Destructor – Compiler Generated : always non-virtual

The compiler never generates a virtual destructor. Such destructors are always non-virtual. This is because C++ compiler cannot decide if in given architecture, the given class needs to become a base class or not. Therefore, in the absence of a “virtual” destructor in base class, the compiler shall not call corresponding function in derived class objects. Since, a non-virtual destructor of base class can not call virtual destructors in derived classes. Even when the given derived classes explicitly declare their destructors as virtual, then also the compiler shall not call them.

Example

In this example, the MFBase class looks empty. This means it shall get compiler-generated destructor. (Actually, it shall get all other compiler-generated special functions- constructor, assignment operator also). However, the MFDerived class has an explicit “virtual” keyword. This generated special functions of MFBase is non-virtual, therefore, this shall never call destructor of MFDerived.

#include <iostream>  //main header
using namespace std; //namespace

class MFBase
{
     //Destructor is compiler-generated
};

class MFDerived :public MFBase
{
    public:
    MFDerived()
    {
        cout << "Derived Constructor" 
             << endl;
    }
    virtual ~MFDerived()  
             //virtual in derived class
    {
        cout << "Derived Destructor" 
             << endl;
    }
};

int main()
{
  MFDerived *d1 = new MFDerived;
  MFDerived *d2 = new MFDerived;

  MFBase *b = d1;

  delete b;  //Problematic
  delete d2; 
  
  return 0;  
}

The compiler destroy the derived class part only in case of “d2”. The output of the program will be:

Derived Constructor    //d1 (or b)
Derived Constructor    //d2
Derived Destructor     //d1 (or b)

In case of “b”, clearly, the compiler destroys only the base class part, i.e. in MFBase class. The MFBase class compiler contains a compiler-generated destructor. Since, this is a “non-virtual” so it shall not call MFDerived. Please note that MFDerived class clearly has explicit virtual keyword.

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