Re: Query regarding iterators and vector

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Fri, 04 Jul 2008 23:39:31 +0200

Message-ID:

<WuGdndZ0656GBfPVnZ2dnUVZ_jidnZ2d@posted.comnet>

* prasadmpatil@gmail.com:

I am new STL programming.
I have a query regarding vectors. If I am iterating over a vector
using a iterator, but do some operations that modify the size of the
vector. Will the iterator recognize this?

No, it's invalidated, meaning UB if you continue using it.

I wrote the following program to test this out.

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main()
{
    vector<int> a;

    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    cout<<"Vector test begin"<<endl;
    vector<int>::iterator iter0;
    for(iter0=a.begin(); iter0!=a.end(); iter0++)
    {
        cout << "\n value: " << (*iter0)<<endl;
        if(*iter0 == 2)
        {
            a.push_back(4);
            a.push_back(5);
        }
    }
    cout<<"Vector test end";
}

I am getting the following output. Could somebody please explain what
is happening in my program.

Formally it's Undefined Behavior. Practically you most likely experienced that
the vector reallocated its internal buffer, and copied over the old values. The
iterator would then be using invalid information about the vector's internal
contents.

I am thoroughly confused.

UB can have that effect, indeed, formally it can have *any* effect.

Cheers, & hth.,

- Alf

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