Explain C++ memory, types, and concurrency fundamentals

Answer the following C++ / systems fundamentals questions: 1) **C++ `class` vs `struct`** - What are the differences between `class` and `struct` in C++? - Can a `class` have a reference (`T&`) as a data member? What are the implications? 2) **Virtual constructor** - Can a constructor be `virtual` in C++? If not, what patterns achieve “polymorphic construction”? 3) **Pointer vs reference** - What are the key differences between pointers and references in C++ (nullability, reseating, object lifetime, ABI, usage)? 4) **Placement new** - What is placement `new`? When would you use it, and what must you do manually after using it? 5) **Performance comparison (cache/memory effects)** - Compare likely performance and CPU/memory behavior of these two loops (assume `data` points to a large `int` array of length `64*1024*1024` allocated on the heap): - Code A: increment `data[0]` in a tight loop `N` times. - Code B: increment `data[i]` for `i = 0; i < N; i += 8`. - Discuss caching, prefetching, write allocation, store buffers, memory bandwidth, and why one might be faster. 6) **Function pointer type** - In `int (*s[10])(int);`, what is the type of `s`? 7) **`unordered_map` vs `map`** - Compare complexity guarantees, ordering, iterator invalidation, memory overhead, and typical use cases. 8) **Overloading vs overriding** - Explain function overloading vs method overriding (virtual dispatch) in C++. 9) **Mutex vs condition variable** - What problems does each solve? Why do condition variables require a mutex and a loop around `wait`? 10) **What causes a physical memory access?** - At a high level, what events force the CPU to access DRAM (vs cache)? Include page faults, TLB misses, cache misses, and DMA. 11) **Shared memory synchronization** - If multiple processes/threads communicate via shared memory, how do you correctly synchronize access? Discuss options (process-shared mutexes, semaphores, futexes, file locks) and pitfalls. 12) **Linux networking and virtualization (high level)** - Describe how a packet travels through the Linux networking stack, and how common virtualization primitives (namespaces, veth pairs, bridges, TAP, iptables/nftables) fit into that path.