Re: [Boost-users] [Review] Formal Review: Boost.Move

Subject: Re: [Boost-users] [Review] Formal Review: Boost.Move
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2010-05-23 08:06:44

On 23/05/2010 11:15, Neil Groves wrote:

> I convinced myself that with static_casts do not break strict aliasing
> rules since the conversion from rv<T>* to T* is valid. I believe that
> section 5.2.9 para 2 applies:

The problem is not downcasting, but the fact that T is not rv<T> (the 
dynamic type of T is T, not rv<T>):

"If the object of type “cv1 B” is actually a subobject of an object of 
type D, the result refers to the enclosing object of type D. Otherwise, 
the result of the cast is undefined."

When we use static_cast<rv<T>>, the object T is not actually a subobject 
of rv<T>, so I think we're in undefined behaviour. Imagine this:

class B
{
   int filler;
};

class D : public B
{ int filler2;}

B b;
D *dp = static_cast<D*>(&b);

//dp is now pointing to garbage

Best,

Ion