Re: [Solved] sequence of inheritance from virtual base class?
Am 20.05.2013 12:16, schrieb Francis Glassborow:
On 20/05/2013 00:39, Wil Evers wrote:
I think the lesson to learn from this thread is that a void pointer
obtained from the address of an X must not be cast back to a
pointer to some other type, even if that other type is closely
related to X.
static_cast<Base> should convert the pointer if necessary,
reinterpret_cast<Base> can fail.
I don't think that static_cast versus reinterpret_cast is relevant in
the here discussed conversion case. The reason for the observed
problem really is (as others have said in different words) that the
language doesn't specify that the conversion sequence (all by means of
static_cast)
D* d1 -> void* v -> B* b -> D* d2
where B* and D* do have an effective offset different from zero, will
return an address value d2 that is equal to the original address value
d1. The constraint is expressed in 5.2.9 p13:
"A prvalue of type ?pointer to cv1 void? can be converted to a prvalue
of type ?pointer to cv2 T,? [..] If the original pointer value
represents the address A of a byte in memory and A satisfies the
alignment requirement of T, then the resulting pointer value
represents the same address as the original pointer value, that is,
A. The result of any other such pointer conversion is unspecified. A
value of type pointer to object converted to ?pointer to cv void? and
back, possibly with different cv-qualification, shall have its
original value."
The behavior of above sequence is undefined, because the assumption
that b points to the same address value as the original address value
of d1 is invalid.
HTH & Greetings from Bremen,
Daniel Kr?gler
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