素数缓冲求法，可重用类

  #include <iostream>
  #include <algorithm>
  #include <vector>
  #include <iterator>
  
  template <typename T>
  class PrimerCache
- {
      typedef T   ValueType;
      typedef std::vector<ValueType> IntArray;
      typedef typename IntArray::const_iterator IntPtr;
  
      static IntArray mCache;
       
  protected:
      bool Test(ValueType num) const
-     {
          // make sure : half  < mCache.back()
          // !!! IncreaseCache(half) promised this !!!        
  
          ValueType half = num / 2 + 1;
         
          for(IntPtr i = mCache.begin(); *i < half && i != mCache.end();  ++i)
-         {
              if( num % (*i) == 0)
-             {
                   return false;
              }
          }         
          return true;
      }
  
      void IncreaseCache(ValueType num) const
-     {
          ValueType upper = mCache.back();
           
          while (upper < num)
-         {
              upper++;             
              if( Test(upper))
-             {
                  mCache.push_back(upper);
              }
               
          }         
      }
  
      bool InCache(ValueType num) const
-     {
          return std::binary_search(mCache.begin(), mCache.end(), num);
      }
  public:
      PrimerCache(ValueType lowerNum = 1000)
-     {
          IsPrimer(lowerNum);
      }
  
      bool IsPrimer(ValueType num) const
-     {
          ValueType biggestPrimer = mCache.back() + 1;
  
          if( biggestPrimer > num)
-         {
              return InCache(num);
          }
          else
-         {
              IncreaseCache(num);
              return Test(num);  
          }
      }
      friend std::ostream& operator<<(std::ostream& out, PrimerCache const& primerCache)
-     {
          IntArray& cache = primerCache.mCache;
          out << "Primer Cache Has " << cache.size() << " elements, they are " << std::endl;  
          std::copy(cache.begin(), cache.end(), std::ostream_iterator<int>(out, ","));
          return out;
      }
  };
  //the one and only cache;
  template <typename T>
  typename PrimerCache<T>::IntArray PrimerCache<T>::mCache = IntArray(1, 2);
  
  template <typename CacheType>
  void TestPrimer(CacheType& cache, int num)
- {
      int test = num;
      std::cout<< test << " is primer? "<< std::boolalpha << cache.IsPrimer(test) << std::endl;
  }
  
  
  int main()
- {
      PrimerCache<int> primer;
      int test;
  
      TestPrimer(primer, 1);
      TestPrimer(primer, 2);
      TestPrimer(primer, 4);
      TestPrimer(primer, 5);
      TestPrimer(primer, 99);
      TestPrimer(primer, 97);
      TestPrimer(primer, 101);
      TestPrimer(primer, 102);
      TestPrimer(primer, 98);
      TestPrimer(primer, 96);
       
  
      std::cout << primer << std::endl;
  
      std::cout << "primer is " << sizeof ( PrimerCache<int>) << std::endl;
  
      PrimerCache<short> p(10);
      
      TestPrimer(p, 1);
      TestPrimer(p, 2);
  
      std::cout << p << std::endl;
      return 0;
  }