CODEWITHSUNDEEP
c++stllistiterator
user3009269
user3009269

Reputation: 442

C++ iterator on a list error

I have in a C++ class the following : I want to use an iterator to have an element of the questions list at a time by calling the getNextQuestion() function after checking if the iterator is still valid by calling isValid(). It gives me the following terrible error :

passing ‘const iterator {aka const std::_List_iterator<domain::Question>}’ as ‘this’ argument of ‘std::_List_iterator<_Tp>::_Self& 
std::_List_iterator<_Tp>::operator++() [with _Tp = domain::Question,std::_List_iterator<_Tp>::_Self = 
std::_List_iterator<domain::Question>]’ discards qualifiers [-fpermissive]

#ifndef TESTREPOSITORY_H_
#define TESTREPOSITORY_H_

#include <iostream>
#include <iterator>
#include <list>
#include <algorithm>
#include <fstream>
#include "../domain/question.h"

using namespace domain;

namespace repository{
template<class T>
class TestRepository{
std::string file;
std::list<T> questions;
typename std::list<T>::iterator it;
public:
TestRepository(std::string& file=""):file(file){
    this->questions = this->getQ();
    this->it = this->questions.begin();
};

std::list<T> getQ() const{
    std::list<T> listq;
    using namespace std;
    string line;
    std::ifstream fin(file.c_str());
    while(fin.good()){
        Question q;
        fin >> q;
        listq.push_back(q);
    }
    fin.close();
    return listq;
}

const bool isValid() const{
    return this->it != this->questions.end();
}

const T getNextQuestion() const{
    T q = (*this->it);
    ++this->it;
    return q;
}

};
}

#endif /* TESTREPOSITORY_H_ */

Here is the code where I call these funcitons,maybe here is the problem coming from :

#include "TestController.h"
#include "../domain/test.h"
#include <iostream>
#include <list>
#include <iterator>

 namespace controller{

TestController::TestController(repository::TestRepository<domain::Question>* repo,int   testId){
this->repo = repo;
this->testId = 0;
}

const test TestController::getCurrentTest() const{
test test(this->testId,0,0);
return test;
}

const bool TestController::isValid() const{
return this->repo->isValid();
 }

const Question TestController::getNextQuestion() const{
return this->repo->getNextQuestion();
}



}

Upvotes: 0

Views: 149

Answers (2)

BartoszKP
BartoszKP

Reputation: 35891

Here:

const T getNextQuestion() const{
    T q = (*this->it);
    ++this->it;
    return q;
}

You are changing the field it and you're not supposed to, because the method is const. If you want to use const only in the meaning that your "repository" is not modified, but its internal iterator is irrelevant you can use the mutable keyword:

mutable typename std::list<T>::iterator it;

Upvotes: 1

quantdev
quantdev

Reputation: 23793

You are trying to modify a member in a const member function, which is forbidden (thats the point of const member functions) :

const T getNextQuestion() const{
    T q = (*this->it);
    ++this->it;   // << Here
    return q;
}

This method should be non const, or consider having your member iterator be mutable :

mutable typename std::list<T>::iterator it;

Upvotes: 0

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