CODEWITHSUNDEEP
c++c++11shared-ptrsmart-pointers
kgf3JfUtW
kgf3JfUtW

Reputation: 14918

Reference count for a reference to shared_pointer

I understand that the shared_ptr class automatically manages dynamic objects.

Here, I have a function f that returns a const shared_ptr<int> to an int 2.

I have two versions of main that differ in one place. Version A saves the return value of f into a shared pointer, while version B saves into shared pointer reference.

 using namespace std;
    const std::shared_ptr<int> f() {
        const std::shared_ptr<int> ret = std::make_shared<int>(2);
        return ret;
    }

    int main () {
        const std::shared_ptr<int> p = f();     // A
        // const std::shared_ptr<int> &p = f(); // B
        cout << p.use_count() << *p << endl; // prints 1 2
        return 0;
    }

Both versions print 1 2. I am okay with version A because p is the last shared_ptr pointing at the int, therefore use_count is 1.

Question: Why is use_count equal to 1 for version B? Where is the last existing shared_ptr?

Upvotes: 0

Views: 244

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275385

In C++ if you assign a temporary directly into a && or const&, reference lifetime extension kicks in and the temporary lasts as long as reference does.

This only works when your reference is a local variable1.

1 Or in certain cases in certain compilers where you use aggregate construction of structs. But don't use that, because it is one of the spots where a "transparent" forwarding constructor behaves fundamentally different than an aggregate, so otherwise innocuous changes later can break your code.

Upvotes: 1

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