What is a virtual destructor in C++ and why is it needed? Provide an example of incorrect behavior in the absence of a virtual destructor.

Answer.

A virtual destructor is a destructor declared with the virtual keyword in the base class. This ensures that the derived class's destructor is called when deleting an object through a pointer to the base class.

class Base {
public:
    virtual ~Base() { /* ... */ }
};
class Derived : public Base {
public:
    ~Derived() override { /* ... */ }
};

Base* ptr = new Derived();
delete ptr; // Both destructors will be called

Without a virtual destructor, the behavior may be incorrect — the derived class's destructor will not be called, leading to resource leaks!

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Trick Question.

Is it necessary to declare the destructor virtual if your base class has no virtual functions?

Answer: Yes, if the class is intended for inheritance with deletion via a pointer to the base type. Even if there are no other virtual functions, the virtuality of the destructor is necessary for the correct deletion of descendants.

class Shape {
public:
    virtual ~Shape() {}
};

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Examples of real errors due to ignorance of the topic's subtleties.

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Story 1

In a large graphics library project, OpenGL resources (buffers, textures) leaked when deleting drawable objects through a pointer to the base class — the corresponding releases were in the destructors of the derived classes, which were not called.

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Story 2

In a network library for the TCP protocol, when deleting sessions, the destructor of the descendants managed through a common base interface was not called. This led to widespread socket hangs and exhaustion of descriptor limits.

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Story 3

In an embedded system in C++, a wrongly declared (non-virtual) destructor of the base class caused memory leaks for system resources of input-output controllers, and devices failed after several driver update cycles.