Re: strict aliasing

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Wed, Nov 07, 2007 at 05:22:01AM +1100, skaller wrote:
> > One way to do this in C++ is to derive the different representations that
> > might appear in your "union" from a common base class, and use placement
> > new to lay them out. 
> 
> I don't understand. You cannot put ANY constructable types
> in a union.

That's why I said "union", with the quotes.  We're not talking
about a union keyword, we're talking about how to get the
effect you want, legally, as opposed to the illegal way that
you are doing it.

If you have a pointer or reference to Base, the object might
really be any class derived from Base.  With placement new,
you make a buffer big enough to hold the object, you can
then construct an object of the right type there.  This kind
of code could be auto-generated by your compiler and would
be quick and type-safe.  The actual type of the storage
is an array of char, which by the rules of the language may
alias to any type.

But this is off-topic.