C++ code works fine on desktop but not on the laptop

Question

This simple code for some reason works perfectly fine on my desktop but when I try it on my laptop only the first part (printing the elements of vector) works then the program ends and instead of saying "Process finished with exit code 0" it says
"Process finished with exit code -1073741819 (0xC0000005)". I don't know what's wrong with my laptop. Can anyone help me?

#include <iostream>
#include <vector>

using namespace std;

int main() {
    vector<int> array{1, 2, 3, 4, 5};
    vector<int>::iterator it;
    int arraysize;
    for (int i = 0; i < array.size(); i++) {
    cout << array[i] << endl;
    }
    cout << " " << endl;

    for (it = array.begin(); it < array.end(); it++) {
        if(*it%2==0){
            array.erase(it);
            it--;
        }
    }
    arraysize=array.size();
    cout<<"size:"<<arraysize<<endl;
    for (int i = 0; i < array.size(); i++) {
        cout << array[i] << endl;
    }



    return 0;
}

score 3 · Answer 1 · answered Jun 24 '20 at 08:38

3

This happens because of iterator invalidation, when you erase an element of the vector the iterator it becomes invalidated leading to undefined behaviour, you can read more about this here Iterator invalidation rules

answered Jun 24 '20 at 08:38

SzymonO

432
3
15

score 3 · Accepted Answer · answered Jun 24 '20 at 08:40

The problem is not with the computer but with the code.

array.erase(it); invalidates the iterator it, and any subsequent use of it has undefined behaviour.
The worst kind of undefined behaviour is the one that appears to work.

erase returns an iterator to the element after the erased one, and you should use that.

for (it = array.begin(); it < array.end(); it++) {
    if(*it%2==0){
        it = array.erase(it);
        it--;
    }
}

or

it = array.begin(); 
while (it < array.end()) {
    if(*it%2==0){
        it = array.erase(it);
    }
    else {
        it++;
    }
}

Because undefined behaviour can be anything, including exhibiting no apparent problems whatsoever. It can be different on different machines, or on different days of the week, or seemingly completely random. — molbdnilo, Jun 24 '20 at 08:54

Caleth · Answer 3 · 2020-06-24T09:16:39.033

2

Your program has undefined behaviour, you decrement an invalid iterator

array.erase(it); // it becomes invalid
it--; // Undefined

You can avoid this by removing elements with the "erase-remove" pattern

auto is_even = [](int i) { return i%2==0; };
array.erase(std::remove_if(array.begin(), array.end(), is_even), array.end());

Or in C++20

std::erase_if(array, is_even);

edited Jun 24 '20 at 09:16

answered Jun 24 '20 at 08:41

Caleth

52,200
2
44
75

While other answers talk about the quick fix, your answer respectfully doesn't even go there. You have instead shown the _correct_ way to erase multiple items from a vector! Nice one. :) – paddy Jun 24 '20 at 09:03

lukeg · Answer 4 · 2020-06-24T08:43:45.377

1

There is nothing wrong with your laptop. The problem is with the code. When you erase something from the vector, it invalidates the preexisting iterators following the erased elements. You may want to use the return value of erase, which references the new reallocated location of the erased element's successor.

edited Jun 24 '20 at 08:43

answered Jun 24 '20 at 08:39

lukeg

4,189
3
19
40

Note that only iterators pointing to or past the erased element are invalidated. Iterators pointing to previous elements should remain intact. – iBug Jun 24 '20 at 08:40

C++ code works fine on desktop but not on the laptop

4 Answers4