C / C++ / MFC

// PolymorphismA.cpp

#include "stdafx.h"

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <sstream>
#include <iomanip>
#include <string>

using namespace std;

// We want, inasmuch as possible, to work only with CBase pointers
// but we have a hierarchy based on CBase derived classes

class CBase // CBase is our base class
{
private:
   CBase() // default ctor is protected, we do not want to default construct
   { cout << "CBase() default ctor called" << endl; 
     _data = -1; _isvalid = false; };

public:
   explicit CBase(const int data) // explicit Construction
   { cout << "CBase(const int data) explicit ctor called" << endl; 
     _data = data; _isvalid = true; };
   virtual ~CBase() // polymorphism, virtual dtor
   { cout << "~CBase() dtor called" << endl; };

public:
   // CBase and derived objects are copy-constructible and assignable through base pointers
   CBase(const CBase& iOther) // it cannot be declared as virtual
   { cout << "CBase(const CBase& iOther) copy ctor called" << endl; 
     copyBase(iOther); // copy the base
   };
   virtual CBase& operator=(const CBase& iOther) // we declare it as virtual
   { cout << "CBase& CBase::operator=(const CBase& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        copyBase(iOther); // copy the base
     }
     return *this;
   };

private:
   // Aux Method to copy the base class, common to copy ctor and assignment op.
   void copyBase(const CBase& iOther)
   { cout << "CBase::copyBase() copy called" << endl; 
     _data = iOther._data; _isvalid = iOther._isvalid; };

public: // we also have real methods
   int getData() const // Accessor
   { return _data; };
   void setData(const int data) // Setter
   { _data = data; };
   bool isValid() const // Accessor
   { return _isvalid; };
   void setValid(const bool isvalid) // Setter
   { _isvalid = isvalid; };

private:
   int _data;     // CBase state
   bool _isvalid; // CBase state
};

class CDerivedA : public CBase // CDerivedA is derived from CBase interface
{
private:
   CDerivedA() // default ctor is protected, we do not want to default construct
      : CBase((int) -1)
   { cout << "CDerivedA() default ctor called" << endl; 
     _dataA = -1.0; setValid(false); };

public:
   explicit CDerivedA(const double dataA) // explicit Construction
      : CBase((int) 0) // explicitly invoke base constructor
   { cout << "CDerivedA(const double dataA) explicit ctor called" << endl; 
     _dataA = dataA; };
   virtual ~CDerivedA() // polymorphism, virtual dtor
   { cout << "~CDerivedA() dtor called" << endl; };

public:
   // Also CDerivedA are copy-constructible and assignable in case we work directly with pointers to CDerivedA
   // We need Specific to this class copy ctor & assignment op.
   CDerivedA(const CDerivedA& iOther) // it cannot be declared as virtual
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedA(const CDerivedA& iOther) copy ctor called" << endl; 
     _dataA = iOther._dataA; // our specific data copy
   };
   virtual CDerivedA& operator=(const CDerivedA& iOther) // declared as virtual for further derivation
   { cout << "CDerivedA& CDerivedA::operator=(const CDerivedA& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        // We may call the base class assignment operator... as in
        this->CBase::operator=(iOther);
        // ... or we may call the base copy ctor
        // this->CBase::CBase(iOther);
        _dataA = iOther._dataA; // our specific data copy
     }
     return *this;
   };
   // Define a new copy ctor if we are passed the base class pointer
   CDerivedA(const CBase& iOther)
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedA(const CBase& iOther) copy ctor called" << endl;
     // our specific data copy
     // here we do the derived class copy, but we have a CBase object to deal with...
     // we need to see if we are of the same kind.
     buildVirtualCopy(iOther);
   };
   // Override the base class assignment operator
   virtual CBase& operator=(const CBase& iOther) // declared as virtual in base class
   { cout << "CBase& CDerivedA::operator=(const CBase& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        // We may call the base class assignment operator... as in
        this->CBase::operator=(iOther);
        // ... or we may call the base copy ctor
        // this->CBase::CBase(iOther);
        // our specific data copy
        // here we do the derived class copy, but we have a CBase object to deal with...
        // we need to see if we are of the same kind.
        buildVirtualCopy(iOther);
     }
     return *this;
   };

