C ++，使用const_iterator进行运算符=时遇到问题

Question

There is a compilation error which occurs when I write the following:

const_iterator it = cp.begin();

const_iterator is my own class for const iterator .
cp is an object of a class ConjuntoPreguntas (see below).
The error is:

mainprueba.cpp:30:6: error: no match for ‘operator=’ (operand types are ‘ConjuntoPreguntas::const_iterator’ and ‘ConjuntoPreguntas::iterator’)
  cit = CP.begin();
      ^
mainprueba.cpp:30:6: note: candidate is:
In file included from mainprueba.cpp:2:0:
conjuntopreguntas.h:258:21: note: ConjuntoPreguntas::const_iterator& ConjuntoPreguntas::const_iterator::operator=(const ConjuntoPreguntas::const_iterator&)
     const_iterator& operator=(const const_iterator& cit){
                     ^
conjuntopreguntas.h:258:21: note:   no known conversion for argument 1 from ‘ConjuntoPreguntas::iterator’ to ‘const ConjuntoPreguntas::const_iterator&’

The code:

class ConjuntoPreguntas{
private:
    map<int,Pregunta> preguntas;

public:     
    class const_iterator;

    class iterator{
    private:
          map<int,Pregunta>::iterator it;

    public:
          iterator & operator++(){
           ++it;
          }

          iterator & operator--(){
           --it;
          }

          pair<const int,Pregunta> &operator *(){
           return *it;
          }

          bool operator ==(const iterator &i){
           return i.it==it;
          }   

          bool operator !=(const iterator &i){
           return i.it!=it;
          }

          friend class ConjuntoPreguntas;
          friend class const_iterator;
    };


    class const_iterator{
    private:
          map<int,Pregunta>::iterator it;
    public:
          const_iterator(){ 
          }

          const_iterator & operator++(){
           ++it;
          }

          const_iterator & operator--(){
           --it;
          }

          pair<const int,Pregunta> &operator *(){
           return *it;
          }

          bool operator ==(const const_iterator &i){
           return i.it==it;
          }   

          bool operator !=(const const_iterator &i){
           return i.it!=it;
          }

          const_iterator& operator=(const const_iterator& cit){
          }

          friend class ConjuntoPreguntas;

    };

    iterator begin(){
        iterator i;
        i.it=preguntas.begin();
        return i;
    }   

    iterator end(){
        iterator i;
        i.it=preguntas.end();
        return i;
    }
/* other code, irrelevant to the problem */                             
};   

If anyone can help me, I will be very grateful.

Answer 1

Your most immediate problem is because you don't have a const version of begin :

   const_iterator begin() const
   {
        const_iterator i;
        i.it = preguntas.begin();
        return i;
    }

But then also your const_iterator class uses map's iterator which is going to result in another problem.

If you're writing a container-looking class, there's nothing for it but to write const-correct iterator and const_iterator classes and provide the const-correct members.

If however you are NOT writing a container, you may not want to do this. The best case is to provide a container-agnostic interface to the class, where for example you provide meaningful names rather than direct container access. Alternately provide const-only access via the map const_iterator (don't write your own iterator class).

Answer 2

If you want to fully encapsulate std container inside your class (daunting and usually unneccessary task) you need to make sure you also define all conversions. In particlar, standard container iterators have a constructor which creates const_iterator from iterator (but not the other way around!). You will have to create it yourselfa as well.

However, a much better design choice would be to simply expose the member map variable.