Xcode vs VS2013 Part1 - Implementation

[Deliberation]

Hello,
I have exactly same code, which ran at both Xcode and VS.
While VS doesn't have any problem running this program, Xcode shows three error messages.

Line 89: Member initializer 'current' does not name a non-static data member or base class
Line 93: Use of undeclared identifier 'retrieve'
Line 103 & 116: Use of undeclared identifier 'current'

Does anyone know what makes Xcode stop running?
Thank you.

#include <iostream>
#include <stddef.h>
using namespace std;



#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

#endif


 

[helios]

89: A derived class cannot initialize members of its base class in its initializer list. VS is incorrect in not rejecting that code.
93: You need to provide a non-const overload for const_iterator::retrieve() const.
103 & 116: I don't see it. Maybe the compiler got confused at this point.

[Deliberation]

Hi Helios,
thank you for pointing them out.
So for a constructor in line 89, I change it to

iterator() {} // default constructor 

And right below that line, I created a code for a non-const overload for const_iterator::retrieve() const. Is a code below right for the non-const overload?

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

[JLBorges]

> So for a constructor in line 89, I change it to
> iterator() {} // default constructor

Or just declare it as explicitly defaulted: iterator() = default ;


> And right below that line, I created a code for a non-const overload for const_iterator::retrieve() const.

This is completely unnecessary. You could just remove that overload.


The other errors are caused by dependant names being looked up during the first phase of the two phase lookup. We need to tell the compiler that the name - say current - is a dependant name, to be looked up during the second phase of the two phase lookup.
http://www.cplusplus.com/forum/general/140421/#msg741857

Otherwise, the code appears to be fine. Well done! (I've tested only a few functions.)

With those corrections (and a few suggestions thrown in):
clang++, g++: http://coliru.stacked-crooked.com/a/8dc1283cb6530fa5
msc++ (VS 2103, does not support noexcept): http://rextester.com/HFJTVN46969

[Deliberation]

Hi JLBorges,

Thank you so much for your help!

This program is not written by me. It has been taken from my CS professor's reading assignment. I have only 2 months of programming experience :)

With your advice, I put a following code right above line 89 and program ran successfully.

using const_iterator::current;

I also read a link you provided, but how can I know that current is a dependent name in this program?

[JLBorges]

> I also read a link you provided, but how can I know that current is a dependent name in this program?

This was linked to from within the link in the earlier post:

In a template that we write, there are two kinds of names (identifiers) that could be used - dependant names and non- dependant names. A dependant name is a name that depends on a template parameter; a non-dependant name has the same meaning irrespective of what the template parameters are.

For example:

template< typename T > void foo( T& x, int value )
{
    ++ T::static_member_variable ; // 'static_member_variable' is a dependant name
    ++value ; // 'value' is a non- dependant name
}

What a dependant name refers to could be something different for each different instantiation of the template. As a consequence, C++ templates are subject to "two-phase name lookup". When a template is initially parsed (before any instantiation takes place) the compiler looks up the non-dependent names. When a particular instantiation of the template takes place, the template parameters are known by then, and the compiler looks up dependent names.

http://www.cplusplus.com/forum/beginner/103508/#msg557627


In our case, we know that the name 'current', used in the definition of a member of List<T>::iterator is a dependent name. It refers to the name in the base class List<T>::const_iterator (of type 'pointer to List<T>::Node), which is dependant on the template parameter T.

However, when a dependent name refers to a name in a base class, the compiler needs to be informed that it is a dependent name. This is so, because the IS specifies:

In the definition of a class or class template, the scope of a dependent base class is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.

For instance:

int name_one = 0 ;

template < typename T > struct base { int name_one = 0 ; int name_two = 0 ; } ;

template < typename T > struct derived : base<T>
{
     void foo()
     {
          name_one = 7 ; // 'name_one' is treated as a non-dependant name
                         // (the scope of the dependent base class base<T> is not examined)
                         //  assigns to ::name_one at namespace scope (and not to base<T>::name_one)

         // name_two = 7 ; // **** error: 'name_two' is not found

         // we can fix the error in several ways
         base<T>::name_two = 7 ; // fine: 'name_two' is a dependant name in the dependent base class
         derived<T>::name_two = 7 ; // fine: 'name_two' is a dependant name, to be looked up during phase two
         this->name_two = 7 ; // fine: same as above
         
         using name_two = base<T>::name_two ; // when we use the unqualified name 'name_two'
                                              // we mean the dependant name 'name_two' in the dependent base class
         name_two = 7 ; // fine: refers to the dependant name 'base<T>::name_two' ;                                    
     }
};

> This program is not written by me. It has been taken from my CS professor's reading assignment.

Did he/she/it actually dump a using namespace std; in the header file?
And favoured <stddef.h> over <cstddef> and then blithely proceeded to sprinkle NULL liberally all over the code?

Career teacher, I suppose. This is the fundamental tragedy of C++:
those who can, do; those who can't (with a few notable exceptions) teach.
(At least in the US and (alas) in my own country. Perhaps things are not as bad elsewhere. In this regard, I do envy the education system of the medical profession.)