It fails to define a const_iterator from it's counterpart iterator

Question

I am extending stl container with a self defined container so as to provide more flexible control on the operations of the elements

class MyContainer; template <typename T> class myiterator :public iterator<bidirectional_iterator_tag, T> { friend class MyContainer; private: T *pointer; myiterator(T *pt):pointer(pt) {} public: T& operator*() {return (*pointer);} const myiterator<T>& operator++() { pointer->current_iterator++; return *this; } bool isEnd(void) const { return pointer->current_iterator == pointer->data.end(); } }; class MyContainer { friend class myiterator<MyContainer>; public: typedef myiterator<MyContainer> iterator; typedef myiterator<MyContainer const> const_iterator; private: map<int, int> data; map<int, int>::const_iterator current_iterator; public: MyContainer() {current_iterator = data.begin(); } void addDataPair(int key, int value) {data[key] = value;} int first() const {return (*current_iterator).first;} int second() const {return (*current_iterator).second;} iterator begin() { current_iterator = data.begin(); return iterator(this); } const_iterator begin() const { return const_iterator(this); } };

This code run ok, if I use iterator as follow

MyContainer h; h.addDataPair(1, 1); h.addDataPair(2, 2); h.addDataPair(3, 3); for (MyContainer::iterator it=h.begin(); !it.isEnd(); ++it) { cout << (*it).first() << " " << (*it).second() << endl; }

But it won't compile if I change iterator to const_iterator. I read some article, which mention that to define constant iterator, we simply replace the value_type from X to X const, that's how I did in my code. But I soon find that it might won't work in my case because the reference returned by an iterator is the container itself in my case. I don't know then how to make the const_iterator work without duplicate the coding.

In addition, my iterator is derived from std::iterator but I found that I cannot override the constructor for my iterator. Is that any way that I can pass more than one parameters into my iterator besides the T *pt ? Thanks.

This is a bit odd. MyContainer appears to be both a container and an iterator. — Vaughn Cato
– Vaughn Cato, Commented Apr 20, 2012 at 4:23
I know :) Because I don't want the code to modify the map<int, int>'s element directly so I write a wrapper to control that while they are trying to access the map data with operator[], operator++ , etc. I was going to inherit std::map to do so but seems that it is not recommended to do so because of the virtual destructor. Anyway, this is the only way I can think of to keep the interface as close as the map and add some controls. — user1285419
– user1285419, Commented Apr 20, 2012 at 4:32
Do you want there to be any difference between a const_iterator and an iterator of your container? — Vaughn Cato
– Vaughn Cato, Commented Apr 20, 2012 at 5:07
Hi Vaughn, since the data stored in the map is really sensitive and I don't want them to be changed for some case, so I need a const_iterator in that siutation. — user1285419
– user1285419, Commented Apr 20, 2012 at 5:58

Jonathan Wakely · Accepted Answer · 2012-06-16 02:35:33Z

First problem:

If you change this:

for (MyContainer::iterator it=h.begin(); !it.isEnd(); ++it)

to

for (MyContainer::const_iterator it=h.begin(); !it.isEnd(); ++it)

then you get a non-const iterator from begin() and end() and try to initialize a const_iterator from it, but that's a different type and your my_iterator template doesn't have a constructor that allows construction from a different type.

You can fix that by adding:

template<typename> friend class myiterator; template<typename T2> myiterator(myiterator<T2> const& i) : pointer(i.pointer) { }

You should also make operator* const (it doesn't alter the iterator to dereference it.)

But there's still a bigger problem, a const_iterator points to a const MyContainer, but const_iterator::operator++ needs to alter that object, which it can't because it's const. So you can't increment your const_iterator i.e. can't use it to iterate! You might want to rethink that design.

Collectives™ on Stack Overflow

It fails to define a const_iterator from it's counterpart iterator

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related