Re: [boost] aligned_storage in unions

Subject: Re: [boost] aligned_storage in unions
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-09-22 15:49:34

On 09/22/10 14:04, Frank Mori Hess wrote:
> On Wednesday 22 September 2010, Larry Evans wrote:
[snip]
>> Rethought that answer because I don't (yet)
>> see where pointers to the 2 different types
>> exist at the same time in the operator= for
> 
> I don't see how that gets around the anti-aliasing rules.  The compiler is 
> allowed to assume incompatible pointers won't _ever_ alias.  So just because 
> your program does things in a certain order doesn't mean the compiler isn't 
> allowed to use the anti-aliasing assumptions to reorder things in a nasty 
> way.  Couldn't it look at the destruction of the old object and the 
> construction of the new object as two completely independent actions it is 
> free to reorder or interleave?
> 
I hadn't thought of the compiler reordering the calls.
Just to be clear, the compiler, using the anti-aliasing rules,
could reorder:

  316	        destroy();
  317	        assign_copy(from);

to:

  317	        assign_copy(from);
  316	        destroy();

?  Or maybe just do everything in destroy before the lhs->~Lhs() call,
then execute assign_copy(from) and then do lhs->~Lhs().  Hmm...
OK, I think I got it, but am surprised.

So now, using the C++0x non-pod in union capability; however, would one
generate the attribute for Spirit's alternative parser (a | b):

http://www.boost.org/doc/libs/1_44_0/libs/spirit/doc/html/spirit/qi/quick_reference/compound_attribute_rules.html

?  Maybe one could do what fusion does for vector, i.e. using
boost/preprocessory, generates:

  template<T1,T2,...,Tn>
  struct vector_n
  {
      T1 m1;
      T2 m2;
      ...
      Tn mn;
  };

Only for the union, it would be:

  template<T1,T2,...,Tn>
  struct union_n
  {
      union
      {
        T1 m1;
        T2 m2;
        ...
        Tn mn;
      } u;
  };

Or something like that?
Otherwise, I don't see a workaround to this strict aliasing
problem :(

-Larry