Vector of classes containing pointers not working

Question

I have this C++ code:

#include <vector>
using namespace std;

struct Test {
    int* member = 0;

    Test() {}
    Test(const Test& o) { member = new int(*o.member); }
    ~Test() { delete member; }
};

int main()
{
    vector<Test> vecTest = {
        Test(), Test(), Test()
    };

    vecTest.erase(vecTest.begin());
}

This program builds properly, but it crashes with exit code -1073741819. What am I doing wrong?

Q: What happens in your copy constructor if `o.member` is null? — Mat, May 30 '21 at 18:59
you should read here: https://stackoverflow.com/questions/4172722/what-is-the-rule-of-three — 463035818_is_not_an_ai, May 30 '21 at 19:02
why is `member` a pointer? and why are includes not included? — 463035818_is_not_an_ai, May 30 '21 at 19:02
Includes are not shown to make the snippet shorter (the hidden part is just `#include using namespace std;`). `member` is a pointer because of performance (I know, `int` is small, this is just simplified example of something bigger, where `member` is bigger object) @463035818_is_not_a_number. — FireFragment, May 30 '21 at 19:06
please read about [mcve]. Removing those two lines makes it more difficult for others to reproduce and understand your issue, not easier — 463035818_is_not_an_ai, May 30 '21 at 19:08

Remy Lebeau · Accepted Answer · 2021-05-30T21:22:43.133

-1073741819 is hex 0xC0000005. That is the exit code for an uncaught Access Violation exception. Which means your code is accessing invalid memory.

Your vector is initialized with Test objects that are holding null pointers. When you erase the 1st object, the remaining objects have to be moved down in the vector. But, since your class does not implement move semantics, the objects have to be copied instead. Your copy constructor is not handling the case when o.member is nullptr. And your class does not implement a copy assignment operator at all, thus violating the Rule of 3/5/0, which can lead to memory leaks and multiple objects ending up holding the same pointer causing double-deletes of memory.

Try this instead:

class Test {
    int* member;
public:
    Test() { member = new int(); }
    Test(const Test& o) { member = new int(*(o.member)); }
    ~Test() { delete member; }

    Test& operator=(const Test& rhs) {
        if (&rhs != this) {
            *member = *(rhs.member);
        }
        return *this;
    }
};

Or, with move semantics added:

class Test {
    int* member;
public:
    Test() { member = new int(); }
    Test(const Test& o) { member = new int(*(o.member)); }
    Test(Test&& o) { member = o.member; o.member = nullptr; }
    ~Test() { delete member; }

    Test& operator=(Test rhs) {
        std::swap(member, rhs.member);
        return *this;
    }
};

That being said, consider using std::unique_ptr to help with memory management:

class Test {
    std::unique_ptr<int> member;
public:
    Test() { member = std::make_unique<int>(); }
    Test(const Test& o) { member = std::make_unique<int>(*(o.member)); }
    Test(Test&& o) { member = std::move(o.member); }

    Test& operator=(Test rhs) {
        std::swap(member, rhs.member);
        return *this;
    }
};

maybe worth mentioning that its not the kind of exception one can `catch` — 463035818_is_not_an_ai, May 30 '21 at 19:21
then I am missing something, tried it here: https://godbolt.org/z/YEz14Gj5n — 463035818_is_not_an_ai, May 30 '21 at 19:26
@463035818_is_not_a_number it is compiler-specific. Some compilers can catch OS errors with `catch`. Some can't — Remy Lebeau, May 30 '21 at 19:39
@RemyLebeau, Thanks for a great answer. I'd like to have 1 question. You wrote that the OP's copy constructor is not handling the case when o.member is nullptr. I wonder if your copy constructor handles that case ? I can see some differences between these 2 copy constructors but I am not 100% sure how that case is handled in your copy constructor. Thanks. — Job_September_2020, May 30 '21 at 19:54
@Job_September_2020 if you look more carefully, I had updated the default constructor to ensure `member` is never `nullptr` to begin with. — Remy Lebeau, May 30 '21 at 21:18

Vector of classes containing pointers not working

1 Answers1