Question

Is the following well-defined:

char* charPtr = new char[42];
int* intPtr = (int*)charPtr;

charPtr++;
intPtr = (int*) charPtr;

The intPtr isn’t properly aligned (in at least one of the two cases). Is it illegal just having it there? Is it UB using it at any stage? How can you use it and how can’t you?

Answer 1

First, of course: the pointer is guaranteed to be aligned in the
first case (by §5.3.4/10 and §3.7.4.1/2), and may be correctly
aligned in both cases. (Obviously, if sizeof(int) == 1, but
even when this is not the case, an implementation doesn’t
necessarily have alignment requirements.)

And to make things clear: your casts are all reinterpret_cast.

Beyond that, this is an interesting question, because as far as
I can tell, there is no difference in the two casts, as far as
the standard is concerned. The results of the conversion are
unspecified (according to §5.2.10/7); you’re not even guaranteed
that converting it back into a char* will result in the
original value. (It obviously won’t, for example, on machines
where int* is smaller than a char*.)

In practice, of course: the standard requires that the return
value of new char[N] be sufficiently aligned for any value
which may fit into it, so you are guaranteed to be able to do:

intPtr = new (charPtr) int;

Which has exactly the same effect as your cast, given that the
default constructor for int is a no-op. (And assuming that
sizeof(int) <= 42.) So it’s hard to imagine an implementation
in which the first part fails. You should be able to use the
intPtr just like any other legally obtained intPtr. And the
idea that converting it back to a char* would somehow result
in a different value from the original char* seems
preposterous.

In the second part, all bets are off: you definitely can’t
dereference the pointer (unless your implementation guarantees
otherwise), and it’s also quite possible that converting it back
to char* results in something different. (Imagine a word
addressed machine, for example, where converting a char* to an
int* rounds up. Then converting back would result in
a char* which was sizeof(int) higher than the original. Or
where an attempt to convert a misaligned pointer always resulted
in a null pointer.)