/*
 * cloneable-strostrup.cc
 */
#include <iostream>
using namespace std;

//
// Every class having Has_Clone as a (private) base class must invoke
// the Has_Clone<T> default constructor, which does nothing but test
// the constraint. If the constraint test fails the compilers will emit
// an error message.
//
// Compiling with g++ and option -Wall will emit some warnings:
// 
// warning: unused variable 'pc'
//
//   Adding, in analogy with what's done in constraints() for 'test'
//   will only result in the same warning as for 'test', i.e. 
//   "statement has no effect".
//
// warning: statement has no effect
//
//   Refers to the expression statement 'test;', which will normally
//   will be optimized away.
//
template<typename T>
class Has_Clone 
{
   public:
      Has_Clone() { void (*p)() = constraints; }
   private:
      static void constraints() {
	 T* (T::*test)() const = &T::clone;
	 test;  // supress some warnings
      }
};

//
// A test class C, having Has_Clone as a private base class, so
// Has_Clone::constraints() will not add to the public interface
// for C.
//
template<typename T>
class C : Has_Clone<T>
{
   public:
      C() {}
   private:
};

//
// Two test classes, one with a clone() function and one without:
//
template<typename T>
class Cloneable 
{
   public:
      Cloneable();
      virtual ~Cloneable() {}
      virtual Cloneable* clone() const { return new Cloneable(*this); }
};

template<class T>
class Not_Cloneable 
{
   public:
      Not_Cloneable();
      virtual ~Not_Cloneable() {}
};

int main()
{
   C< Cloneable<int> >    c;

   // error: `clone' is not a member of `Not_Cloneable<int>'
// C<Not_Cloneable<int> > nc;

   C< Cloneable<int> >* pc = new C< Cloneable<int> >;

   // error: `clone' is not a member of `Not_Cloneable<int>'
// C< Not_Cloneable<int> >* pn = new C< Not_Cloneable<int> >;

   delete pc;

   return 0;
}

template<typename T>
Cloneable<T>::Cloneable()
{
   cout << "Cloneable created" << endl;
}

template<typename T>
Not_Cloneable<T>::Not_Cloneable()
{
   cout << "Not_Cloneable created" << endl;
}