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Why does a polymorphic base need a virtual destructor when deleting through a base pointer?

Explain virtual destruction order, undefined behavior without it, and the rule that every polymorphic base owns a virtual destructor.

TRT practice prompt — not a verified question from a named employer.

The Runtime Theory Team2 min read
Learn it firstVirtual Destructors: Closing a Hierarchy SafelyPro — members onlyDestroy derived objects through base pointers without leaks by declaring a virtual destructor, and trace the construction and destruction order it guarantees.

A strong answer

A strong candidate frames Why does a polymorphic base need a virtual destructor when deleting through a base pointer? around the exact memory layout, compiler semantics, and hardware execution model covered in Virtual Destructors: Closing a Hierarchy Safely. Explain virtual destruction order, undefined behavior without it, and the rule that every polymorphic base owns a virtual destructor.

The program prints four lines in order: Base constructor, Derived constructor, Derived destructor, Base destructor. Remove virtual from the base destructor and the third line can vanish. That vanished line is the bug this lesson removes for good.

A virtual destructor is a base destructor declared with virtual so that deletion through a base pointer selects the destructor by object type. Each polymorphic object carries a hidden pointer to a per-class table, and the destruction call indexes that table. As the trace above shows, the same delete statement destroys the full object when the destructor is virtual and may destroy only the base part when it is not.

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 why a base needs a virtual destructor.
  • Mastery of core invariant: Predict destructor order for deletion through a base pointer.
  • Mastery of core invariant: Apply the virtual destructor rule to polymorphic bases.
  • 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.

This answer walks

Practice follow-ups

  1. 01What output proves full teardown when you delete through a base pointer?
  2. 02Why must a base destructor avoid calling virtuals that need derived state?

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