Class deconstructor and pointers

Question

We were writing a code in our university (learning about Object Oriented Program). We wrote this class and the issue is that, as per my understanding and our teacher the program should crash in the following condition, but in my case it doesn't.

The thing is this line is the culprit

DynamicArray d, f;
f = d;

according to him(rest of the code is attached), since when main ends, deconstructor deletes f and then goes for d, but since pointer was pointing to same mem location in both cases, now it should give error when it tries to delete it, since there is nothing there....but my compiler runs the code perfectly. I am using gcc compiler. Denconstructor at the end of class, rest is to fill dynamic array.

#include <iostream>
#include <ctime>

class DynamicArray{

private:
    int *arr;
    int size, cap; //cap is the physical size, size is number of elements

public:
    DynamicArray(){
        arr = nullptr;
        size = cap = 0;
    }
    DynamicArray(int i){
        cap = i;
        size = 0;
        arr = new int[cap];
    }
    void pushback(int j){

        if(cap == 0){
            arr = new int[cap];
            arr[0] = j;
            size++;
            cap++;
        }
        else if(size < cap){
            arr[size] = j;
            size++;
        }
        else if(size == cap){
            int *arr2 = new int[cap * 2];
            int i;
            cap *= 2;

            for(i = 0; i < size; i++){
                arr2[i] = arr[i];
            }
            arr2[i++] = j;

            delete[] arr;
            arr = arr2;
        }
    }
    void print(){
        for(int i = 0; i < size; i++)
            std::cout << arr[i] << " ";
    }
    ~DynamicArray(){
        if(arr != nullptr)
            delete[] arr;
    }
};

int main(){

    DynamicArray d, f;

    srand(time(nullptr));
    int n = rand() % 5;

    for(int i = 0; i < n; i++){
        d.pushback(rand() % 10);
    }

    f = d;

    f.print();
    std::cout << std::endl;
    d.print();

    return 0;
}

*Undefined behavior* (as caused in this case by calling operator `delete[]` on an invalid pointer) does not guarantee anything. In debug mode many compilers will flag a double free — UnholySheep, Mar 06 '18 at 13:19
Technically, it doesn't need to _crash_. `delete`ing the same memory multiple times is just undefined behavior. Which means - anything could happen, and _crash_ is just one of the possibilities. — Algirdas Preidžius, Mar 06 '18 at 13:20
Possible duplicate of [What happens in a double delete?](https://stackoverflow.com/questions/9169774/what-happens-in-a-double-delete) — UnholySheep, Mar 06 '18 at 13:21
@Osama Asif C Infinitum Pay attention to that the class definition has several errors as for example if(cap == 0){ arr = new int[cap]; arr[0] = j;.:) — Vlad from Moscow, Mar 06 '18 at 13:22
Why would two default constructed objects, holding a `nullptr` each, cause problems? Your example is lacking. — StoryTeller - Unslander Monica, Mar 06 '18 at 13:24
@sama Asif C Infinitum Change the teacher or the compiler.:) — Vlad from Moscow, Mar 06 '18 at 13:25
@VladfromMoscow hahaha will think about it! :) and as far as i can tell there is no error in the code you mentioned two comments up.. — Osama Asif C Infinitum, Mar 06 '18 at 13:58
@OsamaAsifCInfinitum You are wrong about the code. You are trying to allocate a memory wuth the size equal to 0 and then trying to write to this memory. — Vlad from Moscow, Mar 06 '18 at 14:00

score 4 · Accepted Answer · answered Mar 06 '18 at 13:21

Your code causes so-called "undefined behaviour". Basically, that means anything can happen, including that nothing happens or that whatever happens isn't easily observable. For a more precise definition of "undefined behaviour", you'd have to check e.g. the C++ standard, but there are also many discussions concerning this term here.

Try running your code using e.g. valgrind, it will tell you that your code is broken.

Class deconstructor and pointers

1 Answers1