Iterator Implementation - (Linked List Queue) - c++

Question

I am implementing an iterator for a Queue data type, but the iterator is being initialized as a const inside of the class implementation for some reason. I cannot figure out why the constructor is causing the iterator to return itself as a const.
Any feedback as to what the intricasies of the C++ language that may be causing my problem could be, would be very helpful.
The Error I am receiving from Eclipse which seems to be coming from my begin() method is:
../src/linked_queue.hpp:315:35: error: invalid conversion from
'const ics::LinkedQueue<int>*' to 'ics::LinkedQueue<int>*' [-fpermissive]
Interface:
#ifndef LINKED_QUEUE_HPP_
#define LINKED_QUEUE_HPP_
#include <string>
#include <iostream>
#include <sstream>
#include <initializer_list>
#include "ics_exceptions.hpp"
namespace ics {
template<class T> class LinkedQueue {
public:
//Destructor/Constructors
~LinkedQueue();
LinkedQueue ();
LinkedQueue (const LinkedQueue<T>& to_copy);
explicit LinkedQueue (const std::initializer_list<T>& il);
template <class Iterable>
explicit LinkedQueue (const Iterable& i);
//Queries
bool empty () const;
int size () const;
T& peek () const;
std::string str () const; //supplies useful debugging information; contrast to operator <<
//Commands
int enqueue (const T& element);
T dequeue ();
void clear ();
template <class Iterable>
int enqueue_all (const Iterable& i);
//Operators
LinkedQueue<T>& operator = (const LinkedQueue<T>& rhs);
bool operator == (const LinkedQueue<T>& rhs) const;
bool operator != (const LinkedQueue<T>& rhs) const;
template<class T2>
friend std::ostream& operator << (std::ostream& outs, const LinkedQueue<T2>& q);
private:
class LN;
public:
class Iterator {
public:
~Iterator();
T erase();
std::string str () const;
LinkedQueue<T>::Iterator& operator ++ ();
LinkedQueue<T>::Iterator operator ++ (int);
bool operator == (const LinkedQueue<T>::Iterator& rhs) const;
bool operator != (const LinkedQueue<T>::Iterator& rhs) const;
T& operator * () const;
T* operator -> () const;
friend std::ostream& operator << (std::ostream& outs, const LinkedQueue<T>::Iterator& i) {
outs << i.str();
return outs;
}
friend Iterator LinkedQueue<T>::begin () const;
friend Iterator LinkedQueue<T>::end () const;
private:
LN* prev = nullptr;
LN* current;
LinkedQueue<T>* ref_queue;
int expected_mod_count;
bool can_erase = true;
Iterator(LinkedQueue<T>* iterate_over, LN* initial);
};
Iterator begin () const;
Iterator end () const;
private:
class LN {
public:
LN () {}
LN (const LN& ln) : value(ln.value), next(ln.next){}
LN (T v, LN* n = nullptr) : value(v), next(n){}
T value;
LN* next = nullptr;
};
LN* front = nullptr;
LN* rear = nullptr;
int used = 0; //Cache for number of values in linked list
int mod_count = 0; //For sensing any concurrent modifications
//Helper methods
void delete_list(LN*& front);
};
Implementation (I've included only a section of my Iterator code):
template<class T>
auto LinkedQueue<T>::begin () const -> LinkedQueue<T>::Iterator {
return Iterator(this, this->front);
}
template<class T>
auto LinkedQueue<T>::end () const -> LinkedQueue<T>::Iterator {
// return Iterator(this, this->rear);
}
template<class T>
LinkedQueue<T>::Iterator::Iterator(LinkedQueue<T>* iterate_over, LN* initial) {
ref_queue = iterate_over;
expected_mod_count = iterate_over->mod_count;
current = initial;
}

Answer 1

The error is because Iterator begin () const; is marked as const, hence the this used in the function is const as well. So in turn, the compiler gives the error that it cannot convert from a const to non-const.
Given that the function signatures are fixed in this case, to resolve the error, added const will help solve the issue.
You could add const to the constructor signature
Iterator(LinkedQueue<T> const* iterate_over, LN* initial);
And make the member const as well
LinkedQueue<T> const* ref_queue;
Adding the const where required to ensure that members and functions remain const as required is known as being const correct.