to use those attributes correctly. The C# compiler doesn't have that
> rule, and it's much more strict. I think this makes learning the usage of
> those attributes faster.
>
> Bye,
> bearophile
>

I think one benefit of the current approach is that we'll be able to
use free functions which could be called as if they belong to a class
(if they have that class as the first parameter), since we could use
the uniform function call (UFC) syntax. But UFC doesn't work with
classes yet, otherwise we might be able to do this:

module foo;
import std.stdio;

class Foo {
   private int _x, _y;
   this(int x, int y) {
       _x = x;
       _y = y;
   }
}

int sumXY(Foo foo) {
   return foo._x + foo._y;
}

module main;
import foo;
import std.stdio;

void main() {
   auto obj = new Foo(10, 10);
   writeln(obj.sumXY());  // using UFC, but doesn't work yet
   //~ writeln(obj._x + obj._y);  // not allowed
}

We could have a bunch of templated functions in the foo module which
could work with any class inside that module. So it might help out
against the common God class problem. What do you think?
February 11, 2011
Re: Inheritance problem
== Auszug aus Andrej Mitrovic (andrej.mitrovich@gmail.com)'s Artikel
> On 2/11/11, bearophile <bearophileHUGS@lycos.com> wrote:
> > Steven Schveighoffer:
> >
> >> Any code can access any members defined in the current module,
regardless
> >>
> >> of access attributes
> >
> > I am not sure if Walter understands how much this rule makes it
hard for
> > people not already used to protected/private attributes to
understand and
> > learn to use those attributes correctly. The C# compiler doesn't
have that
> > rule, and it's much more strict. I think this makes learning the
usage of
> > those attributes faster.
> >
> > Bye,
> > bearophile
> >
> I think one benefit of the current approach is that we'll be able to
> use free functions which could be called as if they belong to a
class
> (if they have that class as the first parameter), since we could use
> the uniform function call (UFC) syntax. But UFC doesn't work with
> classes yet, otherwise we might be able to do this:
> module foo;
> import std.stdio;
> class Foo {
>     private int _x, _y;
>     this(int x, int y) {
>         _x = x;
>         _y = y;
>     }
> }
> int sumXY(Foo foo) {
>     return foo._x + foo._y;
> }
> module main;
> import foo;
> import std.stdio;
> void main() {
>     auto obj = new Foo(10, 10);
>     writeln(obj.sumXY());  // using UFC, but doesn't work yet
>     //~ writeln(obj._x + obj._y);  // not allowed
> }
> We could have a bunch of templated functions in the foo module which
> could work with any class inside that module. So it might help out
> against the common God class problem. What do you think?

Looks really interesting - would be a nice feature :)
February 11, 2011
Re: Inheritance problem
On 02/11/2011 11:26 PM, Andrej Mitrovic wrote:
> On 2/11/11, bearophile<bearophileHUGS@lycos.com>  wrote:
>> Steven Schveighoffer:
>>
>>> Any code can access any members defined in the current module, regardless
>>>
>>> of access attributes
>>
>> I am not sure if Walter understands how much this rule makes it hard for
>> people not already used to protected/private attributes to understand and
>> learn to use those attributes correctly. The C# compiler doesn't have that
>> rule, and it's much more strict. I think this makes learning the usage of
>> those attributes faster.
>>
>> Bye,
>> bearophile
>>
>
> I think one benefit of the current approach is that we'll be able to
> use free functions which could be called as if they belong to a class
> (if they have that class as the first parameter), since we could use
> the uniform function call (UFC) syntax. But UFC doesn't work with
> classes yet, otherwise we might be able to do this:
>
> module foo;
> import std.stdio;
>
> class Foo {
>      private int _x, _y;
>      this(int x, int y) {
>          _x = x;
>          _y = y;
>      }
> }
>
> int sumXY(Foo foo) {
>      return foo._x + foo._y;
> }
>
> module main;
> import foo;
> import std.stdio;
>
> void main() {
>      auto obj = new Foo(10, 10);
>      writeln(obj.sumXY());  // using UFC, but doesn't work yet
>      //~ writeln(obj._x + obj._y);  // not allowed
> }
>
> We could have a bunch of templated functions in the foo module which
> could work with any class inside that module. So it might help out
> against the common God class problem. What do you think?

Interesting. UFC syntax is also criticised (don't remmber on which points 
exactly), but it brings with nice side-features.

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
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