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.

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;
  
}

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;
}

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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:

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:

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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