Re: understanding strict aliasing

From:
Joshua Maurice <joshuamaurice@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Mon, 22 Nov 2010 00:13:31 CST
Message-ID:
<6d6e9309-5e43-425c-b289-fb6b16d6ecf2@j1g2000vbl.googlegroups.com>
On Nov 20, 5:22 pm, Alberto Griggio <alberto.grig...@gmail.com> wrote:

Ok, let's see if I am understanding you correctly then.
Given what you write above (and after having read also the relevant parts of
the standard again) and your previous example, if I understood everything
correctly, then this should work:

     Pool()
     {
          chunks = new Chunk();
          char *start = chunks->mem;
          head = new (start) Link();

          char *last = &(start[(4-1) * sizeof(Link)]);
          for (char *p = start; p < last; p += sizeof(Link)) {
              Link *n = new (p + sizeof(Link)) Link();
              reinterpret_cast<Link *>(p)->next = n;
          }
          reinterpret_cast<Link *>(last)->next = 0;
     }

     void *alloc()
     {
          Link *p = head;
          head = head->next;
          return p;
     }

     void free(void *p)
     {
         Link *l = new (p) Link();
         l->next = head;
         head = l;
     }

When Pool::free is called, p is a pointer to a memory block that must be
freed, and therefore it will not be accessed from the rest of the program
before a subsequent call to Pool::alloc. Therefore, p is not aliased
anywhere else in the program. Then, I'm starting the life of the POD Link
object by using a placement new. Therefore, it is safe to access l->next in
Pool::free. A similar situtation occurs also in Pool::alloc, since I've used
a placement new in Pool::Pool() for setting up the linked list.
Or am I still wrong?


If you post some complete code, I might be better able to determine
it's correctness. Are you using the same definitions of chunk and
link?

At the very least, you still have alignment problems for a generic
implementation, which is undefined behavior, which could cause it to
crash. It's possible that you could have a data type, say double,
which requires a certain alignment, and that char array member sub-
object may not be correctly aligned to hold a double object.

I'm not actually sure the best way offhand to deal with the alignment
problem. I would probably first find out what the biggest basic data
type is, pointer, double, whatever, on the hardware which I care
about, and just return memory to the user which is aligned on that
alignment so the user can put whatever object he wants into it. (This
is also a requirement for the type of function the C++ standard calls
"an allocation function". I don't remember much offhand.)

Also, I'm not quite sure what you're trying to do with that new code.
I would probably take my code, ensure it's actually bug free, fix the
alignment issues, and use that. Notice how mine has exactly one cast,
the static_cast in free to get the void pointer back to the Link
pointer - whereas your code has many more casts. Casts should be kept
to a minimum in code, and generally casting is a sign of a bad design.
In this case, I would argue that malloc and free are an example of bad
design, a lack of type safety. This is related to how C also lets
void* be cast to any pointer type implicitly in order to make malloc
not excessively annoying to use, and other similar generic data
structures and algorithms. In C++, this mostly isn't needed because of
new and delete, its type safe containers based on templates. With C+
+0x with variadic templates, void* casting hackery will be needed even
less.

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