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Explain C++ virtual dispatch and object lifetime

J.P. MorganData Scientist · Onsite
MediumData ScientistOnsiteSoftware Engineering Fundamentals

Last updated

This question tests the candidate's understanding of C++ memory management, the mechanics of the virtual method table (vtable), and the importance of virtual destructors in polymorphic inheritance.

The Problem

You are working on a high-performance quantitative trading library where object lifetime and memory safety are critical. Analyze the following C++ code snippet and answer the questions provided below.

class Base {
public:
    Base() { std::cout << "Base Ctor"; }
    ~Base() { std::cout << "Base Dtor"; }
    virtual void process() { std::cout << "Base Process"; }
};

class Derived : public Base {
public:
    Derived() { std::cout process();
    delete ptr;
}

Your Tasks:

  1. Virtual Dispatch: Explain how ptr->process() calls Derived::process() even though the pointer type is Base*.
  2. Memory Safety: Describe the issue with the code above regarding object destruction. What will happen when delete ptr is called, and how do you fix it?
  3. Lifetime: Briefly explain the difference in lifetime and management between creating an object on the stack (Derived d;) versus the heap (new Derived();).

Hints

Hint 1

Consider how the compiler uses a hidden pointer (often called a vptr) inside an object to resolve virtual functions at runtime.

Hint 2

Think about what the compiler generates for a non-virtual destructor when the object is deleted through a base class pointer.

Hint 3

Contrast the automatic scope-based cleanup of stack variables with the manual memory management required for heap-allocated pointers.

Follow-up questions

  • How do smart pointers like std::unique_ptr or std::shared_ptr mitigate the risk of memory leaks in this specific example?
  • What are the performance trade-offs associated with using virtual functions in a latency-sensitive application?
  • How does the behavior of the code change if we pass the object by value (`void func(Base b)`) instead of using a pointer or reference?