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Re: Problem with -O3 and int to pointer (64bits)

On 11/3/06, Andrew Haley <> wrote:
Benjamin Grange writes:
 > On 11/3/06, Andrew Haley <> wrote:
 > > Benjamin Grange writes:
 > >  > Do you see a problem in this line?:
 > >  > (void *)  (unsigned long)(*(((unsigned int *)&a)));
 > >
 > > Yes, I can: it's not legal C++.  What are you really tring to do that
 > > needs this?
 > I'm trying to make this code which is not from me to work  with O3.
 > In the real code, there is a list class which use  void * to save its elements.
 > A lot of other list are derived from this one to avoid doing casting
 > of void * to the original pointer. They all use real pointer, so no
 > problem.

 > But there is a special derived list which converts its elements
 > into int and save the int as a void * in order to avoid doing heap
 > allocation. Exactly like the code I have posted.

If you know that the data will all fit within a void*, you can simply do

  class A {
          A(int n) { x = n; }
          int x;

     int n;
     void *p;
  } u;

   u.n = a.x;
   return u.p;

... or whatever, depending on your pointer size.

 > What make my code not legal C++?
 > Will it be better if I do something like that for put():
 > listA[i] = (void *)a.getInt();
 > and get:
 > return A((int)listA[i])

OK, the rule is this: you cannot access an object of type A through a
pointer that is not type compatible with A.

So, if you declare:

   short *p;
   int a = 9;
   p = (short*)&a;
   return p;

then your code will not work, and current versions of gcc will warn you:

 $ gcc /mnt/zebedee/p.c -O2 -Wall
/mnt/zebedee/p.c: In function 'main':
/mnt/zebedee/p.c:7: warning: dereferencing type-punned pointer will break strict-aliasing rules
Ok thank you. I will use the getInt() version since it's legal C++ and
pointer size independent.

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