What do basic, strong, and nothrow guarantees promise after a throw?

A strong answer

A strong candidate frames What do basic, strong, and nothrow guarantees promise after a throw? around the exact memory layout, compiler semantics, and hardware execution model covered in Exception Safety: Guarantees and RAII Discipline. Distinguish valid, unchanged, and never-throw promises and show copy-and-swap with RAII discipline.

The prior lesson made throws precise. This lesson makes their aftermath predictable. Consider the bulk import from the hook: forty inserts succeed, the forty-first validates badly and throws, and the catalog now holds forty unlogged additions. The caller cannot retry because retry duplicates, and cannot continue because state is unknown. The defect is not the throw but the missing promise about what a throw leaves behind.

An exception-safety guarantee is that promise. The basic guarantee says the object remains valid, with no leaks and an intact invariant, though its value may have changed. The strong guarantee says the call either completes fully or leaves state exactly as before. The nothrow guarantee says the operation never throws and always completes. The trace above demonstrates strong behavior for a catalog update: fallible work happens on a side copy, and only a non-throwing step touches the live object.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Distinguish basic, strong, and nothrow guarantees with examples.
• Mastery of core invariant: Write mutating operations that stay coherent under exceptions.
• Mastery of core invariant: Mark destructors and moves noexcept to protect unwinding.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.