private:
   // aux method
   void buildVirtualCopy(const CBase& iOther)
   {
      cout << "CDerivedA::buildVirtualCopy copy ctor called" << endl;
      // try to down-cast to our type
      const CDerivedA* pOther = dynamic_cast<const CDerivedA*> (&iOther);
      if ( nullptr != pOther ) // we are of the same kind
      {
         // do the copy
         _dataA = pOther->_dataA;
      } 
      else // deal with different object kind
      {
         cout << "CDerivedA: explicit conversion from other type is forbidden" << endl;
         setValid(false); // here we just set the copy as invalid
         // but we could also have thrown an exception
         // ... or defined an explicit conversion policy from other types
      }
   };

public: // we have also real methods
   double getDataA() const // Accessor
   { return _dataA; };
   void setDataA(const double dataA) // Setter
   { _dataA = dataA; };

private:
   double _dataA; // CDerivedA state
};

class CDerivedB : public CBase // CDerivedB is derived from CBase interface
{
private:
   CDerivedB() // default ctor is protected, we do not want to default construct
      : CBase((int) -1)
   { cout << "CDerivedB() default ctor called" << endl; 
     _dataB = ""; setValid(false); };

public:
   explicit CDerivedB(const string dataB) // explicit Construction
      : CBase((int) 1) // explicitly invoke base constructor
   { cout << "CDerivedB(const string dataB) explicit ctor called" << endl; 
     _dataB = dataB; };
   virtual ~CDerivedB() // polymorphism, virtual dtor
   { cout << "~CDerivedB() dtor called" << endl; };

public:
   // Also CDerivedB are copy-constructible and assignable in case we work directly with pointers to CDerivedB
   // We need Specific to this class copy ctor & assignment op.
   CDerivedB(const CDerivedB& iOther) // it cannot be declared as virtual
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedB(const CDerivedB& iOther) copy ctor called" << endl; 
     _dataB = iOther._dataB; // our specific data copy
   };
   virtual CDerivedB& operator=(const CDerivedB& iOther) // declared as virtual for further derivation
   { cout << "CDerivedB& CDerivedB::operator=(const CDerivedB& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        // We may call the base class assignment operator... as in
        this->CBase::operator=(iOther);
        // ... or we may call the base copy ctor
        // this->CBase::CBase(iOther);
        _dataB = iOther._dataB; // our specific data copy
     }
     return *this;
   };
   // Define a new copy ctor if we are passed the base class pointer
   CDerivedB(const CBase& iOther)
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedB(const CBase& iOther) copy ctor called" << endl;
     // our specific data copy
     // here we do the derived class copy, but we have a CBase object to deal with...
     // we need to see if we are of the same kind.
     buildVirtualCopy(iOther);
   };
   // Override the base class assignment operator
   virtual CBase& operator=(const CBase& iOther) // declared as virtual in base class
   { cout << "CBase& CDerivedB::operator=(const CBase& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        // We may call the base class assignment operator... as in
        this->CBase::operator=(iOther);
        // ... or we may call the base copy ctor
        // this->CBase::CBase(iOther);

        // our specific data copy
        // here we do the derived class copy, but we have a CBase object to deal with...
        // we need to see if we are of the same kind.
        buildVirtualCopy(iOther);
     }
     return *this;
   };

