how to compute offset of data member at compile time (virtual class)

[skaller]

On Thu, 2007-10-25 at 17:57 +0100, Andrew Haley wrote:
> John Gateley writes:
>  > I need to compute the offset of a data member at compile time.
>  > The class has virtual functions, and so the offsetof macro does
>  > not work (why?).
> 
> Because it's a non-POD type.  See Section 9, Classes:

An mistake by WG21.

>  > Is there a way to do this?
> 
> No.  Consider what happens with multiple and virtual inheritance.

It is irrelevant. Given any struct, every data member has a fixed
offset within it. The following formula always calculates it:

	typedef unsigned char uchar;
	T x;
	ptrdiff_t offsetof_m_in_T = 
		(uchar*)(void*)&x.m - (uchar*)(void*)&x
	;

provided there are no access violations and no ambiguity.
Replacing 'x' with *(T*)0 isn't conforming but will usually
work :)

Note: with virtual bases, pointer to member *casts* can
be indeterminate, but there are no casts in offsetof:
the struct type is complete.

I use offsetof() on constructible types with virtual functions
extensively in Felix to calculate the offsets of every pointer
in every heap allocated data type. Thankfully gcc has a switch
now which allows this, -Wno-invalid-offsetof, because it is the 
only way to get the information required, and the 
information is mandatory for my garbage collector. It works fine
on MSVC++ too.




-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net