Iterator traits

test_iterator.cc

#include <iterator>
#include <numeric>

class array{
	int d[5] = { 1, 2, 3, 4, 5 };

public:
	struct it{
		// traits section
		using difference_type = unsigned;
		using value_type = int;
		using pointer = const value_type *;
		using reference = value_type &;
		using iterator_category = std::bidirectional_iterator_tag;
		
		int p;
		array const &a;
		
		constexpr it(int const p, array const &a) : p(p), a(a){}
		
		bool operator ==(it const other) const{
		return p == other.p;
		}
		
		constexpr bool operator !=(it const other) const{
		return p != other.p;
		}
		
		constexpr int operator *() const{
		return a.d[p];
		}
		
		it &operator ++(){
		++p;
		return *this;
		}
		
		it &operator --(){
		--p;
		return *this;
		}
	};

public:
	constexpr it begin() const{
		return { 0, *this };
	}
	
	constexpr it end() const{
		return { 5, *this };
	}
};

int main0(){
	constexpr array a;
	
	int sum = 0;
	
	// works without traits
	for(auto const &x : a)
		sum += x;
	
	return sum;
}

int main1(){
	constexpr array a;
	
	// works without traits
	return std::accumulate(std::begin(a), std::end(a), 0);
}

constexpr int main2(){
	constexpr array a;

	// NOT works without traits
	using len = std::iterator_traits<array::it>::difference_type;
	
	len const count = std::distance(std::begin(a), std::end(a));

	return count;
}