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Comments on When to use custom iterators versus pointers

Post

When to use custom iterators versus pointers

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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:

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:

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...

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2 comment threads

This all stuff actually looks like re-inventing the wheel to me. You might do that for practicing pur... (1 comment)
Wrapper (4 comments)
Wrapper
Lundin‭ wrote 2 months ago

"However, while writing this code, I couldn't help but notice that I am essentially writing a wrapper around pointer arithmetic." Indeed. When writing C++, one often comes to the conclusion "WTF am I even doing, this is nothing but bloated meta programming". In many cases, an iterator is nothing but a glorified pointer/loop iterator. The motivation for introducing it to the language might have been something like: "not all containers store their data in a linear, indexed fashion". Imagine a plain old linked list as one such example.

Lundin‭ wrote 2 months ago · edited 2 months ago

The language design mistake from there was to enforce this unison iterator interface to all classes, even those that do not benefit from it what-so-ever. That's poor OO design of the language itself - proper OO would distinct from containers which are "linear" and those who aren't. There's no reason why some array class should implement std::copy (requiring iterators) any different than old school memcpy for example. Whereas some linked list class would implement it very differently.

mr Tsjolder‭ wrote 2 months ago

Are you saying that using pointers is just fine and custom iterators are only useful if iteration becomes more complex?

Lundin‭ wrote 2 months ago

Some would probably disagree and saying that "pointers are dangerous". However, if the programmer can't even write correct code for iterating over a linear sequence of data, then what correct code can they write... It way easier to write than to implement some manner of iteration template monstrosity with "functors" and what else you'd need to compare items.