What is the correct way of using C++11's range-based for?

Question

What is the correct way of using C++11\'s range-based for?

What syntax should be used? for (auto elem : container), or for (auto&

Answer 1

While the initial motivation of the range-for loop might have been ease of iterating over the elements of a container, the syntax is generic enough to be useful even for objects that are not purely containers.

The syntactic requirement for the for-loop is that range_expression support begin() and end() as either functions -- either as member functions of the type that it evaluates to or as non-member functions what take an instance of the type.

As a contrived example, one can generate a range of numbers and iterate over the range using the following class.

struct Range
{
   struct Iterator
   {
      Iterator(int v, int s) : val(v), step(s) {}

      int operator*() const
      {
         return val;
      }

      Iterator& operator++()
      {
         val += step;
         return *this;
      }

      bool operator!=(Iterator const& rhs) const
      {
         return (this->val < rhs.val);
      }

      int val;
      int step;
   };

   Range(int l, int h, int s=1) : low(l), high(h), step(s) {}

   Iterator begin() const
   {
      return Iterator(low, step);
   }

   Iterator end() const
   {
      return Iterator(high, 1);
   }

   int low, high, step;
}; 

With the following main function,

#include 

int main()
{
   Range r1(1, 10);
   for ( auto item : r1 )
   {
      std::cout << item << " ";
   }
   std::cout << std::endl;

   Range r2(1, 20, 2);
   for ( auto item : r2 )
   {
      std::cout << item << " ";
   }
   std::cout << std::endl;

   Range r3(1, 20, 3);
   for ( auto item : r3 )
   {
      std::cout << item << " ";
   }
   std::cout << std::endl;
}

one would get the following output.

1 2 3 4 5 6 7 8 9 
1 3 5 7 9 11 13 15 17 19 
1 4 7 10 13 16 19