~/ntl-11.4.2/doc/LazyTable.cpp.html

    /***************************************************************************

    LazyTable<T,MAX>: 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.

    Usage:

       LazyTable<T,MAX> tab; // declaration of the lazy table,
                             // with max size == MAX

        ...

       do {
          LazyTable<T,MAX>::Builder builder(tab, n); // request length n
          long amt = builder.amt();
          if (!amt) break;

          ... initialize elements i = n-amt..n-1
              using builder.move(p), where p is a UnqiuePtr<T>
              note that each move application appends one element

       } while(0);    // When this scope closes,
                      // the table is fully initialized to length n

       const T* val = table[i];  // read-only access to table elements 0..n-1

    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.

    ****************************************************************************/

    template<class T, long MAX>
    class LazyTable {
    public:
       LazyTable();
       ~LazyTable();

       const T * const  operator[] (long i) const;
       // element access -- currently no range checking

       long length() const;
       // current table length

       class Builder {
          Builder(const LazyTable&, long request);
          // EXCEPTIONS: may throw an exception if request is out of range
          // or if alocation of table fails

         ~Builder()

          long amt() const;

          void move(UniquePtr<T>& p);
          // EXCEPTIONS: throws exception of move is not allowed.
          // Provides strong ES guarantee.
       };

    private:
       LazyTable(const LazyTable&);             // disabled
       LazyTable& operator=(const LazyTable&);

    };

    // 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 LazyTable
    // 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.