Pointer tracking fails to properly account for object address changes with (multiple) inheritance

[naalit]

Describe the bug Empirical's pointer tracking infrastructure assumes that object addresses don't change, so when an object is deleted it looks up its address in a table of allocated addresses. But C++ sometimes does change object addresses, specifically with multiple inheritance but this should also happen sometimes with particularly large single inheritance situations although I haven't found an example where that actually happens with single inheritance. Specifically, in the example below the compiler moves the pointer to C forward a little bit to get the B pointer, and the virtual destructor knows how to get the original pointer back but Empirical doesn't.

To Reproduce Here's a minimal example that fails with -DEMP_TRACK_MEM, but is valid C++ and passes AddressSanitizer:

#include "emp/base/Ptr.hpp"

class A {
public:
  virtual ~A() {}
};

class B {
public:
  virtual ~B() {}
};

class C : public A, public B {
  size_t c;
};

void cleanup(emp::Ptr<B> b) { b.Delete(); }

int main() {
  emp::Ptr<C> c = emp::NewPtr<C>();
  cleanup(c);
}

Expected behavior The above program doesn't actually contain any memory management errors (as far as I know, and AddressSanitizer agrees), so pointer tracking should not produce any.

Toolchain:

• OS: Arch Linux
  • Compiler: GCC 12.2.0
• Empirical Version: current master, but I originally found this problem in Symbulation with 1fe7f9008c

[naalit]

I just found out that dynamic_cast<void*>(ptr) will obtain the base address of the real object allocation, so using that in Ptr::Delete() and similar methods should hopefully fix this problem. I've made that change in PR #477.