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C++ Polymorphism & Destructors


Example

If a class is intended to be used polymorphically, with derived instances being stored as base pointers/references, its base class' destructor should be either virtual or protected. In the former case, this will cause object destruction to check the vtable, automatically calling the correct destructor based on the dynamic type. In the latter case, destroying the object through a base class pointer/reference is disabled, and the object can only be deleted when explicitly treated as its actual type.

struct VirtualDestructor {
    virtual ~VirtualDestructor() = default;
};

struct VirtualDerived : VirtualDestructor {};

struct ProtectedDestructor {
  protected:
    ~ProtectedDestructor() = default;
};

struct ProtectedDerived : ProtectedDestructor {
    ~ProtectedDerived() = default;
};

// ...

VirtualDestructor* vd = new VirtualDerived;
delete vd; // Looks up VirtualDestructor::~VirtualDestructor() in vtable, sees it's
           // VirtualDerived::~VirtualDerived(), calls that.

ProtectedDestructor* pd = new ProtectedDerived;
delete pd; // Error: ProtectedDestructor::~ProtectedDestructor() is protected.
delete static_cast<ProtectedDerived*>(pd); // Good.

Both of these practices guarantee that the derived class' destructor will always be called on derived class instances, preventing memory leaks.