Hi!
Can anyone please explain what -f[no-]strict-aliasing and
-W[no-]strict-aliasing do? (Yes, gcc accepts -Wno-strict-aliasing!) I'm
experiencing some mild nausea because of type-punning in a library I'm
maintaining. Gcc's documentation is not very clear, talking only about
types being "almost the same". And unfortunately I have no copy of the
holy standard lying around.. Also, Google appears to turn up only
confused and confusing arguments, to this issue... :-(
I was under the impression that pointers to some data structures are
"almost the same", but apparently this is wrong:
class Base {
public:
int x;
};
class Derived : public Base {
public:
Derived() { x = 24; }
virtual ~Derived() {} // (1)
};
class Manager {
public:
union {
void* pointer;
Base* basepointer;
};
};
#include <iostream>
using namespace std;
int main()
{
Derived* mp = new Derived;
Manager obj;
obj.pointer = mp; // (2)
cout << mp->x << endl;
cout << obj.basepointer->x << endl;
}
When compiled with GCC 3.x or 4.x, this results in:
24
134515352
With GCC 2.95, it resulted in:
24
24
Okay, removing the vtable in Derived (1) or adding one in Base (by adding
a virtual dtor) appears to fix the program. Obviously, assigning to
basepointer, as opposed to pointer (2) fixes it, too.
But: As it stands, is the program really wrong? And: Why does
-f[no-]strict-aliasing not make a difference. And should not
-Wstrict-aliasing issue a warning?
Regards
-richy.
--
Richard B. Kreckel
<http://www.ginac.de/~kreckel/>
