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.