vector iterator in c++

Question

I am attempting to use iterators to walk over a vector<char*> in c++. I have built a dummy program that is supposed to start at the end, and step backward (toward the beginning, or rend()) on a number >0, and forward (toward the end, or rbegin()) on a number <0, and exit on 0. if the iterator has reached either of the ends and the user attempts to step further, it should repeat the element at that end and not move the iterator. My problem is that, rather than doing that, if the user tries to run over the end, I just get a segfault. here's my code:

#include <iostream>
#include <vector>
#include <stdio.h>

using namespace std;

int main(){
    vector<char*> vect;
    char* tmp;
    for (int i=1; i<=5; i++){
        tmp = new char[7];
        sprintf(tmp, "hello%d", i);
        vect.push_back(tmp);
    }

    vector<char*>::const_reverse_iterator it = vect.rbegin();

    int a;
    cin >> a;

    while (a!=0){
        if (a>0){
            if (it < vect.rend()){
                cout << *(++it) << endl;
            } else{
                cout << *it << endl;
            }
        } else{
            if (it > vect.rbegin()){
               cout << *(--it) << endl;
            } else{
                cout << *it << endl;
            }
        }
        cin >> a;
    }

    return 0;
}

Can anyone identify the problem?

EDIT

I forgot that I made a minor change. my previous code did not populate tmp in the initializing for loop. that has been fixed

yea I saw that, but it doesnt really matter as this is just dumnmy code — ewok
– ewok, Commented Aug 13, 2012 at 15:17

Andriy · Accepted Answer · 2012-08-13 15:00:37Z

7

The problem is that the rend iterator points one item past the (reversed) end of sequence. Dereferencing it causes a segfault:

    if (it < vect.rend()){
        cout << *(++it) << endl;
    } else{
        cout << *it << endl;    // <---- segfault
    }

A minimal fix could be

if (it+1 < vect.rend())
{
    cout << *(++it) << endl;
} else{
    cout << *it << endl;   
}

edited Aug 13, 2012 at 15:00

answered Aug 13, 2012 at 14:54

Andriy

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2 Comments

Chad Over a year ago

This is correct, but it's a little difficult to see exactly where rend() is being dereferenced from a casual glance at the code. The suggested line should replace if(it < vect.rend()).

Andriy Over a year ago

@Chad: Thank you, I added a clarification

Pete Becker · Accepted Answer · 2012-08-13 15:56:26Z

0

Since the goal, effectively, is to not use the past-the-end position, I'd recast the problem: it needs two iterators, one pointing to the first element in the desired range, and one pointing to the last one. Then the mechanics become easy:

if (it != end)
    ++it;
cout << *it << endl;

Similarly, going the other direction:

if (it != begin)
    --it;
cout << *it << endl;

Where begin and end are defined like this:

typedef vector<char*>::reverse_iterator iter;
iter begin = vect.rbegin();
iter end = --vect.rend();  // assumes that vect is not empty

answered Aug 13, 2012 at 15:56

Pete Becker

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