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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:
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...
1 answer
However, I suspect that most iterators can be written as iterating over an array.
From my experience, I wouldn't expect this to be the case. From the standard library containers, only std::vector and std::array can work with iterators implemented as pointers. All the other containers have internal structures different from a C-style array.
After all, a pointer is by definition a contiguous_iterator, and most containers (not just std ones) are structured such that they don't (and can't) support contiguous iterators.
I've written many various iterator classes; mostly over ranges, but some over containers too. Not once was there a single contiguous array in the underlying the data structure such that a pointer could be used in place of the iterator.
Is there any reason to wrap an iterator for array-like classes in a custom iterator class?
Nothing major. If I was writing a container/range that could use a pointer as an iterator, I'd probably use it and invest the saved development time elsewhere.
One possible reason to wrap a pointer in a class is that you can introduce custom behaviour, such as assert-based bounds checking which can catch some bugs in debug builds without slowing down production code.
If yes, is there any class in the standard library that implements this?
I am not aware of anything in the standard library. However, when writing your own iterators, it's well worth it to look into the Boost.Iterator library; in particular, you should consider checking out the boost::iterator_facade class template.
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