Create contiguous_iterator for custom class - c++

Question

Summary
I have a custom array class:
template<typename T, int SIZE>
class Array {
private:
T mArray[SIZE];
};
To enable support for std algorithms, with ranges, I want to create an iterator for this class. It would seem that std::contiguous_iterator would be the optimal choice since I can guarantee contiguous memory layout for the data. Following the iterator tutorial I should create a class inside this class. However, I should somehow be (quoted) "For example, instead of the std::forward_iterator_tag tag you would mark your iterator with the std::forward_iterator concept.".
I have a hard time figuring out what the syntax would look like for this, and I have been unable to find a post on the web showcasing this.
Question
How do I complete the following code snippet to implement std::contiguous_iterator for my Array<T,S> class?:
import <iterator>;
template<typename T, int SIZE>
class Array {
public:
const T& operator[](int i) { return mArray[i]; }
T& operator[](int i) { return mArray[i]; }
private:
T mArray[SIZE];
public:
struct Iterator {
Iterator(T* ptr) : mPtr(ptr) {}
private:
T* mPtr;
};
Iterator begin() { return Iterator(&mArray[0]); }
Iterator end() { return Iterator(&mArray[SIZE]); }
};
NOTE: There is a lot of operator overloads. An answer is not required to provide all of them. I just need an example syntax for where to place the concept, then I can probably figure out the rest.

Answer 1

As far as I could tell you need typedefs on the iterator class, so simply using a pointer was not sufficient. Here is an example:
#include <iterator>
#include <algorithm>
template<typename T, int SIZE>
class Array {
public:
const T& operator[](int i) const { return mArray[i]; }
T& operator[](int i) { return mArray[i]; }
private:
T mArray[SIZE];
public:
struct iterator
{
using difference_type=std::ptrdiff_t;
using value_type=std::remove_cv_t<T>;
using pointer=T*;
using reference=T&;
using iterator_category=std::random_access_iterator_tag;
using iterator_concept=std::contiguous_iterator_tag;
using self_type=iterator;
iterator(T *x) : ptr(x) {}
T operator*() { return *ptr; }
T operator->() { return ptr; }
difference_type operator-(const iterator& rhs) { return ptr-rhs.ptr; }
iterator& operator ++() { ++ptr; return *this;}
bool operator !=(const iterator& rhs) { return ptr != rhs.ptr; }
private:
T * ptr;
};
iterator begin() { return &mArray[0]; }
iterator end() { return &mArray[SIZE]; }
};
int foo(Array<int, 7>& a)
{
int sum;
for (auto x : a)
{
sum+=x;
}
return sum;
}
int goo(Array<int, 7>& a, int x)
{
auto ret=std::find(a.begin(), a.end(), x);
if (ret!=a.end()) return *ret;
return 0;
}
Note that you would likely need const_iterator and reverse_iterators for const and non-const ...

