When should you use unique_ptr, shared_ptr, or weak_ptr for ownership?

A strong answer

A strong candidate frames When should you use unique_ptr, shared_ptr, or weak_ptr for ownership? around the exact memory layout, compiler semantics, and hardware execution model covered in Smart Pointers: unique_ptr, shared_ptr and weak_ptr. Compare exclusive ownership, counted sharing, and non-owning observation with modern creation.

The starting program for this lesson, 0_Language/oops/smart_pointer.cpp, already demonstrates the three smart pointers correctly, but it builds the unique owner with new inside the pointer constructor and prints raw addresses as proof of ownership. That style works in a demo and leaks in real code, because any expression that allocates with new before ownership is established can throw in between and strand the allocation. The trace above shows the modern replacement: each object is born already owned, moves are explicit, and observation never extends lifetime.

An owning pointer decides when its memory is released. A raw pointer never decides; its destruction does nothing to the pointee. A smart pointer closes the gap by tying release to destruction, so unwinding releases automatically on every path.

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: Transfer exclusive ownership with unique_ptr and move semantics.
• Mastery of core invariant: Share ownership with shared_ptr while tracking the use count.
• Mastery of core invariant: Break cycles with weak_ptr and observe without extending lifetime.
• 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.