Lazy<T>: template class for lazy initialization of objects whose
values do not change after initialization.
In a multi-threaded environment, this makes use of "double checked locking"
for an efficient, thread-safe solution.


   Lazy<T> obj; // declaration of the lazy object


   do {
      Lazy<T>::Builder builder(obj);
      if (!builder()) break; // if we are not building, the break out

      UniquePtr<T> p;  // create a pointer 


      builder.move(p); // move p into the object to complete the initialization
                       // We can then complete the initialization process.
   } while(0);  // When this scope closes, the object is fully initialized.
                // subsequent attempts to build the object will yield
                // !builder.built()

   T objCopy = *obj;   // *obj returns a read-only reference
                       // one can also use -> operator

It is important to follow this recipe carefully.  In particular,
the builder must be enclosed in a scope, as it's destructor
plays a crucial role in finalizing the initialization.

NOTE: if p is null in builder.move(p), the object is still considered


template<class T, class P=DefaultDeleterPolicy>
class Lazy {

   Lazy(const Lazy&);
   Lazy& operator=(const Lazy&);
   // deep copies using T's copy constructor
   // EXCEPTIONS: may throw (but provides strong ES guarantee)

   const T& operator*()  const;     // pointer access
   const T* operator->() const;
   const T* get() const;

   operator fake_null_type() const;
   // allows test against 0


   kill();  // destroy and reset

   bool built() const; // test if already built

   class Builder {
      Builder(const Lazy&);

      bool operator()() const; // test if we are building

      void move(UniquePtr<T,P>&);
      // EXCEPTIONS: may throw an exception if the move is not allowed
      // (i.e., not building or already moved).
      // Provides strong ES guarantee. 

// EXCEPTIONS: except where noted, no exceptions are thrown

// NOTE: For more on double-checked locking, see
// http://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/

// NOTE: when compiled with the NTL_THREADS option, the Lazy
// class may contain data members from the standard library
// that may not satisfy the requirements of the Vec class
// (i.e., relocatability).  One can wrap it in a pointer 
// class (e.g., CopiedPtr) to deal with this.

// NOTE: The optional parameter P is used as in the specification
// of the UniquePtr class.  The default should work fine in 
// most cases.  It was introduced mainly to allow for the PIMPL
// paradigm.