Re: Is this portable? [static pointer casts, char* arithmetic]
* SG:
I can't work around this ugly char* pointer arithmetic. Maybe you
can. If you can my hat is off to you. I was thinking of alternatives
but haven't come up with working ones so far.
For example, if C++ provided a feature (magic library function) that
lets me adjust a void pointer so that the following two pieces are
equivalent:
1. A* pa = ...;
B* pb = pa; // A derives from B
2. void adjust(void *&, typeinfo const& from, typeinfo const&
to);
void adjust(void const*&, typeinfo const& from, typeinfo const&
to);
A* pa = ...;
void* pvoid = pa;
adjust(pvoid,typeid(A*),typeid(B*));
B* pb = static_cast<B*>(pvoid);
then I wouldn't need this char* arithmetic anymore. The difference
between (1) and (2) is that (2) can be used with typeinfo objects that
are only known at runtime. I could use (2) because I don't know
anymore that typeid(*paw) is a concrete_wrapper<A> but I could get the
void* and typeinfo object through the type-agnostic abstract wrapper
interface.
As I wrote I think redesign, so that the required information is directly
available, is better, and by that I mean simpler and more efficient.
But for the currently present pointer adjustment, have you considered a member
pointer, since, as your comment stated, it's a pointer to a member? ;-)
C++ member pointers are not really pointers: they're more like offsets,
automating the kind of trickery that you attempted to do via casts.
It won't work when the "member" is an element of an array, and you also need a
typed full object pointer.
But chances are that that's what you have.
Cheers & hth.,
- Alf
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