How can I use a overloaded const_iterator inside a class? - c++

Question

I'm doing a University project in C++, whose purpose is to learn how to use the diferent STL containers and their iterators.
In my program, I have a class with a set:
class ConjuntoDeLetras{
private:
set<Letra> letras;
public:
ConjuntoDeLetras();
···
};
Inside the class, I have two nested class, iterator and const_iterator. (I don't know if it's the best way to make class iterators, but the teacher tell us that we need to do at this way):
class iterator{
private:
set<Letra>::iterator it;
public:
iterator();
Letra operator*();
ConjuntoDeLetras::iterator& operator++();
ConjuntoDeLetras::iterator& operator+=(int num);
bool operator==(const ConjuntoDeLetras::iterator &i);
bool operator!=(const ConjuntoDeLetras::iterator &i);
friend class ConjuntoDeLetras;
};
class const_iterator{
private:
set<Letra>::const_iterator it;
public:
const_iterator();
Letra operator*();
ConjuntoDeLetras::const_iterator& operator++();
ConjuntoDeLetras::const_iterator& operator+=(int num);
bool operator==(const ConjuntoDeLetras::const_iterator &i);
bool operator!=(const ConjuntoDeLetras::const_iterator &i);
friend class ConjuntoDeLetras;
};
Both iterator class methods works well. In ConjuntoDeLetras class we have the begin and end methods:
ConjuntoDeLetras::iterator begin();
ConjuntoDeLetras::const_iterator begin() const;
ConjuntoDeLetras::iterator end();
ConjuntoDeLetras::const_iterator end() const;
The problem is here. When I'm going to use the const_iterator we have problems:
ConjuntoDeLetras::const_iterator itL;
for(itL=L.begin(); itL!=L.end(); ++itL){
CantidadLetras aux;
aux.frecuenciaAbsoluta = 0;
aux.frecuenciaRelativa = 0;
aux.letra = (*itL).getLetra();
salida.push_back(aux);
}
When I execute this code the compiler says that I don't have operator= for const_iterator to iterator. I know the reason of the problem, it's because the object L is a not const variable and uses the normal iterator begin() and end(). I thought about removing the final const of the functions but I can't overload the function only with the return type. I don't know what is the best solution. Here is the compiler error:
error: no match for ‘operator=’ (operand types are ‘ConjuntoDeLetras::const_iterator’ and ‘ConjuntoDeLetras::iterator’)
for(itL=L.begin(); itL!=L.end(); ++itL){
^
And the same error with the end().

Answer 1

You are missing the other ++ operator and the required typedefs to be an iterator.
Plus, you need =default copy/move/assignments.
Finally you need a converting constructor from your iterator to your const_iterator.
Optionaly add two const_iterator::operator=(iterator) overloads (copy assign and move assign), plus const_iterator::operator==(iterator const&) and iterator==const_iterator similarly.