private:
   // aux method
   void buildVirtualCopy(const CBase& iOther)
   {
      cout << "CDerivedB::buildVirtualCopy copy ctor called" << endl;
      // try to down-cast to our type
      const CDerivedB* pOther = dynamic_cast<const CDerivedB*> (&iOther); 
      if ( nullptr != pOther ) // we are of the same kind
      {
         // do the copy
         _dataB = pOther->_dataB;
      } 
      else // deal with different object kind
      {
         // let us look for a type we know of (cross-cast)
         // try to down-cast to this other type
         const CDerivedA* pOtherA = dynamic_cast<const CDerivedA*> (&iOther); 
         if ( nullptr != pOtherA ) // we know about this type
         {
            // do a conversion copy
            stringstream ss;
            ss << setw(4) << pOtherA->getDataA();
            _dataB = ss.str();
            cout << "CDerivedB: explicit conversion from CDerivedA type" << endl;
         } 
         else // deal with different object kind
         {
            setValid(false); // here we just set the copy as invalid
            // but we could also have thrown an exception
         }
      }
   };

public: // we have also real methods
   string getDataB() const // Accessor
   { return _dataB; };
   void setDataB(const string& dataB) // Setter
   { _dataB = dataB; };

private:
   string _dataB; // CDerivedB state
};

// PolymorphismB.cpp

#include "stdafx.h"

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <sstream>
#include <iomanip>
#include <string>

using namespace std;

// We want, inasmuch as possible, to work only with CBase pointers
// but we have a hierarchy based on CBase derived classes

class CBase // CBase is our base class
{
private:
   CBase() // default ctor is protected, we do not want to default construct
   { cout << "CBase() default ctor called" << endl; 
     _data = -1; _isvalid = false; };

public:
   explicit CBase(const int data) // explicit Construction
   { cout << "CBase(const int data) explicit ctor called" << endl; 
     _data = data; _isvalid = true; };
   virtual ~CBase() // polymorphism, virtual dtor
   { cout << "~CBase() dtor called" << endl; };

public:
   // CBase and derived objects are copy-constructible and assignable through base pointers
   CBase(const CBase& iOther) // it cannot be declared as virtual
   { cout << "CBase(const CBase& iOther) copy ctor called" << endl; 
     copyBase(iOther); // copy the base
     // it does not know about other types, No need to call virtual copy
   };
   CBase& operator=(const CBase& iOther) // invoking virtual copy method
   { cout << "CBase& CBase::operator=(const CBase& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        copyBase(iOther); // copy the base
        buildVirtualCopy(iOther); // Delegate to virtual copy if needed
     }
     return *this;
   };

private:
   // Aux Method to copy the base class
   void copyBase(const CBase& iOther)
   { cout << "CBase::copyBase() copy called" << endl; 
     _data = iOther._data; _isvalid = iOther._isvalid; };

protected:
   // Aux Method for derived classes copy
   virtual void buildVirtualCopy(const CBase& iOther) // do nothing in base class
   { cout << "CBase::buildVirtualCopy copy ctor called, it does nothing" << endl; };

public: // we also have real methods
   int getData() const // Accessor
   { return _data; };
   void setData(const int data) // Setter
   { _data = data; };
   bool isValid() const // Accessor
   { return _isvalid; };
   void setValid(const bool isvalid) // Setter
   { _isvalid = isvalid; };

private:
   int _data;     // CBase state
   bool _isvalid; // CBase state
};

class CDerivedA : public CBase // CDerivedA is derived from CBase interface
{
private:
   CDerivedA() // default ctor is protected, we do not want to default construct
      : CBase((int) -1)
   { cout << "CDerivedA() default ctor called" << endl; 
     _dataA = -1.0; setValid(false); };

public:
   explicit CDerivedA(const double dataA) // explicit Construction
      : CBase((int) 0) // explicitly invoke base constructor
   { cout << "CDerivedA(const double dataA) explicit ctor called" << endl; 
     _dataA = dataA; };
   virtual ~CDerivedA() // polymorphism, virtual dtor
   { cout << "~CDerivedA() dtor called" << endl; };

