On 11/3/06, Andrew Haley <aph@xxxxxxxxxx> wrote:
Benjamin Grange writes:
> On 11/3/06, Andrew Haley <aph@xxxxxxxxxx> 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 {
public:
A(int n) { x = n; }
int x;
};
union
{
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.
--
Benjamin
