constructors and conversions of classes - c++

Question

I have 2 classes:
class iterator {
Node<K,V>* n;
public:
iterator(){
std::cout<<"new empty iterator"<<std::endl;
};
iterator(const iterator& iter):n(iter.n){
std::cout<<"copy iterator"<<std::endl;
}
explicit iterator(Node<K,V>* nodePtr):n(nodePtr) {
std::cout<<"create iterator with Node Ptr"<<std::endl;
}
iterator& operator=(const iterator& iter);
void operator++();
Node<K,V>& operator*();
bool operator!=(const iterator& iter);
K& operator[](const Key& k)const;
V& operator[](const Val& v)const;
Node<K,V>* getNodePtr()const{
std::cout<<"get Node Ptr"<<std::endl;
return n;
}
friend class Map;
};
class const_iterator :public iterator {
const Node<K,V>* n;
public:
const_iterator(){
std::cout<<"new empty const_iterator"<<std::endl;
};
const_iterator(const iterator& iter):
n(iter.getNodePtr()){
std::cout<<"convert"<<std::endl;
}
const_iterator(const const_iterator& iter):n(iter.n){}
explicit const_iterator(const Node<K,V>* node):n(node){}
const_iterator& operator=(const const_iterator& iter){
n = iter.n;
return *this;
}
const Node<K,V>& operator*(){
return *n;
}
friend class Map;
};
iterator begin()const{
return iterator(firstKey);
}
iterator end()const{
return iterator(dummyKey);
}
I want it to make an automatic conversion between the 2 classes using:
const_iterator(const iterator& iter):
n(iter.getNodePtr()){
std::cout<<"convert"<<std::endl;
}
to do something like this for example:
Map<int,int>::const_iterator it = m.begin();
now, the thing is, that this constructor calls for some reason iterator(); in the first class and I can't figure out why.
I know begin() and end() have some standart versions, but I can't use any of it here. I also can't make Map to be const, or write a conversion function.
any help?

Answer 1

You've made iterator a base class of const_iterator, so an iterator constructor will (and should) be called as part of constructing a const_iterator.

Answer 2

If you will define data storage as
const Map<int, int> data;
instead of
Map<int, int> data;
const version of begin() and end() will be invoked.
Also, C++11 standard containers add cbegin() and cend() for that purpose. You could re-difine it as follows:
const_iterator cbegin()const{
return const_iterator(firstKey);
}
const_iterator cend()const{
return const_iterator(dummyKey);
}