Re: Calling a non-virtual member function for a NULL instance

Matt Curtis wrote:


But that's so easy! :-)

The work around is to make Cell a proxy class that holds a (possibly
NULL) pointer to a CellContent object that does the real work. The
proxy has a non-virtual function which tests whether the internal
pointer is null and either dispatches the call to the pointed-to object
or returns the default value.

#include <vector>
#include <numeric>
#include <iostream>

// handy utility
template < class T >
struct Destroyer {
   Destroyer( T& container ): mContainer( container ) {}
   ~Destroyer()
   {
      for (typename T::iterator i = mContainer.begin();
           i != mContainer.end();
           ++i) {
         i->Destroy();
      }
   }
   T& mContainer;
};

// base class for real cell content
class CellContent {
public:
   virtual double GetValue() { return 2.5; }
};

// proxy class for storage in container
class Cell {
public:
   Cell(): pData(0) {}
   Cell( CellContent* p ): pData(p) {}
   double GetValue() const {
      return pData ? (pData->GetValue()) : 0.0;
   }
   void Destroy() { delete pData; }
private:
   CellContent* pData;
};

struct AddUp {
   double operator() (double d, const Cell& c) const
   {
      return d + c.GetValue();
   }
};

int main()
{
   std::vector<Cell> cells (100);
   Destroyer< std::vector<Cell> > x (cells);
   cells[10] = Cell( new CellContent );
   cells[20] = Cell( new CellContent );
   double d = std::accumulate( cells.begin(), cells.end(), 0.0, AddUp()
);
   std::cout << d << std::endl;
}

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