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I went with the three-pointer layout (begin, end, end-of-capacity) which felt natural.
Start by sketching the memory layout: a pointer to a dynamically allocated array, a size, and a capacity. Then implement the methods, emphasizing the fast path of push_back (when size < capacity) and the growth strategy (e.g., doubling) that ensures amortized O(1). Explain the amortized analysis using the aggregate method or accounting method.
Pro tip: Mention that the growth factor should be between 1 and 2 (e.g., 1.5 or 2) to balance memory usage and performance, and that using a factor of 2 can lead to memory fragmentation, while 1.5 is often used in practice (e.g., in some standard libraries).
Draw a diagram showing a pointer to the heap-allocated buffer, the current size (number of elements), and the capacity (allocated space). Mention that the buffer is contiguous.
These are trivial: return the size, return the capacity, and return a reference to the element at the given index (with optional bounds checking in debug mode).
If size < capacity, construct the new element at buffer[size] and increment size. This is O(1).
When capacity is full, allocate a new buffer of larger capacity (e.g., double or 1.5x), copy/move elements, and deallocate the old buffer. Discuss trade-offs of different growth factors.
Use the aggregate method: sum the costs of n push_backs. The total cost is O(n) because each element is copied at most a constant number of times when doubling. Thus, amortized cost per operation is O(1).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by explaining the fundamental problem: `new T[n]` default-constructs n objects, which is incorrect for a vector that needs to manage uninitialized storage and control construction timing. Then demonstrate the correct approach using raw memory allocation (e.g., `operator new` or `std::allocator`), placement new for construction, and explicit destructor calls for cleanup. Finally, implement the rule of five (destructor, copy constructor, copy assignment, move constructor, move assignment) ensuring exception safety and proper resource management.
Pro tip: Emphasize that `new T[n]` requires T to be default-constructible and immediately constructs all elements, which is inefficient and semantically wrong for a vector that should only construct elements when needed. Also, mention that using `std::allocator` or `operator new` separates allocation from construction, enabling strong exception guarantees and support for non-default-constructible types.
Discuss that `new T[n]` default-constructs n objects immediately, which is wasteful and incorrect for a vector that needs to manage uninitialized memory and construct elements only when added. It also requires T to be default-constructible and doesn't allow separate allocation from construction.
Demonstrate allocating raw, uninitialized memory using `operator new` (e.g., `static_cast<T*>(::operator new(n * sizeof(T)))`) or `std::allocator<T>::allocate(n)`. Explain that this only reserves memory without constructing objects.
Use placement new to construct elements into the raw storage: `new (ptr + i) T(args...)`. Show how to handle exceptions during construction by destroying already-constructed elements and deallocating memory.
Explicitly call destructors for each constructed element (e.g., `ptr[i].~T()`) and then deallocate the raw memory using `operator delete` or `std::allocator<T>::deallocate`. Ensure this happens in the destructor and during exception cleanup.
Write the destructor, copy constructor, copy assignment operator, move constructor, and move assignment operator. Ensure deep copying, move semantics (stealing resources), self-assignment safety, and exception safety (e.g., copy-and-swap idiom).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Probably the hardest part of the whole interview.
Explain that if T's copy constructor throws during reallocation, the vector must remain unchanged to provide the strong exception guarantee. Describe the implementation strategy: allocate new buffer, copy elements one by one, and only commit (swap pointers, update size/capacity) after all copies succeed; if any copy throws, deallocate the new buffer and propagate the exception, leaving the original vector intact.
Pro tip: Mention that this is why std::vector::push_back provides the strong exception guarantee when T's copy constructor doesn't throw, but only the basic guarantee if it can throw. Also note that move semantics (if noexcept) can be used to achieve the strong guarantee even for throwing copy constructors, as in C++11 and later.
State that the vector should provide the strong exception guarantee: if an exception is thrown, the vector remains unchanged (no effects).
Explain that reallocation involves allocating a new buffer, copying existing elements to it, and then destroying the old elements and deallocating the old buffer.
If a copy constructor throws partway through, catch the exception (or use RAII), destroy any successfully copied elements in the new buffer, deallocate the new buffer, and rethrow. The original vector remains untouched.
After all elements are successfully copied, swap the new buffer with the old one (or update pointers), update size and capacity, and then destroy old elements and deallocate old buffer.
Mention that if T's copy constructor can throw, push_back may not offer the strong guarantee in all implementations (e.g., if it uses move_if_noexcept). Also note that using move semantics (if noexcept) can preserve the strong guarantee.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Explain that the decision hinges on the type's move semantics and exception guarantees, and that std::move_if_noexcept is the standard utility that automates this choice. Emphasize that this matters for strong exception safety: if moving can throw, copying preserves the original buffer, allowing rollback.
Pro tip: Mention that std::move_if_noexcept returns an rvalue reference only if the move constructor is noexcept or if the type is not copyable; otherwise it returns a const lvalue reference, forcing a copy. This shows you understand the precise conditions and the trade-off between performance and safety.
