ClassIdioms

The Iterator Protocol

When you want to use a user-defined type in a range-based for-loop, your user-defined type has to implement the Iterator Protocol.

 ClassIdioms

Here is the question I want to answer: What interface must a user-defined type support to be usable in a range-based for-loop.

Requirements of a Range-Based for-Loop

Let me start with a simple experiment and use  std::array in C++ Insights. Here is a simple example:

// iteratorProtocol.cpp

#include <array>

int main() {
   
    std::array<int, 5> myArr{1, 2, 3, 4, 5};
    for (auto a: myArr) a;
  
}

 

 

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    C++ Insights creates the following code out of it:

    #include <array>
    
    int main()
    {
      std::array<int, 5> myArr = {{1, 2, 3, 4, 5}};
      {
        std::array<int, 5> & __range1 = myArr;
        int * __begin1 = __range1.begin();
        int * __end1 = __range1.end();
        for(; __begin1 != __end1; ++__begin1) {
          int a = *__begin1;
          a;
        }
        
      }
      return 0;
    }
    

     

    Let me write it more generally: When you use a range-based for-loop (for(range_declaration : range_expression)), the compiler creates the following code:

    {
      auto && __range = range_expression ;
      auto __begin = begin_expr;      
      auto __end = end_expr;          
      for (;__begin != __end; ++__begin) {
        range_declaration = *__begin;
        loop_statement
      }
    }
    

     

    I marked the essential parts in red:

    • begin_expr and end_expr: return an iterator object

    • Iterator object
      • operator++: incrementing the iterator
      • operator*: dereferencing the iterator and accessing the current element
      • operator!=: comparing the iterator with another iterator

    begin_expr and end_expr call the essential begin and end on the range_expression. begin and end could either be member functions or free functions on range_expression.

    Let me apply the theory and create a number generator.

    A Generator

    My first implementation supports the Iterator Protocol

    The Iterator Protocol

    The following class Generator supports the elementary Iterator Protocol.

    // iterator.cpp
    
    #include <iostream>
    
    class Generator {
        int begin_{};
        int end_{};
    
    public:
        Generator(int begin, int end) : begin_{begin}, end_{end} {}
    
        class Iterator {
            int value_{};
        public:
            explicit Iterator(int pos) : value_{pos} {}
    
            int operator*() const { return value_; }           // (3)
    
            Iterator& operator++() {                           // (4)
                ++value_;
                return *this;
            }
    
            bool operator!=(const Iterator& other) const {      // (5)
                return value_ != other.value_;
            }
        };
    
        Iterator begin() const { return Iterator{begin_}; }     // (1)
        Iterator end() const { return Iterator{end_}; }         // (2)
    };
    
    int main() {
    
       std::cout << '\n';
        
       Generator gen{100, 110};
       for (auto v : gen) std::cout << v << " ";
    
       std::cout << "\n\n";
    
    }
    

     

    The class Generator has member functions begin and end, (lines 1 and 2) returning iterator objects, initialized with begin_ and end_. begin_ and end_ stand for the range of created numbers. Let me analyze the inner class Iterator which keeps track of the generated numbers:

    • operator* returns the current value
    • operator++ increments the current value
    • operator!= compares the current value with the end_ marker.

    Finally, here is the output of the program:

    iterator

     

    Let me generalize the iterator returned by begin() and end() and make it a forward iterator. Afterward, the class Generator can be used in most of the algorithms of the Standard Template Library. To put it differently, the unordered associative containers support a forward iterator.

    A Forward Iterator

    The following improved Generator has an inner class Iterator that is a forward iterator.

     

    // forwardIterator.cpp
    
    #include <iostream>
    #include <numeric>
    
    class Generator {
        int begin_{};
        int end_{};
    
     public:
        Generator(int begin, int end) : begin_{begin}, end_{end} {}
    
        class Iterator {
            using iterator_category = std::forward_iterator_tag;    // (1)
            using difference_type   = std::ptrdiff_t;
            using value_type        = int;
            using pointer           = int*;
            using reference         = int&;
            int value_{};
         public:
            explicit Iterator(int pos) : value_{pos} {}
    
            value_type operator*() const { return value_; }
            pointer operator->() { return &value_; }                // (2)         
    
            Iterator& operator++() {                           
                ++value_;
                return *this;
            }
            Iterator operator++(int) {                              // (3)
                Iterator tmp = *this; 
                ++(*this); 
                return tmp; 
            }
                                                                    // (4)
            friend bool operator==(const Iterator& fir, const Iterator& sec) {      
                return fir.value_ == sec.value_;
            }
            friend bool operator!=(const Iterator& fir, const Iterator& sec) {      
                return fir.value_ != sec.value_;
            }
        };
    
        Iterator begin() const { return Iterator{begin_}; }     
        Iterator end() const { return Iterator{end_}; }         
    };
    
    int main() {
    
        std::cout << '\n';
        
        Generator gen{1, 11};
        for (auto v : gen) std::cout << v << " ";                  // (5)
    
        std::cout << "\n\n";
                                                                   // (6)
        std::cout << "sum:  " << std::accumulate(std::begin(gen), std::end(gen), 0);
    
        std::cout << "\n\n";
                                                                    // (7)
        std::cout << "prod: " << std::accumulate(gen.begin(), gen.end(), 1, 
                                                 [](int fir, int sec){ return fir * sec; });
    
        std::cout << "\n\n";
    
    }
    

     

    First, Iterator needs several type aliases in the following member function declarations. Additionally, to the previous Iterator implementation in the program iterator.cpp, the current Iterator supports the following member functions: the arrow operator (operator-> in line 2), the post-increment operator (operator++(int) in line 3), and the equal operator (operator== in line 4).

    That was it already. Now, I can use my improved Generator still in a range-based for-loop (line 5), but also in the STL algorithm std::accumulate. Line 6 calculates the sum of all numbers from 1 to 10; line 7 does a similar job by multiplying numbers from 1 to 11. In the first case, I choose the neutral element 0 for the summation, and in the second case the neutral element 1 for the multiplication.

    There is a subtle difference between the first and the second call of std::accumulate. The first call uses the non-member functions std::begin and std::end on the Generator: std::accumulate(std::begin(gen), std::end(gen), 0), but the second call the Generator's member functions begin() and end() directly which I implemented.

    Finally, here is the output of the program:forwardIterator

    What’s Next?

    In my next post, I will write about the Covariant Return Type. The Covariant Return Type of a member function allows an overriding member function to return a narrower type. This is particularly useful when you implement the creational design pattern Prototype.

     

     

     

     

     

     

     

     

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