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Item 7: Declare destructors virtual in polymorphic base classes

Concept​

When a derived class object is deleted through a pointer to a base class with a non-virtual destructor, the behavior is undefined — typically the derived part of the object is never destroyed. Any class with virtual functions should almost certainly have a virtual destructor. Conversely, classes not designed to be base classes or not designed to be used polymorphically should not declare virtual destructors, because virtual functions add the overhead of a vptr (virtual table pointer).

Code Example​

class TimeKeeper {
public:
    TimeKeeper() {}
    virtual ~TimeKeeper() {}  // virtual destructor is essential
    virtual void getCurrentTime() const = 0;
};

class AtomicClock : public TimeKeeper {
public:
    ~AtomicClock() override { /* cleanup atomic clock resources */ }
    void getCurrentTime() const override { /* ... */ }
};

// Correct usage with polymorphic deletion
TimeKeeper* ptk = new AtomicClock();
delete ptk;  // Correctly calls ~AtomicClock() then ~TimeKeeper()

// Without virtual destructor, this would be undefined behavior!

Full source code

Things to Remember​

  • Polymorphic base classes should declare virtual destructors. If a class has any virtual functions, it should have a virtual destructor.
  • Classes not designed to be base classes or not designed to be used polymorphically should not declare virtual destructors.
  • Don't inherit from classes with non-virtual destructors (e.g., std::string, STL containers).
  • Item 5 — Compiler-generated destructors are non-virtual by default
  • Item 9 — Virtual function behavior during construction and destruction
  • Item 34 — Differentiate between inheritance of interface and implementation