How to browse a list<Class> In c-CodePudding

I'm having a problem here and I thought I would find an easy answer on the Internet, but it's been 1 hour and I can't solve it. Seems so simple, but I can't find a way...

I've got 2 classes:

#include <iostream>
#include <list>
using namespace std;

class classB;

class classA{
private :
    string name;
    list<classB*> listClassB;

public:
    void getListClassB() const;
};

class classB{
private:
    string name;
    list<classA*> listClassA;

public:
    void getListClassA() const;
};

What I do on the getListClassB() method is:

void classA::getListClassB() const {
    for(list<classB*>::iterator it = listClassB.begin(); it != listClassB.end; it  ){
        //Stuff
    }
}

Visual Studio Code tells me that there is an error is on listClassB from list<classB*>::iterator it = listClassB.begin()

The complete error about that is:

there is no appropriate user-defined conversion of 
"std::_List_const_iterator<std::_List_val<std::conditional_t<true, std::_List_simple_types<classB *>, std::_List_iter_types<classB *, size_t, ptrdiff_t, classB **, classB *const *, classB *&, classB *const &, std::_List_node<classB *, void *> *>>>>\" in \"std::_List_iterator<std::_List_val<std::conditional_t<true, std::_List_simple_types<classB *>, std::_List_iter_types<classB *, size_t, ptrdiff_t, classB **, classB *const *, classB *&, classB *const &, std::_List_node<classB *, void *> *>>>>"

EDIT:

Ok, so thanks again for all your time, but this error makes me crazy.

I completed a bit the code to be more explicit about my work.

I don't go deeper because it's college work in France, and it's about UML classes, so it's class linked with others classes...

CodePudding user response：

First, iterator is a pointer semantic object, so you have to dereference it to access the actual object. it.str makes no sense, since std::list<>::iterator has no str member (and even if it did, that would have nothing to do with str member of Whatever class.

Second, your list elements are pointers themselves, so this means you need a second indirection in order to access the str member:

std::cout << (*(*it)).str;

More appealing syntax would be:

std::cout << (*it)->str;

CodePudding user response：

The error message is difficult to read with all the template stuff in it, but it basically boils down to this:

there is no appropriate user-defined conversion of 
"std::_List_const_iterator<>" in "std::_List_iterator<>"

getListClassB() is marked as const, so its this pointer is typed as const classA *, thus the listClassB member is implicitly const when accessed via this.

When list::begin() is called on a const list object, it returns a list::const_iterator, which your loop is trying to assign to a list::iterator, which is not allowed, hence the error. list::iterator allows the list data to be modified, whereas list::const_iterator does not. Which is why a const_iterator cannot be assigned to an iterator.

That being said, your for loop has another typo in it. You need to change listClassB.end to listClassB.end() instead (notice the extra parenthesis), ie you need to call end() and compare its return value to it, not compare end itself to it.

Try this instead:

void classA::getListClassB() const {
    for(list<classB*>::const_iterator it = listClassB.begin(); it != listClassB.end();   it){
        //Stuff
    }
}

Note, however, that the const_iterator/iterator mismatch could have been avoided if you had used auto instead to let the compiler deduce the appropriate type for it, eg:

void classA::getListClassB() const {
    for(auto it = listClassB.begin(); it != listClassB.end();   it){
        //Stuff
    }
}

But, a safer way to avoid both mistakes would be to use a range-for loop instead, eg:

void classA::getListClassB() const {
    for(classB *ptr : listClassB){
        //Stuff
    }
}