Preserving class member-variable order?

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3 total messages Started by Michael Thu, 07 Aug 2003 23:12

#714

Author: Michael
Date: Thu, 07 Aug 2003 23:12

31 lines
2285 bytes

Hi,

I'm dev'ing in C++ using Codewarrior 8.3, on OS 9.2 & X.

I'm trying to find out if there's a way to preserve the order of class
member variables, ie the order in which they are listed in the class
declaration. I'm hoping to be able to reference a class member indirectly by
deref'ing another class member that is a known distance away. I've done this
before on Windows (VC++) and it works nicely, but the same approach doesn't
seem to work in OS 9 (or maybe because of the CW compiler?). I haven't tried
yet on OS X but I imagine it might be the same on OS 9?

A simple example is below. This simple example might work, but I've been
trying to get it to work in a much larger class. I haven't tested
extensively, thinking that someone might have a quick answer for me.

class MyClass{
    char interestingValue;
    char placeHolder;
}

MyClass obj;

char val = *(&(obj.placeHolder)-1);

I was thinking there might be a compiler option or pragma I could use to fix
the order of the members during compilation. Does anyone have any
suggestions?

Many thanks,
Michael

Re: Preserving class member-variable order?

#715

Author: John Harrison
Date: Fri, 08 Aug 2003 08:18

52 lines
2649 bytes

"Michael" <mstauff@bellatlantic.net> wrote in message
news:r1BYa.1145$014.1012@nwrdny02.gnilink.net...
> Hi,
>
> I'm dev'ing in C++ using Codewarrior 8.3, on OS 9.2 & X.
>
> I'm trying to find out if there's a way to preserve the order of class
> member variables, ie the order in which they are listed in the class
> declaration. I'm hoping to be able to reference a class member indirectly
by
> deref'ing another class member that is a known distance away. I've done
this
> before on Windows (VC++) and it works nicely, but the same approach
doesn't
> seem to work in OS 9 (or maybe because of the CW compiler?). I haven't
tried
> yet on OS X but I imagine it might be the same on OS 9?
>
> A simple example is below. This simple example might work, but I've been
> trying to get it to work in a much larger class. I haven't tested
> extensively, thinking that someone might have a quick answer for me.
>
> class MyClass{
>     char interestingValue;
>     char placeHolder;
> }
>
> MyClass obj;
>
> char val = *(&(obj.placeHolder)-1);
>
> I was thinking there might be a compiler option or pragma I could use to
fix
> the order of the members during compilation. Does anyone have any
> suggestions?
>
> Many thanks,
> Michael
>

I don't think you need any pragmas or compiler options.

#include <stddef.h>

char val = *(&(obj.placeHolder) - (offsetof(MyClass, placeHolder) -
offsetof(MyClass, interestingValue)));

offsetof is a macro defined in <stddef.h>.There are some restrictions on
what sort of class you can use offsetof with.

john

Re: Preserving class member-variable order?

#724

Author: Michael
Date: Mon, 11 Aug 2003 18:40

61 lines
3129 bytes

"John Harrison" <john_andronicus@hotmail.com> wrote in message
news:bgvm8q$smqin$1@ID-196037.news.uni-berlin.de...
>
> "Michael" <mstauff@bellatlantic.net> wrote in message
> news:r1BYa.1145$014.1012@nwrdny02.gnilink.net...
> > Hi,
> >
> > I'm dev'ing in C++ using Codewarrior 8.3, on OS 9.2 & X.
> >
> > I'm trying to find out if there's a way to preserve the order of class
> > member variables, ie the order in which they are listed in the class
> > declaration. I'm hoping to be able to reference a class member
indirectly
> by
> > deref'ing another class member that is a known distance away. I've done
> this
> > before on Windows (VC++) and it works nicely, but the same approach
> doesn't
> > seem to work in OS 9 (or maybe because of the CW compiler?). I haven't
> tried
> > yet on OS X but I imagine it might be the same on OS 9?
> >
> > A simple example is below. This simple example might work, but I've been
> > trying to get it to work in a much larger class. I haven't tested
> > extensively, thinking that someone might have a quick answer for me.
> >
> > class MyClass{
> >     char interestingValue;
> >     char placeHolder;
> > }
> >
> > MyClass obj;
> >
> > char val = *(&(obj.placeHolder)-1);
> >
> > I was thinking there might be a compiler option or pragma I could use to
> fix
> > the order of the members during compilation. Does anyone have any
> > suggestions?
> >
> > Many thanks,
> > Michael
> >
>
> I don't think you need any pragmas or compiler options.
>
> #include <stddef.h>
>
> char val = *(&(obj.placeHolder) - (offsetof(MyClass, placeHolder) -
> offsetof(MyClass, interestingValue)));
>
> offsetof is a macro defined in <stddef.h>.There are some restrictions on
> what sort of class you can use offsetof with.
>
> john

Thanks John! I'll look into that.

Cheers,
Michael

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