Question

I get this warning. I would like defined behavior but i would like to keep this code as it is. When may i break aliasing rules?

warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

String is my own string which is a POD. This code is called from C. S may be an int. String is pretty much struct String { RealString*s; } but templated and helper functions. I do a static assert to make sure String is a pod, is 4bytes and int is 4bytes. I also wrote an assert which checks if all pointers are >= NotAPtr. Its in my new/malloc overload. I may put that assert in String as well if you suggest

Considering the rules i am following (mainly that string is a pod and always the same size as int) would it be fine if i break aliasing rules? Is this one of the few times one is breaking it right?

void func(String s) {
    auto v=*(unsigned int*)&s;
    myassert(v);
    if(v < NotAPtr) {
        //v is an int
    }
    else{
        //v is a ptr
    }
}

Answer 1

The safe way of treating a variable as two different types is to turn it into a union. One part of the union can be your pointer, the other part an integer.

struct String
{
    union
    {
        RealString*s;
        int i;
    };
};