Why a Virtual Destructor is Needed in C++

In C++, when you have a base class pointer pointing to a derived class object, deleting through the base pointer can cause problems if the base class destructor is not virtual.

Example

struct Base {
  // No virtual destructor
};

struct Derived : public Base {
  int* data;
  ~Derived() {
      delete data; // clean up allocated memory
  }
};

Base* obj = new Derived();
delete obj; // Problem! Only Base destructor is called

In this example, the Derived destructor is never called because the destructor in Base is not virtual. As a result, any resources allocated by Derived (like data) are not freed, causing a memory leak.

Solution: Virtual Destructor

struct Base {
  virtual ~Base() = default; // Virtual destructor ensures proper cleanup
};

struct Derived : public Base {
  int* data;
  ~Derived() {
      delete data;
  }
};

Base* obj = new Derived();
delete obj; // Correct! Derived destructor is called first, then Base destructor

By marking the base class destructor as virtual:

• C++ creates a vtable that stores the actual destructor to call.
• Deleting a derived object through a base pointer will call the derived destructor first, then the base destructor.
• All resources in the derived class are properly cleaned up, avoiding memory leaks.

Rule of Thumb

If a class has any virtual functions and is intended to be used polymorphically (deleted through a base pointer), its destructor should always be virtual.