Great job, cocks-for-brains. You've parrotted C programmers for so long, you almost sound like one.
Fact is, the C standard most certainly describes the behavior of const void * access to structures between the struct data * and the (struct data *)+sizeof(struct data)
You can replace read() with fread() or readv() or even i_suck_cocks() if you want. The C standard guarantees that your function will be able to load data into your structure safely.
Not so with C++, which doesn't guarantee that method pointers nor vtables won't be dispatched. Thanks to operating systems relocating your program or library, you simply aren't guaranteed to be able to do that in C++.
Replace brains with cocks, and we've got you.