Question

I’m interested to know why an assignment like &a = &b is not permitted in C++.

I understand the risks in doing so, but it’s not a good enough reason to forbid it completely, to me at least. The reason why I thought about that was because I was looking for a smart way to swap big objects without copying, and something like that seemed like a good idea:

void ptr_swap( ptrdiff_t &a, ptrdiff_t &b )
{
    a = a ^ b;
    b = a ^ b;
    a = a ^ b;
}

int main()
{
    double a = 157648.13;
    double b = 96871.84;

    printf("%.4f %.4f\n", a, b);

    ptr_swap( reinterpret_cast<ptrdiff_t>(&a), reinterpret_cast<ptrdiff_t>(&b) );

    printf("%.4f %.4f\n", a, b);
}

.. but apparently, the compiler doesn’t think so. 🙁

EDIT: I understand why this doesn’t work. Maybe my question would be clearer like that: I don’t know how many properties a variable has in a program, but at least a name, a value, an address, and some indication of its lifetime perhaps. The thing is that to me, swapping is essentially renaming two existing values stored in memory, and it’s nonsense that it would incur a copy.

With this “horrific” code example that I gave up there, what I’m trying to do is to say to the compiler: “From now on, b is named a, and vice-versa”. Why is such a thing not possible? The motivation is that it’s already possible to “instruct the compiler” in a way, via TMP for instance, so why not like that too?

Answer 1

It seems to me like you want to change the address of an object, right? That is impossible. Every object has a fixed address through its entire lifetime. You cannot give a different address to an object, ever. The address of an object is implicit in the machine code that’s generated, it’s not stored anywhere per se.