Answer 2

Credits
Thanks to #glenn-teitelbaum for pointing me in the right direction. I think I managed to figure out how to do this. It took a long time, so this will hopefully save someone else that trouble.
[Answering my own question]
The iterator should comply with the std::contiguous_iterator concept, so I looked at cppreference for the necessary parts. The concept is defined like this:
template<class I>
concept contiguous_iterator =
std::random_access_iterator<I> &&
std::derived_from</*ITER_CONCEPT*/<I>, std::contiguous_iterator_tag> &&
std::is_lvalue_reference_v<std::iter_reference_t<I>> &&
std::same_as<
std::iter_value_t<I>, std::remove_cvref_t<std::iter_reference_t<I>>
> &&
requires(const I& i) {
{ std::to_address(i) } ->
std::same_as<std::add_pointer_t<std::iter_reference_t<I>>>;
};
So in order to implement this concept, I must first also implement std::random_access_iterator, std::is_derived_from<[...]>, std::is_lvalue_reference_v<[...]>, and so on. This is a recursive process, especially since std::random_access_iterator builds on top of 4 other iterator types. Since this is a C++20 question, I aim to use C++20 features as much as possible (since it greatly simplifies the implementation). This is the complete iterator implementation:
template<typename T, int SIZE>
class Array
{
public:
class Iterator
{
public:
using iterator_category = std::contiguous_iterator_tag;
using iterator_concept = std::contiguous_iterator_tag;
//using difference_type = std::ptrdiff_t; // Likely the same
using difference_type = typename std::iterator<
std::contiguous_iterator_tag, T>::difference_type;
//using value_type = T;
using value_type = std::remove_cv_t<T>; // Using `T` seems sufficient
using pointer = T*;
using reference = T&;
// constructor for Array<T,S>::begin() and Array<T,S>::end()
Iterator(pointer ptr) : mPtr(ptr) {}
// std::weakly_incrementable<I>
Iterator& operator++() { ++mPtr; return *this; }
Iterator operator++(int) { Iterator tmp = *this; ++(*this); return tmp; }
Iterator() : mPtr(nullptr/*&mArray[0]*/) {} // TODO: Unsure which is correct!
// std::input_or_output_iterator<I>
reference operator*() { return *mPtr; }
// std::indirectly_readable<I>
friend reference operator*(const Iterator& it) { return *(it.mPtr); }
// std::input_iterator<I>
// No actions were needed here!
// std::forward_iterator<I>
// In C++20, 'operator==' implies 'operator!='
bool operator==(const Iterator& it) const { return mPtr == it.mPtr; }
// std::bidirectional_iterator<I>
Iterator& operator--() { --mPtr; return *this; }
Iterator operator--(int) { Iterator tmp = *this; --(*this); return tmp; }
// std::random_access_iterator<I>
// std::totally_ordered<I>
std::weak_ordering operator<=>(const Iterator& it) const {
return std::compare_three_way{}(mPtr, it.mPtr);
// alternatively: `return mPtr <=> it.mPtr;`
}
// std::sized_sentinel_for<I, I>
difference_type operator-(const Iterator& it) const { return mPtr - it.mPtr; }
// std::iter_difference<I> operators
Iterator& operator+=(difference_type diff) { mPtr += diff; return *this; }
Iterator& operator-=(difference_type diff) { mPtr -= diff; return *this; }
Iterator operator+(difference_type diff) const { return Iterator(mPtr + diff); }
Iterator operator-(difference_type diff) const { return Iterator(mPtr - diff); }
friend Iterator operator+(difference_type diff, const Iterator& it) {
return it + diff;
}
friend Iterator operator-(difference_type diff, const Iterator& it) {
return it - diff;
}
reference operator[](difference_type diff) const { return mPtr[diff]; }
// std::contiguous_iterator<I>
pointer operator->() const { return mPtr; }
using element_type = T;
private:
T* mPtr;
};
// === STATIC ASSERTS ===
// - to verify correct Iterator implementation!
static_assert(std::weakly_incrementable<Iterator>);
static_assert(std::input_or_output_iterator<Iterator>);
static_assert(std::indirectly_readable<Iterator>);
static_assert(std::input_iterator<Iterator>);
static_assert(std::incrementable<Iterator>);
static_assert(std::forward_iterator<Iterator>);
static_assert(std::bidirectional_iterator<Iterator>);
static_assert(std::totally_ordered<Iterator>);
static_assert(std::sized_sentinel_for<Iterator, Iterator>);
static_assert(std::random_access_iterator<Iterator>);
static_assert(std::is_lvalue_reference_v<std::iter_reference_t<Iterator>>);
static_assert(std::same_as<std::iter_value_t<Iterator>,
std::remove_cvref_t<std::iter_reference_t<Iterator>>>);
static_assert(std::contiguous_iterator<Iterator>);
const T& operator[](int i) const {
if (i < 0 || i >= SIZE) {
throw std::runtime_error("Array index out of bounds");
}
return mArray[i];
}
T& operator[](int i) {
if (i < 0 || i >= SIZE) {
throw std::runtime_error("Array index out of bounds");
}
return mArray[i];
}
Iterator begin() { return Iterator(&mArray[0]); }
Iterator end() { return Iterator(&mArray[SIZE]); }
private:
T mArray[SIZE];
};
// Check that the Array class can be used as a contiguous_range.
static_assert(std::ranges::contiguous_range<Array<int, 10>>);
NOTE: using element_type = T; was necessary because of a bug in the specification, which might be fixed. I found information about that here. Adding this fixed issue with std::to_address<Iterator> not being able to compile, and was the last missing piece in going from std::random_access_iterator to std::contiguous_iterator.
Testing
I did not perform a complete testing suite with all algorithms, but I chose a few which depend on ranges and std::random_access_iterator. It all runs smoothly. I also depend on building standard library headers as module units, because I want to showcase how C++20 features work together.
import <stdexcept>;
import <iostream>;
import <iterator>;
import <algorithm>;
import <random>;
#include <memory> // fails to build header unit!
template<typename T, int SIZE>
class Array
{
[...]
};
int main()
{
Array<int, 10> arr;
for (int i = 0; i < 10; i++) arr[i] = i;
// I need to call std::ragnes::shuffle since that depends on
// std::random_access_iterator, so that is a minimum.
// https://en.cppreference.com/w/cpp/algorithm/ranges/shuffle
std::random_device rd;
std::mt19937 gen{rd()};
std::cout << "before random shuffle:\n";
for (auto& i : arr) std::cout << i << ' ';
std::ranges::shuffle(arr, gen);
std::cout << "\nafter random shuffle:\n";
for (auto& i : arr) std::cout << i << ' ';
std::cout << '\n';
// Also std::ranges::stable_sort is a good check (also random_access_iterator):
// https://en.cppreference.com/w/cpp/algorithm/ranges/stable_sort
std::cout << "after stable_sort:\n";
std::ranges::stable_sort(arr);
for (auto& i : arr) std::cout << i << ' ';
std::cout << '\n';
auto [min,max] = std::ranges::minmax(arr);
std::cout << "min: " << min << ", max: " << max << '\n';
return 0;
}