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Q&A When to use custom iterators versus pointers

I am working on a toy project where I have a container for which I would like to write an iterator that iterates over the values in the container. Because the values are stored in a (c-style) arra...

1 answer  ·  posted 2mo ago by mr Tsjolder‭  ·  last activity 2mo ago by Angew‭

Question c++ iterator
#1: Initial revision by user avatar mr Tsjolder‭ · 2024-04-18T08:24:36Z (2 months ago)
When to use custom iterators versus pointers
I am working on a toy project where I have a container for which I would like to write an iterator that iterates over the values in the container.
Because the values are stored in a (c-style) array, this would look something like this:

```c++
class MyClass {
	int _data[10] {};
public:
	class MyIterator;

	MyIterator begin();
	MyIterator end();
};

class MyClass::MyIterator final {
	int* _position {};
public:
	using iterator_category = std::contiguous_iterator_tag;
	using value_type = int;
	using difference_type = std::ptrdiff_t;
	using pointer = value_type*;
	using reference = value_type&;

	explicit MyIterator(int* from) : _position { from } {}
	reference operator*() const { return *_position; }
	pointer operator->() const { return _position; }
	MyIterator& operator++() { ++_position; return *this; }
	MyIterator operator++(int) { MyIterator tmp { *this }; operator++(); return tmp; }
	MyIterator& operator--() { --_position; return *this; }
	MyIterator operator--(int) { MyIterator tmp { *this }; operator--(); return tmp; }
	MyIterator operator += (const difference_type& n) { _position += n; return *this; }
	MyIterator operator -= (const difference_type& n) { _position -= n; return *this; }
	MyIterator operator[](const difference_type& n) { return MyIterator(_position + n); }

	friend auto operator<=>(MyIterator, MyIterator) = default;
	friend auto operator+(MyIterator it, difference_type n) { MyIterator result { it }; result += n; return result; }
	friend auto operator-(MyIterator it, difference_type n) { MyIterator result { it }; result -= n; return result; }
	friend auto operator+(difference_type n, MyIterator it) { return it + n; }
};

MyClass::MyIterator MyClass::begin() {
	return MyClass::MyIterator(&_data[0]);
}

MyClass::MyIterator MyClass::end() {
	return MyClass::MyIterator(&_data[0] + 10);
}
```

However, while writing this code, I couldn't help but notice that I am essentially writing a wrapper around pointer arithmetic.
Instead of writing a custom iterator, everything seems to work just as well when using the following, much more concise code:

```c++
class MyClass2 {
	int _data[10] {};
public:
	using MyIterator = int*;

	MyIterator begin();
	MyIterator end();
};

MyClass2::MyIterator MyClass2::begin() {
	return &_data[0];
}

MyClass2::MyIterator MyClass2::end() {
	return &_data[0] + 10;
}
```

I understand that using a raw pointer as an iterator is enabled by the specialisation of `std::iterator_traits`. So why would/should I bother encapsulating the iterator in a full class with trivial code when using a raw pointer works just as well? 

I do understand that there are scenarios where iterators require more complexity than plain pointer arithmetic. However, I suspect that most iterators can be written as iterating over an array. Is there any reason to wrap an iterator for array-like classes in a custom iterator class? If yes, is there any class in the standard library that implements this? I could only find `std::iterator`, but this is deprecated and doesn't really implement anything...