std :: shared_ptr use_count（）值

Question

Could someone please help me out why the output is 2 and not 3? Thank you.

int main()
{
    std::shared_ptr<int> x(new int);
    std::shared_ptr<int> const& y = x;
    std::shared_ptr<int> z = y;
    std::cout << x.use_count() << std::endl;
    return 0;
}

Answer 1

You only have two shared pointers: x and z .

Note that y is a variable but not an object. Its type is a reference type, not an object type.

(In C++, not every object is a variable, and not every variable is an object.)

Maybe the following code illustrates the way in which y does not hold a share of the ownership:

std::shared_ptr<int> x(new int());

std::shared_ptr<int> const& y = x;
assert(y.use_count() != 0);

x.reset();

assert(y.use_count() == 0);

Answer 2

This line:

std::shared_ptr<int> const& y = x; //doesn't increase use_count()

Is declaring y as just a reference to x . It is just like another name for the same object. There is no std::shared_ptr object being created to increment the reference count.