/*
This file illustrates two ways of achieving polymorphism in C++, neither
form is complete satisfactory.  The first method uses macros to do advanced
copy-and-paste (see the define SWAP lines below).  The second methods uses
templates to define type variables.  But further investigation reveals that
this is just a way of automatically telling the compiler to generate several
different versions of the same program. C++ can not tell when a function 
should be compiled as polymorphic. 
*/


#define SWAP(a,b,type,temp)   type temp = a; \
                         a = b;     \
                         b = temp;


#include<iostream.h>


template <typename headType>
class cell
{public:
  headType head;
  cell *tail;
  cell(headType h, cell *t)  { head = h; tail = t; }
  ~cell() { if (tail != 0) delete(tail); }
}; // end polymorphic class cell


template <typename anyType>
int length(cell<anyType> *L)   // length of a polymorphic list
{ if (L == 0)  return 0; // null?
   else return 1 + length(L->tail);
}
 
int main()
{
  cell<int> *intlist;
  cell<char *>  *strlist;
  cell<float> *flist;
 
  intlist = new cell<int>(3, new cell<int>(5, new cell<int>(7,0)));
  strlist = new cell<char *>("abc", new cell<char *>("def", 0));
  flist = new cell<float>(1.1,new cell<float>(2.2,0)); 

  // cout is already polymorphic!
  cout << length(intlist) << "  " << length(strlist) 
       << " " << length(flist) << "\n";


  int x = 3;
  int y = 4;
  float a = 3.14;
  float b = 5.6;

  SWAP(x,y,int,temp1)
  SWAP(a,b,float,temp2)

    cout << " x == " << x << " and a == " << a << "\n";
  


  return 0;
} // end main

// whenever a polymorphic class is refered to in code, it must
// be instantiated.