Microsoft·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Apr 2026

Summary

Microsoft SWE interview where they handed me a C++20 producer-consumer queue implementation and asked me to tear it apart. Pretty technical, no LeetCode, just real code review work.

Questions Asked (1)

You're given a C++20 producer-consumer message queue implementation using modern features like jthread, stop_token, concepts, and ranges. Do a full code review: find correctness bugs (race conditions, missing notifications, lost wake-ups, signed/unsigned issues), performance problems, and style issues. Propose and explain your fixes.

Technical Trade-offsSystem DesignAlgorithms & Data Structures

Author's notes

This was the whole interview basically.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the queue's contract (bounded/unbounded, blocking/non-blocking, shutdown semantics) and then systematically review the code for correctness, performance, and style. Prioritize concurrency bugs like race conditions and lost wake-ups, then address performance and style, explaining each fix with its rationale and trade-offs.

Pro tip: Demonstrate deep C++20 knowledge by discussing how jthread and stop_token interact with condition variables, and mention that you'd add tests (e.g., with TSan) to validate concurrency fixes—this shows you think about verification, not just code changes.

1. Clarify requirements and assumptions

Ask about the queue's intended behavior: bounded vs unbounded, blocking vs non-blocking, shutdown semantics, and exception safety. This ensures your review targets the right contract.

2. Review correctness and concurrency

Inspect for race conditions, missing notifications, lost wake-ups, spurious wake-ups, signed/unsigned mismatches, and proper use of stop_token. Check that all shared state is protected and condition variables are used correctly.

3. Evaluate performance and efficiency

Look for unnecessary copies, lock contention, false sharing, and inefficient use of ranges or concepts. Suggest improvements like move semantics, finer-grained locking, or lock-free techniques if appropriate.

4. Assess style and modern C++ usage

Check for const-correctness, naming, RAII, and proper use of C++20 features (concepts, ranges, jthread). Suggest idiomatic alternatives and highlight maintainability issues.

5. Propose and prioritize fixes

Summarize the most critical bugs first, then performance and style. For each fix, explain the change, why it's needed, and any trade-offs. Mention testing strategies like unit tests and TSan.

Key Points to Mention

Race conditions on shared variables (e.g., queue size, head/tail indices) without proper synchronization.
Lost wake-ups due to notifying without holding the lock or using the wrong condition variable.
Signed/unsigned issues in size comparisons or index arithmetic leading to underflow/overflow.
Proper use of std::stop_token and std::jthread for cooperative cancellation and automatic joining.
Performance improvements: move semantics, reducing lock scope, avoiding false sharing, and using std::condition_variable_any with stop_token.
Testing and verification: unit tests, stress tests, and ThreadSanitizer to catch concurrency bugs.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.