C++ shared+ptr use_count for nullptr

Question

I have 2 shared_ptrs defined and assigned nullptr. In case 1, I'm using default constructor and in case 2, I used constructor with delete method.

shared_ptr<int> sptr2(nullptr);

cout << "sptr2 use_count: " << sptr2.use_count() << endl;

shared_ptr<int> sptr6(nullptr, default_delete<int>());

cout << "sptr6 use_count: " << sptr6.use_count() << endl;

The output is:

sptr2 use_count: 0
sptr6 use_count: 1

I cannot understand why sptr6 has use count of 1 when it does not have any valid pointer.

g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16)

Answer 1

Per [util.smartptr.shared.const] in C++11 & C++14 (I haven't checked C++17), the shared_ptr is "empty" if you pass no argument. Otherwise, the shared_ptr "owns p" even for the case that p is a nullptr_t.

When a deleter is provided this makes sense (you have to store the deleter somewhere, after all), but what the purpose of this is for the single-argument constructor I couldn't say.

I'm apparently not alone, because the C++11/C++14 specification for the actual functions ([util.smartptr.shared]/1) lists constexpr shared_ptr(nullptr_t) : shared_ptr() { }, which suggests this construction (but not the construction that provides a deleter) should result in an "empty" shared_ptr!

But this is in direct contradiction to the listed semantics (which specifically give use_count == 1 as a post-condition for both of your examples), and would therefore appear to be a bug in the standard.

GCC has apparently chosen to side with the function specification (as has cppreference.com). At least your deleter should be a no-op in this case.

To actually use the default constructor, write this:

shared_ptr<int> sptr;

Answer 2

The reason of different behavior caused by the shared_ptr design. SharedPtr refer to "control object", and control object refers to "user object".

If you initialize shared_ptr with nullptr (sptr2), control object is not created. Constructor does nothing.

If you initialize shared_ptr with deleter (sptr6), control object is created to store deleter. So, if control object created, it has to have reference count 1.