C++ How to delete the object pointer only when no others are referring to that pointer directly or indirectly

Question

I have an object O1 which has pointer. I created another object O2 by copy constructor. The pointer address will be copied into the object O2. When i come out of scope of O2, it will be deleted. The destructor will delete the pointer as well. This make the pointers in object O1 become dangling. How to make sure that i delete the pointer only when i come out of O1.

In a short description, how to delete the object pointer only when no others are referring to that pointer directly or indirectly.

class info {
    public:
        char *buf;
        int len;
        info(int l) {
            len = l;
            buf = new char[len];
            buf[0] = 'h';
        }
        ~info() {
            delete[] buf;
        }
};
int main() {
    info O1(100);
    cout << O1.buf << "\n";
    {
        info O2(O1);
    }
    cout << O1.buf << "\n";
}

Here the destructor of O1 at the end of the main will get a SIGABT signal for trying to deallocate a memory referred by O1.buf which is already deleted through O2.

@rob or just use `std::shared_ptr`? We already have a wheel. — Yakk - Adam Nevraumont, Apr 03 '16 at 07:53
[Related reading](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)). — n. 'pronouns' m., Apr 03 '16 at 08:06

Julien Lopez · Answer 1 · 2020-05-06T21:31:08.417

5

Shared pointers are exactly what you need.

I also suggest you have a look at the other smart pointers available in the memory standard library.

Edit:

Note that as explained here you should use delete[] as a destructor for your smart pointers that own an array type.

edited May 06 '20 at 21:31

answered Apr 03 '16 at 07:52

Julien Lopez

1,599
5
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1

You may want to talk about `weak_ptr` too! They may be very helpful in some situations. – jpo38 Apr 03 '16 at 08:20
1

Downvoting because blind use of `shared_ptr` for arrays is undefined behavior. – lisyarus Apr 03 '16 at 08:24
@lisyarus Correct. Thank you for your comment. – Julien Lopez Apr 03 '16 at 08:33

score 1 · Answer 2 · edited Apr 03 '16 at 08:49

1

To be specific about shared_ptr usage, the class info should be:

class info {
    public:
        std::shared_ptr<char> buf;
        int len;
        info(int l) {
            len = l;
            if(!buf) buf = std::shared_ptr<char>(new char[len], std::default_delete<char[]>());
            buf.get()[0] = 'h';
        }
        ~info() {
        }
};
int main() {
    info O1(100);
    cout << O1.buf << "\n";
    {
        info O2(O1);
    }
    cout << O1.buf.get() << "\n";
}

edited Apr 03 '16 at 08:49

Julien Lopez

1,599
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answered Apr 03 '16 at 08:10

lulyon

5,745
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Unfortunately, `std::shared_ptr::operator[]` is C++17. :-( Replace `buf[0] = 'h';` with `buf.get()[0] = 'h';` and you have my up vote. – Julien Lopez Apr 03 '16 at 08:26
@JulienLopez yeah, fixed it. – lulyon Apr 03 '16 at 08:29
1

Please also add `std::default_delete()` as second argument of the constructor of `std::shared_ptr` to avoid undefined behavior. – Julien Lopez Apr 03 '16 at 08:38

score 0 · Answer 3 · edited May 23 '17 at 10:30

0

It seems that what you need are shared pointers. They are a bit tricky to use for arrays, this answer tells how to.

edited May 23 '17 at 10:30

Community

1
1

answered Apr 03 '16 at 08:17

lisyarus

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C++ How to delete the object pointer only when no others are referring to that pointer directly or indirectly

3 Answers3