Re: [boost] low-level sub-allocation/alignment challenge

From: Greg Colvin (greg_at_[hidden])
Date: 2000-08-24 12:17:41

From: Bill Wade <bill.wade_at_[hidden]>
> > From: David Abrahams [mailto:abrahams_at_[hidden]]
> 
> > > struct Rule
> > > {
> > >     unsigned long cost;
> > >     const void* action;
> > >     const char** symbols(){ return (const char**) (&action+1); }
> > > };
> >
> > Okay, but is this still a POD?
> 
> Yes.  The addition of a non-virtual member function (other than 'tors and
> maybe assignment, I don't remember) is not enough to turn a POD struct into
> a non-POD struct.

So this is a POD too?

  struct Rule
  {
     unsigned long cost;
     const void* action;
     char& operator[](size_t i) { return ((char*)(&action+1))[i]; }
     char operator[](size_t i) const { return ((char*)(&action+1))[i]; }
 };

> > And if action were of type char instead of
> > const void*, then your symbols() array would most likely be misaligned.
> 
> Correct.  In that case you'd need to do something extra to ensure alignment.

Why?  Doesn't char have minimal alignment?