public:
   // Also CDerivedA are copy-constructible and assignable in case we work directly with pointers to CDerivedA
   // We need Specific to this class copy ctor & assignment op.
   CDerivedA(const CDerivedA& iOther) // it cannot be declared as virtual
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedA(const CDerivedA& iOther) copy ctor called" << endl; 
     _dataA = iOther._dataA; // our specific data copy
   };
   CDerivedA& operator=(const CDerivedA& iOther)
   { cout << "CDerivedA& CDerivedA::operator=(const CDerivedA& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        // if we call the base class assignment operator... as in
        // this->CBase::operator=(iOther);
        // then we will invoke as well our virtual copy...
        // hence we cannot call the base class assignment operator.
        // but we can do this...
        this->CBase::CBase(iOther); // call the base copy ctor
        _dataA = iOther._dataA; // our specific data copy
     }
     return *this;
   };
   // Define a new copy ctor for the base class pointers
   CDerivedA(const CBase& iOther)
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedA(const CBase& iOther) copy ctor called" << endl; 
     // here we do the derived class copy, but we have a CBase object to deal with...
     // we need to see if we are of the same kind.
     buildVirtualCopy(iOther);
   };

protected:
   // aux method
   virtual void buildVirtualCopy(const CBase& iOther)
   {
      cout << "CDerivedA::buildVirtualCopy copy ctor called" << endl;
      // try to down-cast to our type
      const CDerivedA* pOther = dynamic_cast<const CDerivedA*> (&iOther); 
      if ( nullptr != pOther ) // we are of the same kind
      {
         // do the copy
         _dataA = pOther->_dataA;
      } 
      else // deal with different object kind
      {
         cout << "CDerivedA: explicit conversion from other type is forbidden" << endl;
         setValid(false); // here we just set the copy as invalid
         // but we could also have thrown an exception
         // ... or defined an explicit conversion policy from other types
      }
   };

public: // we have also real methods
   double getDataA() const // Accessor
   { return _dataA; };
   void setDataA(const double dataA) // Setter
   { _dataA = dataA; };

private:
   double _dataA; // CDerivedA state
};

class CDerivedB : public CBase // CDerivedB is derived from CBase interface
{
private:
   CDerivedB() // default ctor is protected, we do not want to default construct
      : CBase((int) -1)
   { cout << "CDerivedB() default ctor called" << endl; 
     _dataB = ""; setValid(false); };

public:
   explicit CDerivedB(const string dataB) // explicit Construction
      : CBase((int) 1) // explicitly invoke base constructor
   { cout << "CDerivedB(const string dataB) explicit ctor called" << endl; 
     _dataB = dataB; };
   virtual ~CDerivedB() // polymorphism, virtual dtor
   { cout << "~CDerivedB() dtor called" << endl; };

public:
   // Also CDerivedB are copy-constructible and assignable in case we work directly with pointers to CDerivedB
   // We need Specific to this class copy ctor & assignment op.
   CDerivedB(const CDerivedB& iOther) // it cannot be declared as virtual
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedB(const CDerivedB& iOther) copy ctor called" << endl; 
     _dataB = iOther._dataB; // our specific data copy
   };
   CDerivedB& operator=(const CDerivedB& iOther)
   { cout << "CDerivedB& CDerivedB::operator=(const CDerivedB& iOther) assignment called" << endl; 
     if( this != &iOther ) // prevents self-assignment
     {
        // if we call the base class assignment operator... as in
        // this->CBase::operator=(iOther);
        // then we will invoke as well our virtual copy...
        // hence we cannot call the base class assignment operator.
        // but we can do this...
        this->CBase::CBase(iOther); // call the base copy ctor
        _dataB = iOther._dataB; // our specific data copy
     }
     return *this;
   };
   // Override the base class copy ctor & assignment op.
   CDerivedB(const CBase& iOther) // it cannot be declared as virtual
      : CBase(iOther) // copy the base by calling the base copy ctor
   { cout << "CDerivedB(const CBase& iOther) copy ctor called" << endl; 
     // copyBase(iOther); // we cannot use directly this base method, otherwise default ctor invoked
     // here we do the derived class copy, but we have a CBase object to deal with...
     // we need to see if we are of the same kind.
     buildVirtualCopy(iOther);
   };

