I don't know whether C++ does the same (there is the issue of where it sticks the vtable, and I'm not sure what else).

I imagine there will be no problem, since we've been compiling the PETSc C code with C++ compilers for a decade now,

but I don't know whether the C++ standard guarantees that.

Jed is the only person I know that actually reads the standard (or maybe it's part of the ABI).