strict aliasing: how to swap pointers

[Evan Jones <evanj at MIT dot EDU>]

I have a function to swap any two pointers (assuming that sizeof(void*) 
== sizeof(T*)). I would like to use it without violating strict aliasing 
rules. I have a workaround, but I would like to verify that this is the 
right way to do this.

The test case:

void* exchange(void** ptr, void* next);

int main() {
    int v1 = 42;
    int v2 = 123;

    int* a = &v1;

    int before = *a;
    exchange(reinterpret_cast<void**>(&a), &v2);
    int after = *a;

    printf("before: %d after: %d\n", before, after);

    return 0;
}


Output from Redhat's version of gcc 4.1.2 20070925:

test.cc: In function ‘int main()’:
test.cc:16: warning: dereferencing type-punned pointer will break 
strict-aliasing rules


The output from the program is correct. My workaround avoids the 
warning, but I think it will still permit GCC to generate undesired 
optimizations, since it says nothing about the *values* pointed to:


    union {
        int** int_ptr;
        void** void_ptr;
    } temp = { &a };
    exchange(temp.void_ptr, &v2);


I believe this may be incorrect, since *int_ptr and *void_ptr have 
distinct types, and hence GCC could decide that they cannot possibly be 
aliases? What is the "right" way to fix this? Thanks for your help,

Evan Jones

--
Evan Jones
http://evanjones.ca/