Re: using offsetof with g++

[Joe Buck <jbuck at synopsys dot com>]

On Thu, Nov 06, 2003 at 09:23:07PM +0100, Gabriel Dos Reis wrote:
> | > I don't know where you put my favorite example that involve
> | > references:

I wrote:
> | Case 4 (it will do the wrong thing).
 
> Well, I can see reasons why it does not fit in that category.  The
> compiler is computing the wrong answer not because the question does
> not make sense, but just because the compiler has an erroneous
> implementation of offsetof.  See the various PRs we have about it.

I was referring to the traditional implementation of offsetof.
 
>   Sure there is no requirement that a reference "exists" in A.  But as
> a matter of fact, it does exists in form of a pointer in GCC
> implementation.  And in nearly all cases, in other implementations, it
> does exist in one form or the other.  The offset of that incanartion
> is constant.

Yes, but that's at the implementation level.  But would traditional
uses of offsetof work correctly if this case were implemented "correctly"?
Maybe, maybe not.  If the code wants to write to a range of character
starting from the address of the object, plus the offset, with a width
determined by the size of the object, you've got another problem:
sizeof(reference) returns the size of the referenced object.  Also,
the user may expect the referenced object to be accessed, or s/he may
expect the reference itself to be accessed.

So, if you propose defining offsetof() for reference fields, you have
the problem that you need to change the definition of sizeof().