# Compare C++ new, malloc, and Placement new Explain what happens when a C++ `new` expression is evaluated, how it differs from `malloc`, what placement `new` does, and when constructing objects in caller-supplied storage can improve performance. Include destruction, alignment, exception safety, and object lifetime. ### Constraints & Assumptions - Discuss non-array objects first and call out where arrays add complexity. - Distinguish a `new` expression from allocation functions named `operator new`. - Assume modern C++ object-lifetime and alignment rules. - Do not treat an allocator or memory pool as automatically faster without a workload. ### Clarifying Questions to Ask - Should custom allocation functions and allocator-aware containers be included? - Is the use case a pool, arena, shared-memory region, or fixed buffer? - Are over-aligned types and exception-free environments in scope? - Who owns the storage and when may it be reused? ### Hints - Allocation and construction are distinct operations. - Placement `new` does not allocate the supplied buffer. - Destruction and deallocation must mirror the way lifetime and storage began. ### What a Strong Answer Covers - Storage allocation, alignment, constructor invocation, failure, and cleanup in a `new` expression. - Raw byte allocation from `malloc`, explicit construction needs, and matching `free`. - Placement construction, caller-owned storage, explicit destruction, reuse, and lifetime rules. - Pools and arenas, locality, batching, fragmentation, determinism, and measurement. - RAII, exception safety, arrays, over-alignment, and undefined behavior from mismatched cleanup. - Preference for standard containers and allocators over ad hoc memory management when possible. ### Follow-up Questions 1. What happens if a constructor throws after storage was allocated by a `new` expression? 2. Why must placement-constructed objects usually be destroyed explicitly? 3. When is `std::launder` relevant to storage reuse? 4. How would you make an object pool safe under concurrency and exceptions?