What is the difference between an abstract class with shared state and a pure interface?

A strong answer

A strong candidate frames What is the difference between an abstract class with shared state and a pure interface? around the exact memory layout, compiler semantics, and hardware execution model covered in Abstract Classes and Pure Interfaces. Contrast pure virtuals, shared helpers, narrow interfaces, and how derived types become concrete by defining every contract function.

The fixed versions of your 0_Language/oops/abstract_class.cpp and 0_Language/oops/pure_abstract_class.cpp, with the missing string and output headers plus small main demos:

Shape cannot be created alone, but Circle can once it defines draw. ILogger cannot be created alone, but ConsoleLogger can once it defines both logging operations.

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: Explain how a pure virtual function makes a type abstract.
• Mastery of core invariant: Contrast an abstract class with a pure interface.
• Mastery of core invariant: Implement a derived type that fulfills a base contract.
• 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.