Explain C++ alignment and ABI stability

You are interviewing for a senior C++ / systems role. The interviewer asks two low-level fundamentals questions: 1. **C++ memory alignment and padding** - Explain what *memory alignment* means in C and C++. - Describe what *padding* is inside and at the end of structs/classes. - Show with a small example how the order of data members can change the `sizeof` a struct due to padding. - Discuss why alignment and padding exist (performance, hardware constraints) and what problems they can cause (e.g., unexpected struct size, binary compatibility issues, undefined behavior if misaligned). - Mention typical ways to inspect or control layout when absolutely necessary (e.g., compiler-specific pragmas/attributes, `static_assert(sizeof(...))`, etc.), and the risks of doing so. 2. **ABI (Application Binary Interface) and its instability** - Define what an ABI (Application Binary Interface) is, and how it differs from a source-level API. - Explain which aspects are covered by an ABI (e.g., calling conventions, data layout, alignment, name mangling, exception handling, vtable layout). - Explain why the C++ ABI is often *not stable* across different compilers or even different versions of the same compiler, while C ABIs tend to be much more stable. - Give concrete examples of problems ABI instability can cause in practice (e.g., linking a program against a shared library built with a different compiler/version, plugin systems, distributing precompiled libraries). - Describe common engineering strategies to mitigate ABI issues in C++ (e.g., using a stable C interface boundary with `extern "C"`, avoiding STL types in public ABI, the PImpl idiom, rebuilding all components with the same toolchain). Explain your answers in a way that shows deep understanding of low-level details and practical trade-offs.