Erase with iterator

[Rajesh1978]

Hi,
I heard that erasing elements using iterator is dangerous.
But I tried the below program in gcc and it is running fine.
Please explain how it is dangerous

#include<vector>
#include<iterator>
#include<iostream>
using namespace std;
int main()
{
    vector<int>vec;
    vec.push_back(123);
    vec.push_back(12);
    vec.push_back(23);
    vec.push_back(123);
    vec.push_back(1123);
    vec.push_back(111);
    vec.push_back(2223);

    copy(vec.begin(),vec.end(),ostream_iterator<int>(cout,"---"));
    cout<<endl;
    vector<int>::iterator itr;
    for(itr=vec.begin();itr!=vec.end();itr++)
    {
        if(*itr == 123)
            vec.erase(itr);
    }

    copy(vec.begin(),vec.end(),ostream_iterator<int>(cout,"---"));
    cout<<endl;
}

output is:
123---12---23---123---1123---111---2223---
12---23---1123---111---2223---

[TheRogue]

Read up on erase.

[laserlight]

Please explain how it is dangerous

The problem is that the call to erase invalidates the iterator (and iterators pointing to subsequent elements). You could avoid this by writing:

for (itr = vec.begin(); itr != vec.end();)
{
    if (*itr == 123)
    {
        itr = vec.erase(itr);
    }
    else
    {
        ++itr;
    }
}

But it would be simpler and safer to write:

vec.erase(remove(vec.begin(), vec.end(), 123), vec.end());

By the way, you forgot to #include <algorithm>.

[Rajesh1978]

Thanks
I understood now.
But I surprised how my code ran without any problem.

[monarch_dodra]

#include<vector>
#include<iterator>
#include<iostream>
using namespace std;
int main()
{
    vector<int>vec;
    vec.push_back(123);
    vec.push_back(12);
    vec.push_back(23);
    vec.push_back(123);
    vec.push_back(123);
    vec.push_back(1123);
    vec.push_back(111);
    vec.push_back(2223);

    copy(vec.begin(),vec.end(),ostream_iterator<int>(cout,"---"));
    cout<<endl;
    vector<int>::iterator itr;
    for(itr=vec.begin();itr!=vec.end();itr++)
    {
        if(*itr == 123)
            vec.erase(itr);
    }

    copy(vec.begin(),vec.end(),ostream_iterator<int>(cout,"---"));
    cout<<endl;
}

output is:

123---12---23---123---123---1123---111---2223---
12---23---123---1123---111---2223---

[Rajesh1978]

Thanks a lot
You people gave a very vivid explanation and made me understood.

Regards

[Paul McKenzie]

Hi,
I heard that erasing elements using iterator is dangerous.
But I tried the below program in gcc and it is running fine.
Please explain how it is dangerous

When you make a mistake in a C++ program, there is no guarantee how your program will run. It may "work", it may crash, it may work sometimes and crash another time, it may work differently if compiled with a different compiler or different options, etc.

So writing code, and then seeing that it runs does not mean that the code is OK. If you're told why something in C++ is undefined, don't try and write code to see what happens -- you don't know what will happen. Unlike other computer languages, C++ doesn't automatically tell you that you did something wrong when you run a program.

Regards,

Paul McKenzie