C++ In Action: Language

Exercise Solutions

1. Here are the basic classes.

   class Sieve
   {
   public:
       virtual int NextNumber () = 0;
   };
   
   class SourceSieve: public Sieve
   {
   public:
       SourceSieve () : _i (1) {}
       int NextNumber ();
   private:
       int _i;
   };
   
   class Sieve2: public Sieve
   {
   public:
       Sieve2 (Sieve & src) : _src (src) {}
       int NextNumber ();
   private:
       Sieve & _src;
   };

   Here's the implementation of NextNumber for SourceSieve and Sieve2:

   int SourceSieve::NextNumber ()
   {
       if (_i > 100)
           return -1; // end
       return _i++;
   }
   
   int Sieve2::NextNumber ()
   {
       int i;
       do
       {
           i = _src.NextNumber ();
       } while (i % 2 == 0 && i != 2 && i != -1);
       return i;
   }

   This is main which prints all prime numbers less than 100:

   int main ()
   {
       SourceSieve src;
       Sieve2 s2 (src);
       Sieve3 s3 (s2);
       Sieve5 s5 (s3);
       Sieve7 s7 (s5);
       int i;
       for (;;)
       {
           i = s7.NextNumber ();
           if (i == -1)
               break;
           std::cout << i <<" ";
       }    
   }