Re: Common base class, reinterpret_cast, and aliasing
On Mar 26, 2:46 pm, guillaume.melqui...@gmail.com wrote:
On 24 mar, 17:51, Francis Glassborow wrote:
very simply, reinterpret_cast tells the compiler to do what you say and
you will accept all the consequences.
[...]
In practice it may work (in fact in the case in point usually will) but
the Standard does not support casting objects into things that they are
not.
The Standard does support it (see 5.2.10/7 for example) and it is
fortunate. What the Standard does not support is accessing an value
through a type that does not correspond to its dynamic type.
Which is what your code does.
I do get your point about static_cast though, as it indeed
appears (5.2.9/6) that the static_cast itself is undefined
behavior, when the dynamic type does not match.
Now this is interesting, because it means that the behavior is defined
or not depending on whether a pointer argument is explicit or not. I
hope everybody will agree that the following function will give a
perfectly specified behavior (5.2.10) to the above code:
int B_doB(B *this_)
{ return reinterpret_cast<A *>(this_)->get(); }
when replacing "b.doB()" with "B_doB(&b)".
Only if this_ was originally created by reinterpret_cast'ing an
A* to a B*. Otherwise, it's undefined behavior.
You seem to be missing something very fundamental in the C++
object model: typing. If an object has a type B, just about any
attempt to access it as an A is undefined behavior. There are
very few exceptions: you can read the raw bytes (and copy them
in and out in a POD), and you can navigate up and down (but not
sideways, at least not directly) in an inheritance graph. But
that's about it.
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