protected:
   // aux method
   virtual void buildVirtualCopy(const CBase& iOther)
   {
      cout << "CDerivedB::buildVirtualCopy copy ctor called" << endl;
      // try to down-cast to our type
      const CDerivedB* pOther = dynamic_cast<const CDerivedB*> (&iOther); 
      if ( nullptr != pOther ) // we are of the same kind
      {
         // do the copy
         _dataB = pOther->_dataB;
      } 
      else // deal with different object kind
      {
         // let us look for a type we know of (cross-cast)
         // try to down-cast to this other type
         const CDerivedA* pOtherA = dynamic_cast<const CDerivedA*> (&iOther); 
         if ( nullptr != pOtherA ) // we know about this type
         {
            // do a conversion copy
            stringstream ss;
            ss << setw(4) << pOtherA->getDataA();
            _dataB = ss.str();
            cout << "CDerivedB: explicit conversion from CDerivedA type" << endl;
         } 
         else // deal with different object kind
         {
            setValid(false); // here we just set the copy as invalid
            // but we could also have thrown an exception
         }
      }
   };

public: // we have also real methods
   string getDataB() const // Accessor
   { return _dataB; };
   void setDataB(const string& dataB) // Setter
   { _dataB = dataB; };

private:
   string _dataB; // CDerivedB state
};

// Now let us play with our types
int _tmain(int argc, _TCHAR* argv[])
{
   cout << "Program starts..." << endl;
   cout << endl;

   {
      CDerivedA* pA0 = new CDerivedA((double) 1.0);
      CDerivedA* pA1 = new CDerivedA((double) 2.0);
      CDerivedB* pB0 = new CDerivedB(string("This is B1"));
      CDerivedB* pB1 = new CDerivedB(string("This is B2"));
      cout << endl;

      CBase* p00 = nullptr;
      CBase* p01 = nullptr;

      // Assignment via base pointers of type A objects
      cout << "Assignment via base pointers of type A objects" << endl;
      p00 = pA0;
      p01 = pA1;
      // A0 <-- A1
      *p00 = *p01;
      cout << "pA0 dataA = " << pA0->getDataA() << endl;
      cout << "pA1 dataA = " << pA1->getDataA() << endl;
      cout << endl;

      p00 = nullptr;
      p01 = nullptr;

      // Assignment via base pointers of type B objects
      cout << "Assignment via base pointers of type B objects" << endl;
      p00 = pB0;
      p01 = pB1;
      // B0 <-- B1
      *p00 = *p01;
      cout << "pB0 dataB = " << pB0->getDataB() << endl;
      cout << "pB1 dataB = " << pB1->getDataB() << endl;
      cout << endl;

      p00 = nullptr;
      p01 = nullptr;

      // "cross assignment": from B type to A type is forbidden
      cout << "Cross Assignment via base pointers from type B objects to type A is forbidden" << endl;
      p00 = pA0;
      p01 = pB0;
      // A0 <-- B0, forbidden
      cout << "p00 is pointing to pA0 " << endl;
      *p00 = *p01;
      cout << "p00 is valid = " << p00->isValid() << endl;
      cout << endl;

      p00 = nullptr;
      p01 = nullptr;

      // "cross assignment": from A type to B type is allowed
      cout << "Cross Assignment via base pointers from type A objects to type B" << endl;
      cout << "is allowed by custom conversion... " << endl;
      p00 = pB1;
      p01 = pA1;
      // B1 <-- A1, custom conversion allowed
      cout << "p00 is pointing to pB1 " << endl;
      *p00 = *p01;
      cout << "p00 is valid = " << p00->isValid() << endl;
      cout << "p00 --> pB1 dataB = " << pB1->getDataB() << endl;
      cout << endl;

      p00 = nullptr;
      p01 = nullptr;

      delete pA0;
      delete pA1;
      delete pB0;
      delete pB1;
      cout << endl;
   }

   // we can deal directly with pointers to derived classes
   {
      CDerivedA* pA0 = new CDerivedA((double) 1.0);
      CDerivedA* pA1 = new CDerivedA((double) 2.0);
      CDerivedB* pB0 = new CDerivedB(string("This is B1"));
      CDerivedB* pB1 = new CDerivedB(string("This is B2"));
      cout << endl;

