Vector, iterators and const_iterator

Question

I have just started learning vectors and iterators. I can't understand 2 things. Why can I change the constant iterator and what is the role of the "*"?

#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
    vector<string> inventory;
    inventory.push_back("inventory1");
    inventory.push_back("inventory2");
    inventory.push_back("inventory3");
    vector<string>::iterator myIterator;
    vector<string>::const_iterator iter;
    cout << "Your items:\n";
    for (iter = inventory.begin(); iter != inventory.end(); iter++)
    {
        cout << *iter << endl;
    }

Any iterator can change value, unless it is declared const. A const_iterator refers to the object being "pointed to" as const. The * is to retrieve the value the iterator "points" to. — owacoder
– owacoder, Commented Sep 21, 2015 at 13:39
const_iterator doesn't mean that the iterator is a constant, but that you cannot change the item it refers to. — Bo Persson
– Bo Persson, Commented Sep 21, 2015 at 13:42
That code is correct. I just cant stand why "iter", which is const_iterator, can change his value and i cant understand what you mean about the"*" — gprex
– gprex, Commented Sep 21, 2015 at 13:43
I guess i should read more to understand what are you trying to say. — gprex
– gprex, Commented Sep 21, 2015 at 13:45
@owacoder - Don't confuse the poor guy with pointers. They are hard! And to be strict, a pointer is a kind of random access iterator. Not the other way round. :-) — Bo Persson
– Bo Persson, Commented Sep 21, 2015 at 13:49

Bo Persson · Accepted Answer · 2015-09-21 13:55:10Z

10

When you do iter = inventory.begin() it makes iter refer to the first string in your vector. iter++ moves it to refer to the next string.

In the output you use *iter as way to access the string that iter refers to. In the first output that will be inventory1.

The slight confusion about the constness is that

vector<string>::const_iterator   iter;

is an iterator that refers to things that are constant, while

const vector<string>::iterator   iter;

would make the iterator itself constant, but allow you to modify the object it refers to.

answered Sep 21, 2015 at 13:55

Bo Persson

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2 Comments

gprex Over a year ago

So, with * i let the iterator to output it ? Before that it only refers to but with * i give him access ?

gprex Over a year ago

But what is the role here ?:'myIterator = inventory.begin(); *myIterator = "inventory4"; cout << "\nYour items:\n"; for (iter = inventory.begin(); iter != inventory.end(); iter++) { cout << *iter << endl; } of the * ?

eerorika · Accepted Answer · 2015-09-21 13:59:45Z

5

Why can I change the constant iterator

You have a non-const iterator of type vector<string>::const_iterator. You can change it because it is non-const. Being a const_iterator means that dereferencing it returns a const reference to an object. Which means that the referenced object cannot be modified.

The iterator is non-const because the type vector<string>::const_iterator does not have a const qualifier.

what is the role of the "*"

It is the dereference operator. It returns the object that the iterator is currently pinting at. More exactly, it returns a reference to the object. Another name for it is indirection operator.

edited Sep 21, 2015 at 13:59

answered Sep 21, 2015 at 13:54

eerorika

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