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
