/////////////////////////////////////////////////
// MT2Q3.cpp - Factory for Singleton           //
//                                             //
// Jim Fawcett, CSE687 - Midterm, Spring 2010  //
/////////////////////////////////////////////////

#include <string>
#include <iostream>
#include <sstream>

template <class B, class D=B>  // You need to use templates to build
class factory                  // one factory to create many deriveds.
{
public:
  B* Create()  // create singleton instance, useful for shared resource
  {
    if(instance != 0)
      return instance;
    return (instance = new D());  // this will succeed only if D derives
  }                               // from B or is B
private:
  static D* instance;
};

template<class B, class D>
D* factory<B,D>::instance = 0;

class Base
{
public:
  Base() { ++count; }
  virtual ~Base() {};
  virtual std::string say()
  {
    return "\n  Base here with count = " + instanceCount();
  }
  std::string instanceCount() 
  {
    std::ostringstream temp;
    temp << count;
    return temp.str(); 
  }
private:
  static size_t count;
};

size_t Base::count = 0;

class Derived1 : public Base
{
public:
  virtual std::string say() 
  { 
    return std::string("\n  Derived1 here with count = ") + instanceCount(); 
  }
};

class Derived2 : public Base
{
public:
  virtual std::string say() 
  { 
    return std::string("\n  Derived2 here with count = ") + instanceCount(); 
  }
};

void main()
{
  std::cout << "\n  MT2Q3 - Factory for Singleton";
  std::cout << "\n ===============================\n";

  try
  {
    factory<Base,Derived1> f1;

    // should get the same static Base::count of 1
    Base* pB1 = f1.Create();
    std::cout << pB1->say();

    Base* pB2 = f1.Create();
    std::cout << pB2->say();

    if(pB1 != pB2)
      throw std::exception("Not a singleton!");
    std::cout << std::endl;

    // should get the same static Base::count of 2
    factory<Base,Derived2> f2;
    Base* pB3 = f2.Create();
    std::cout << pB3->say();
    Base* pB4 = f2.Create();
    std::cout << pB4->say();

    if(pB3 != pB4)
      throw std::exception("Not a singleton!");

    if(pB1 == pB3)
      throw std::exception("new derived class should get new singleton");

    std::cout << std::endl;

    factory<Base> f3;
    Base* pB5 = f3.Create();
    std::cout << pB5->say() << "\n\n";
  }
  catch(std::exception& ex)
  {
    std::cout << "\n\n  " << ex.what() << "\n\n";
  }
}