It is recommended that you read the recipe Using range-based for loops to iterate on a range before continuing with this one if you need to understand how range-based for loops work and what is the code the compiler generates for such a loop.

To show how we can enable range-based for loops for custom types representing sequences, we will use the following implementation of a simple array:

    template <typename T, size_t const Size> 
    class dummy_array 
    { 
      T data[Size] = {}; 

    public: 
      T const & GetAt(size_t const index) const 
      { 
        if (index < Size) return data[index]; 
        throw std::out_of_range("index out of range"); 
      } 

      void SetAt(size_t const index, T const & value) 
      { 
        if (index < Size) data[index] = value; 
        else throw std::out_of_range("index out of range"); 
      } 

      size_t GetSize() const { return Size; } 
    };

The purpose of this recipe is to enable writing code like the following:

    dummy_array<int, 3> arr; 
    arr.SetAt(0, 1); 
    arr.SetAt(1, 2); 
    arr.SetAt(2, 3); 

    for(auto&& e : arr) 
    {  
      std::cout << e << std::endl; 
    }