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Qube Research & Technologies·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Apr 2026

Summary

Technical round at Qube R&T for a software engineering role. The main question was a deep C++ implementation problem that took up most of the session. Pretty intense if you haven't thought about type erasure in a while.

Questions Asked (1)

Q1

Implement a minimal std::function-like callable wrapper in C++ using type erasure through virtual dispatch. Your implementation should handle function pointers, capturing lambdas, and functors. It also needs to support copy and move semantics, operator(), proper destruction, and a bool conversion to check for an empty state. You can skip small-buffer optimization. As a follow-up, explain your exception-safety decisions and compare the time and space costs of your version versus the standard library for invocation and copying.

System DesignTechnical Trade-offsAlgorithms & Data Structures
Author's notes

This one took me a second to get oriented.

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AI HintsAI Generated

Suggested Approach

Start by outlining the type erasure design: an abstract base class with a templated derived class holding the callable, and a wrapper class managing a pointer to the base. Then implement the required operations (constructor, copy, move, operator(), destructor, bool conversion) and discuss exception safety and performance trade-offs.

Pro tip: Mention that you would use a unique_ptr for ownership and implement copy via a virtual clone() method to avoid slicing, and note that without SBO, every construction incurs a heap allocation, which is a key performance difference from std::function.

1. Define the type-erasure interface

Create an abstract base class with pure virtual call operator, clone, and destructor. This provides the polymorphic interface for invoking and copying the stored callable.

2. Implement the templated model

Derive a templated class from the base that stores the callable and forwards the call operator, implements clone to copy the callable, and relies on the default destructor.

3. Build the wrapper class

Manage a pointer to the base (e.g., unique_ptr) and provide constructors, copy/move semantics, operator(), bool conversion, and destructor. Ensure proper forwarding and exception safety.

4. Discuss exception safety

Explain that constructors may throw (e.g., bad_alloc) and that copy operations should provide strong guarantee by cloning before modifying the target. Move operations should be noexcept.

5. Compare with std::function

Analyze time and space: invocation is a virtual call (similar to std::function), but construction/copy always heap-allocate (no SBO), making it slower and less space-efficient for small callables.

Key Points to Mention

  • Type erasure via abstract base class and templated derived class.
  • Use of unique_ptr for exclusive ownership and virtual clone for copying.
  • Copy and move semantics: deep copy on copy, transfer ownership on move.
  • Exception safety: strong guarantee for copy, noexcept move, and handling of allocation failures.
  • Performance: virtual dispatch overhead for invocation, heap allocation per construction/copy without SBO.
  • Comparison with std::function: std::function uses SBO to avoid heap allocation for small callables, making it faster and more space-efficient.

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