Determine whether the type has a noexcept move constructor and whether it is copyable. If moving can throw and copying is available, prefer copying to maintain exception safety.
State that std::move_if_noexcept is the standard utility that conditionally casts to an rvalue reference based on the type's move constructor's noexcept specification and copyability.
Describe how using move_if_noexcept enables the strong exception guarantee: if an exception occurs during relocation, the original buffer remains unchanged because elements were copied, not moved.
Note that when moves are noexcept, move_if_noexcept allows efficient moves; otherwise, it falls back to copying, which may be slower but ensures safety.
Mention that standard containers like std::vector use this technique when reallocating, balancing performance and exception safety.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
std::is_trivially_copyable for the memcpy path, std::is_trivially_destructible to skip destructor loops, and std::is_nothrow_move_constructible for the move-vs-copy decision.
Name the three type traits directly: std::is_trivially_copyable for memcpy, std::is_trivially_destructible for skipping destructors, and std::is_nothrow_move_constructible (or std::move_if_noexcept) for move-vs-copy during reallocation. Then explain how each trait enables a specific optimization and why it's safe, showing you understand the underlying object model.
Pro tip: Mention that these traits are the foundation of std::vector's implementation in major standard libraries, and that using them correctly is essential for exception safety and performance.
State the traits: std::is_trivially_copyable, std::is_trivially_destructible, and std::is_nothrow_move_constructible (or std::move_if_noexcept).
Describe how std::is_trivially_copyable allows using memcpy for copying elements, avoiding per-element copy constructors.
Describe how std::is_trivially_destructible allows skipping destructor calls when destroying elements, improving performance.
Describe how std::is_nothrow_move_constructible (or std::move_if_noexcept) determines whether to move or copy elements during reallocation to maintain strong exception safety.
Mention that these optimizations are used in standard library implementations of std::vector and are key to high-performance code.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Didn't get deep into this one, it was more of a closing discussion.
Start by clarifying the current vector implementation (e.g., contiguous dynamic array) and then propose extending it with insert and erase operations that shift elements to maintain contiguity. Discuss how reallocation may occur when capacity is exceeded, and derive iterator/reference invalidation rules based on whether reallocation happens and which elements are moved.
Pro tip: Emphasize that insert/erase in the middle are O(n) due to shifting, and that reallocation invalidates all iterators/references, while shifting invalidates only those at or after the modification point. This shows you understand performance and safety trade-offs.
Confirm that the vector is a contiguous dynamic array with size, capacity, and a pointer to heap-allocated memory. Mention that it supports random access and amortized O(1) push_back.
To insert at position pos, if size == capacity, reallocate to a larger buffer (e.g., double capacity) and copy/move elements. Then shift elements from pos to end one slot to the right, place the new element, and increment size.
To erase at position pos, shift elements from pos+1 to end one slot to the left, destroy the last element, and decrement size. No reallocation occurs, but elements after pos are moved.
Explain that reallocation happens only when inserting and size == capacity. The new capacity is typically 2x (or 1.5x) to maintain amortized O(1) for push_back, but insert in the middle is O(n) due to shifting.
If reallocation occurs, all iterators and references are invalidated. Otherwise, for insert, iterators/references at or after pos are invalidated (due to shifting); for erase, iterators/references at or after pos are invalidated. Elements before pos remain valid.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
I gave a surface-level answer about wrapping all allocations through allocator_traits::allocate and construct.
Start by explaining the general pattern of templating a container on an allocator and using std::allocator_traits to abstract allocator operations. Then focus on the move-assignment operator, highlighting how allocator propagation traits (propagate_on_container_move_assignment) determine whether you can steal the buffer or must move element-by-element. Conclude with the implications for exception safety and performance.
Pro tip: Mention that if propagate_on_container_move_assignment is false and allocators are unequal, you cannot steal the buffer—you must move elements individually, which is a common pitfall in high-performance code. Also note that std::allocator_traits provides a uniform interface even for allocators that don't define all members.
Add a template parameter `class Allocator = std::allocator<T>` to the vector class and store an allocator instance. Use `std::allocator_traits<Allocator>` for all allocation, deallocation, construction, and destruction operations.
Replace direct calls to `allocator.allocate`, `allocator.deallocate`, etc., with `std::allocator_traits<Allocator>::allocate`, `deallocate`, `construct`, and `destroy`. This ensures compatibility with minimal allocators.
In the move-assignment operator, check `std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value`. If true, move-assign the allocator and steal the buffer. If false, compare allocators: if equal, steal the buffer; if unequal, move elements individually.
Ensure strong exception safety by deallocating the old buffer only after successful allocation or by using copy-and-swap. Also guard against self-move-assignment.
Explain that propagation traits affect performance: stealing the buffer is O(1), while element-wise move is O(n). Mention that unequal allocators force the slower path, which is crucial in latency-sensitive systems like trading.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.