What happens when we reset a shared_ptr when there are other shared_ptr alias constructed from it?

Question

I am currently learning shared_ptr's alias constructor, and I wrote code like this

int main(){
    std::shared_ptr<Father> father = std::make_shared<Father>();
    std::shared_ptr<Son> son(father, &father->son);
    printf("%d\n", father.use_count());
    printf("%d\n", son.use_count());
    father.reset();
    printf("%d\n", father.use_count());
    printf("%d\n", son.use_count());
    printf("%d\n", father.owner_before(son));
    printf("%d\n", son.owner_before(father));
    return 0;
}

And it prints out

And I got lost here. In my opinion, after father.reset(), father should still have use_count = 1 rather than 0, because son is alias constructed from father, and it not destructed. From this post, the author also says father.use_count() is 1.

// the Foo still exists (ref cnt == 1) // so our Bar pointer is still valid, and we can use it for stuff

So why printf("%d\n", father.use_count()); prints out to be 0?

*"From this post, the author also says father.use_count() is 1"* - No such thing is said there. You confusion stems from bad naming. You conflate the pointers with the objects they manage. — StoryTeller - Unslander Monica, Aug 25 '21 at 08:44
Yes, **the Foo**. He does not say the `use_count` of a reset pointer still magically refers to an objects it was told to forget about. — StoryTeller - Unslander Monica, Aug 25 '21 at 08:47
OK, so as soon as `shared_ptr` is reset, it has no way to access `Foo`. So is there any other ways that I can check Foo still exists? For example, trace `Foo::~Foo`? — calvin, Aug 25 '21 at 08:52
Yes. Tracing a d'tor will tell you when the object goes away — StoryTeller - Unslander Monica, Aug 25 '21 at 08:53

score 4 · Accepted Answer · answered Aug 25 '21 at 08:53

4

After father.reset(), father doesn't point to anything. It holds a null value (officially "there is no managed object").

You are printing the use_count of nothing, not the use_count of the Father or Son object, and the use_count of a null pointer is 0.

answered Aug 25 '21 at 08:53

user253751

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What happens when we reset a shared_ptr when there are other shared_ptr alias constructed from it?

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