I want to understand how the reference count of the managed object in a shared_ptr
is affected when a shared_ptr
is assigned to another.
I came across the following statement in C++ primer, 5th edition, that:
For example, the counter associated with a shared_ptr is incremented when ... we use it as the right-hand operand of an assignment... The counter is decremented when we assign a new value to the shared_ptr...
As an example its shown there:
auto p = make_shared<int>(42); // object to which p points has one user
auto q(p); // p and q point to the same object
// object to which p and q point has two users
auto r = make_shared<int>(42); // int to which r points has one user
r = q; // assign to r, making it point to a different address
// increase the use count for the object to which q points
// reduce the use count of the object to which r had pointed
// the object r had pointed to has no users; that object is automatically freed
When I run a similar code, the above is not my observation:
Code:
#include<iostream>
#include<memory>
int main()
{
std::shared_ptr<int> sh1 = std::make_shared<int>(1);
std::shared_ptr<int> sh2 = std::make_shared<int>(2);
sh2 = sh1;
std::cout << "sh1 use count: " << sh1.use_count() << std::endl;
std::cout << "sh2 use count: " << sh2.use_count() << std::endl;
return 0;
}
Output:
sh1 use count: 2
sh2 use count: 2
How can the use_count
of sh2
also 2? Should not it be 0 as per the mentioned text above? Am I missing something here?
Aucun commentaire:
Enregistrer un commentaire