Allow the operator to be invoked on both const and non-const objects.

   
   class Array
   {
   public:
      Array()
      {
         for ( int i = 0; i < Max_Size; ++i )
         {
            x[ i ] = i;
         }
      }
      
      int& operator[] ( const int a )
      {
         std::cout<< "nonconst" << std::endl;
         return x[ a ];
      }
      
      int operator[] ( const int a ) const
      {
         std::cout << "const" << std::endl;
         return x[ a ];
      }
   
   private:
      enum { Max_Size = 10 };
      int x[ Max_Size ];
   };
   
   int main()
   {
      Array a;
      int i = a[ 3 ];   //non-const
      a[ 3 ] = 33;      //non-const
   
      const Array ca;
      i = ca[ 3 ];      //const
      ca[ 3 ] = 33;     //compilation error
   
      return 0;
   }

Effective C++ Item 18;