      // Assignment via pointers of type A objects
      cout << "Assignment via pointers of type A objects" << endl;
      // A0 <-- A1
      *pA0 = *pA1;
      cout << "pA0 dataA = " << pA0->getDataA() << endl;
      cout << "pA1 dataA = " << pA1->getDataA() << endl;
      cout << endl;

      // Assignment via pointers of type B objects
      cout << "Assignment via pointers of type B objects" << endl;
      // B0 <-- B1
      *pB0 = *pB1;
      cout << "pB0 dataB = " << pB0->getDataB() << endl;
      cout << "pB1 dataB = " << pB1->getDataB() << endl;
      cout << endl;

      delete pA0;
      delete pA1;
      delete pB0;
      delete pB1;
      cout << endl;
   }

   // of course we can deal also with base classes
   {
      cout << "Base class Assignment just works." << endl;
      CBase* p00 = nullptr;
      CBase* p01 = nullptr;
      p00 = new CBase((int) 10);
      p01 = new CBase((int) 11);
      *p00 = *p01;
      cout << "p00 is valid = " << p00->isValid() << endl;
      cout << "p00 data = " << p00->getData() << endl;
      cout << "p01 is valid = " << p01->isValid() << endl;
      cout << "p01 data = " << p01->getData() << endl;
      cout << endl;

      delete p00;
      cout << endl;

      cout << "Base class copy ctor just works." << endl;
      p00 = new CBase(*p01);
      cout << "p00 is valid = " << p00->isValid() << endl;
      cout << "p00 data = " << p00->getData() << endl;

      delete p00;
      delete p01;
      cout << endl;
   }

   // what about copy construction ?
   {
      // Copy ctor via pointers of type A objects
      cout << "Copy ctor via pointers of type A objects" << endl;
      CDerivedA* pA1 = new CDerivedA((double) 2.0);
      cout << endl;
      CDerivedA* pA2 = new CDerivedA(*pA1);
      cout << "pA1 dataA = " << pA1->getDataA() << endl;
      cout << "pA2 dataA = " << pA2->getDataA() << endl;
      delete pA1;
      delete pA2;
      cout << endl;

      // Copy ctor via pointers of type B objects
      cout << "Copy ctor via pointers of type B objects" << endl;
      CDerivedB* pB1 = new CDerivedB(string("This is B2"));
      cout << endl;
      CDerivedB* pB2 = new CDerivedB(*pB1);
      cout << "pB1 dataB = " << pB1->getDataB() << endl;
      cout << "pB2 dataB = " << pB2->getDataB() << endl;
      delete pB1;
      delete pB2;
      cout << endl;

      // Copy ctor via base pointers of type A,B objects
      cout << "Copy ctor via base pointers of type A,B objects" << endl;
      CBase* p00 = new CDerivedA((double) 1.0);
      CBase* p01 = new CDerivedB(string("This is B1"));
      cout << endl;

      cout << "A from A via base" << endl;
      CDerivedA* pA3 = new CDerivedA(*p00);
      cout << endl;
      cout << "base from A (slicing)" << endl;
      CBase*     pA4 = new CBase(*p00); // slice it
      cout << endl;

      cout << "B from B via base" << endl;
      CDerivedB* pB3 = new CDerivedB(*p01);
      cout << endl;
      cout << "base from B (slicing)" << endl;
      CBase*     pB4 = new CBase(*p01); // slice it
      cout << endl;

      cout << "A from B via base" << endl;
      CDerivedA* pA5 = new CDerivedA(*p01);
      cout << endl;

      cout << "B from A via base" << endl;
      CDerivedB* pB5 = new CDerivedB(*p00);
      cout << endl;

      delete pA3;
      delete pA4;
      delete pA5;

      delete pB3;
      delete pB4;
      delete pB5;

      delete p00;
      delete p01;

      cout << endl;
   }

   int k = getchar();
   return 0;
}