C++ for Quants: What HFT and Low-Latency Firms Test in C++ Interviews

C++ for Quants: What HFT and Low-Latency Firms Test in C++ Interviews

C++ is the dominant language for low-latency trading systems at HFT firms (Hudson River Trading, Jump Trading, Citadel Securities, Optiver, Tower) and for performance-critical quant infrastructure at hedge funds (Two Sigma, D. E. Shaw, Citadel, Renaissance). For quant-developer interviews at these firms, C++ skills are tested at depth that exceeds typical big-tech expectations: memory model, undefined behavior, modern C++ idioms, template metaprogramming, lock-free data structures. This guide covers what gets tested at the depth needed.

The C++ Skills That Matter

Object lifetime and ownership

Constructors, destructors, copy/move semantics, the rule of zero/three/five. RAII (Resource Acquisition Is Initialization) as the central pattern. std::unique_ptr, std::shared_ptr, std::weak_ptr; when to use each. Copy elision and return value optimization. Strong candidates can reason about exactly when objects are constructed, copied, moved, and destroyed.

Move semantics

rvalue references, std::move, perfect forwarding (std::forward). Why move semantics matter: avoid expensive copies, enable efficient transfer of ownership. When move constructors are noexcept (matters for STL containers). Subtle issue: std::move doesn’t actually move; it casts to rvalue reference. Strong candidates can write move-correct classes from scratch.

Memory model

Stack vs heap, automatic vs dynamic storage. The C++ memory model and atomic operations. Memory ordering (memory_order_relaxed, acquire, release, acq_rel, seq_cst). When to use each in lock-free programming. False sharing and cache line awareness.

Undefined behavior

Knowing what’s UB and why it matters. Common UB: signed integer overflow, dereferencing null pointers, accessing uninitialized variables, modifying string literals, race conditions. Strong candidates can spot UB in code samples and explain what compilers may legally do (anything).

Templates

Function templates, class templates, partial specialization, SFINAE / concepts (C++20). Variadic templates. Common patterns: CRTP (Curiously Recurring Template Pattern), tag dispatch, type traits. Template metaprogramming is a deep topic; you don’t need expert level, but conversational fluency is expected for senior roles.

STL

std::vector, std::array, std::map, std::unordered_map, std::set, std::deque, std::list. Iterator categories. Algorithms: std::sort, std::find, std::transform, std::accumulate. Knowing complexity of STL operations cold (vector::push_back is amortized O(1); list::push_back is O(1); map operations are O(log n); unordered_map operations are O(1) average). Strong candidates can pick the right container in seconds.

Concurrency

std::thread, std::mutex, std::lock_guard, std::unique_lock. Condition variables. std::atomic and lock-free programming. False sharing. Memory ordering (above). Strong candidates can write thread-safe code, recognize race conditions, and discuss when lock-free is appropriate.

Performance considerations

Cache awareness: data structures fit in cache vs span cache lines. Branch prediction. SIMD intrinsics for tight numerical loops. Profile-guided optimization. Compiler optimization (constexpr, inline, attributes). Strong candidates think in cache lines, not just asymptotic complexity, for performance-critical code.

Common C++ Interview Questions

Implement a thread-safe queue

Mutex-protected std::deque with push and pop. Discuss spurious wakeups in condition variables. Strong candidates discuss alternatives: lock-free SPSC queue, MPMC queue. Warn about unbounded growth; discuss bounded variant.

Implement smart pointers

“Implement a basic unique_ptr.” Class template; raw pointer; destructor calls delete; deleted copy constructor and copy assignment; defined move constructor and move assignment. Strong candidates write all five member functions correctly with appropriate noexcept.

Detect memory leaks

Tools (Valgrind, AddressSanitizer, LeakSanitizer). Common patterns that leak: forgetting to delete, exception-skipping cleanup (RAII fixes this), circular references with shared_ptr (use weak_ptr for back-references). Walk through debugging workflow.

Discuss virtual functions vs templates

Runtime polymorphism (virtual functions, vtable lookup) vs compile-time polymorphism (templates, no runtime dispatch). When each is appropriate. Performance: templates avoid vtable lookup but can bloat code size; virtual is uniform overhead per call. For trading systems, templates often win because the inner loop runs at higher frequency than the cost of code bloat.

Implement a custom allocator

Pool allocators, arena allocators, stack allocators. When generic new/delete is too slow. Walk through allocator concept; discuss alignment and overaligned types. This is real for trading-system roles where allocation overhead matters.

Discuss false sharing

Two threads writing to adjacent memory locations on the same cache line cause cache coherency traffic that destroys parallelism. Fix: pad data structures to cache-line size, or align to cache lines. Strong candidates know this affects high-performance code regardless of algorithmic correctness.

Discuss memory ordering

“What’s memory_order_relaxed and when would I use it?” Used when atomicity matters but ordering doesn’t (e.g., counters incremented from many threads). Acquire/release pairs are used for lock implementations. seq_cst is the strongest and slowest; default for std::atomic operations.

Avoid undefined behavior

“This code has undefined behavior. Spot it.” Examples include signed overflow in checks (use unsigned or check before), strict-aliasing violations, modifying const objects, incorrect placement new use. Strong candidates spot UB and explain consequences.

Modern C++ (C++17, C++20, C++23)

C++17

Structured bindings (auto [x, y] = pair). std::optional, std::variant, std::any. constexpr if. Inline variables. Filesystem library. fold expressions for variadic templates.

C++20

Concepts (constrained templates). Ranges and views. Coroutines (low-level; libraries built on top). Modules (slow adoption). std::span. Three-way comparison (operator). Calendar / time-zone library.

C++23

Various improvements; most quant firms still on C++17 or C++20. Strong candidates know the major C++20 features (concepts, ranges, span) and can discuss when they’re useful.

Things That Surprise C++ Candidates

Returning local objects is fine

Return value optimization (RVO) and copy elision mean returning a local object doesn’t always copy. Modern C++ (C++17 mandatory copy elision) makes this guaranteed in many cases. Don’t manually use std::move on return values; it can disable copy elision.

std::vector reallocation invalidates iterators

If push_back triggers a reallocation, all existing iterators, pointers, and references to elements are invalidated. Reserve capacity if you’ll iterate during insertion.

Constructors and exceptions

If a constructor throws, the destructor doesn’t run (because the object isn’t fully constructed). RAII subobjects already constructed are properly destroyed. Acquire resources with RAII members so exception safety is automatic.

const-correctness

Const member functions can’t modify the object (except mutable members). Const propagates through references and pointers but not through smart pointers’ pointee. Strong const-correctness is a marker of senior C++ engineers.

Frequently Asked Questions

See also: SWE to Quant Dev Transition • Hudson River Trading Interview Guide • Jump Trading Interview Guide