invalid offsetof from non-POD type

[Nathan Sidwell]

John Quigley wrote:
> Perhaps it would be useful if I described how the code that I'm working on 
> uses offsetof.  It is the Torque game engine from www.garagegames.com.  The 
> engine has an internal c-like scripting language.  The fields and methods of 
> the c++ classes in the engine can be exposed to this scripting language.  For 
> each field, the engine uses offsetof to compute the offset of the field in 
> the class.  Later on, when the scripts read or write to the field, the script 
> runtime locates the field by adding the offset to the base pointer of the 
> object in question.
yeah, I understand the idiom. what you really want is a pointer to void data
member. One way to get what you want is something like
#define myoff(T,m) ((size_t)(&((T *)1)->m) - 1)
that
a) circumvents g++'s invalid offsetof check
b) allows m to be from a base, and DTRT if the base is not at offset zero
c) DTWT for virtual bases and reference members

BTW, do people do things like
	offsetof (T, m.sub_m)
? I don't think it blessed by the C/c++ std, but falls out of the common
implementation of offsetof.

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
          The voices in my head said this was stupid too
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