Return Value and Location of function

[Deliberation]

Hi,
I have short questions about part of codes in the program below.

(Line number 103)
Q1. What does the syntax mean?

(Line numbers 171 to 174)
Q2. What is the return value of this?
Q3. Why didn’t a programmer include this function inside a class iterator?

Than you.

#ifndef LIST_H // // multiple inclusion guard
#define LIST_H

template <typename Object>
class List
{
private:
	struct Node // private member structure (could be class instead)
	{
		Object data;
		Node* prev;
		Node* next;
		Node(const Object& d = Object(), Node* p = NULL, Node* n = NULL) // constructor
			: data(d), prev(p), next(n) {}
		Node(Object&& d, Node* p = NULL, Node* n = NULL) // move constructor (C++11)
			: data(move(d)), prev(p), next(n) {}
	};

public:
	class const_iterator // **********************************************
	{
	public:
		const_iterator() : current(NULL) {} // default constructor

		const Object& operator*() const // const de-referencing
		{
			return retrieve();
		}

		const_iterator& operator++() // pre-increment
		{
			current = current->next;
			return *this;
		}

		const_iterator operator++(int) // post-increment
		{
			const_iterator old = *this;
			++(*this);
			return old;
		}

		const_iterator& operator--() // pre-decrement
		{
			current = current->prev;
			return *this;
		}

		const_iterator operator--(int) // post-decrement
		{
			const_iterator old = *this;
			--(*this);
			return old;
		}

		bool operator== (const const_iterator& rhs) const
		{
			return current == rhs.current;
		}

		bool operator != (const const_iterator& rhs) const
		{
			return !(*this == rhs);
		}

	protected:
		Node* current;

		Object& retrieve() const
		{
			return current->data;
		}

		const_iterator(Node* p) : current(p) {} // constructor

		friend class List<Object>;
	}; // end class const_iterator // **********************************************

	class iterator : public const_iterator // **************************************
	{
	public:
		iterator() : current(NULL) {} // default constructor

		Object& operator*() // de-referencing
		{
			return retrieve();
		}

		const Object& operator*() const // const de-referencing
		{
			return const_iterator::operator*();
		}

		iterator& operator++() // pre-increment
		{
			current = current->next;
			return *this;
		}

		iterator operator++(int) // post-increment
		{
			iterator old = *this;
			++(*this);
			return old;
		}

		iterator& operator--() // pre-decrement
		{
			current = current->prev;
			return *this;
		}

		iterator operator--(int) // post-decrement
		{
			iterator old = *this;
			--(*this);
			return old;
		}

	private: // we are not expecting to derive from iterator...
		iterator(Node* p) : const_iterator(p) {} // constructor

		friend class List<Object>;
	};// end class iterator // **********************************************

	// back to class List:
public:
	// constructors
	List() // default
	{
		init();
	}

	List(const List& rhs) // copy constructor
	{
		init();
		for (const Object& x : rhs) // range-based for loop (C++11)
			push_back(x);
	}

	List(List&& rhs) // move constructor (C++11)
		: theSize(rhs.theSize), head(rhs.head), tail(rhs.tail)
	{
		rhs.theSize = 0;
		rhs.head = NULL;
		rhs.tail = NULL;
	}

	// destructor
	~List()
	{
		clear();
		delete head;
		head = NULL;
		delete tail;
		tail = NULL;
	}

	// public assignment operator
	List& operator=(const List& rhs)
	{
		if (this != &rhs) // prevent assignment to itself
		{
			this->clear();
			for (const_iterator itr = rhs.begin(); itr != rhs.end(); ++itr)
				this->push_back(*itr);
		}
		return *this;
	}

	iterator begin()
	{
		return iterator(head->next);
	}

	const_iterator begin() const
	{
		return const_iterator(head->next);
	}

	iterator end()
	{
		return iterator(tail);
	}

	const_iterator end() const
	{
		return const_iterator(tail);
	}

	size_t size() const
	{
		return theSize;
	}

	bool empty() const
	{
		return size() == 0;
	}

	void clear()
	{
		while (!empty())
			pop_front();
	}

	Object& front()
	{
		return *begin();
	}

	const Object& front() const
	{
		return *begin();
	}

	Object& back()
	{
		return *--end();
	}

	const Object& back() const
	{
		return *--end();
	}

	void push_front(const Object& x)
	{
		insert(begin(), x);
	}

	void push_back(const Object& x)
	{
		insert(end(), x);
	}

	void pop_front()
	{
		erase(begin());
	}

	void pop_back()
	{
		erase(--end());
	}

	// Insert copy of x on the free store before itr:
	iterator insert(iterator itr, const Object& x)
	{
		Node* p = itr.current;
		theSize++;
		try
		{
			// new Node allocated on the freestore
			p->prev = p->prev->next = new Node(x, p->prev, p);
		}
		// if memory allocation failed:
		catch (...) // executes in case of exception (error)
		{
			cout << "Error encountered... quitting, sorry!\n";
			system("pause");
			exit(EXIT_FAILURE); // quit
		}
		return iterator(p->prev);
	}

	// Erase item at itr:
	iterator erase(iterator itr)
	{
		if (!empty())
		{
			Node* p = itr.current;
			iterator retVal(p->next);
			p->prev->next = p->next;
			p->next->prev = p->prev;
			delete p;
			p = NULL;
			theSize--;
			return retVal;
		}
		else
		{
			cout << "List empty!\n";
			return NULL;
		}
	}

	// Erase items from start to end:
	iterator erase(iterator start, iterator end)
	{
		for (iterator itr = start; itr != end;)
			itr = erase(itr);
		return end;
	}

private:
	size_t theSize;
	Node* head;
	Node* tail;

	void init()
	{
		theSize = 0;
		try
		{
			head = new Node; // on the free store
			tail = new Node; // on the free store
		}
		// if memory allocation failed:
		catch (...) // executes in case of exception (error)
		{
			cout << "Error encountered... quitting, sorry!\n";
			system("pause");
			exit(EXIT_FAILURE); // quit
		}
		head->next = tail;
		tail->prev = head;
	}
};// end class List

[MiiNiPaa]

Q1: this is literally "this": pointer to current object. This line applies a preincrement to current object.
http://www.learncpp.com/cpp-tutorial/87-the-hidden-this-pointer/

Q2: it constructs an iterator from head->next pointer (calling constructor defined on line 82)

Q3: Because that function does not belong to class iterator and is pointless for it. It is just happen to return iterator. It is like asking "why candies are sol in shops and not on sugar beet farm"

[Deliberation]

Thank you so much MiiNiPaa!