Delete raw pointer after creating a shared_ptr from it

Question

If I do the following,

int* p = new int(10);
std::shared_ptr<int>(p);
delete p;

What happens here? Is the shared_ptr invalid after deletion of the raw pointer? Is there any way to ensure memory access safety in such a scenario?

Answer 1

The code in your question contains 2 conflicting definitions of p. I'm assuming you meant to post something like

int* p = new int(10);
std::shared_ptr<int> p1(p);
delete p;

When the shared_ptr goes out of scope and its reference count falls to zero it will attempt to delete p;, leading to double deletion and undefined behavior.

You've passed ownership of the dynamically allocated int to the shared_ptr, so let it do its job, and don't go about deleting the int yourself.

---

If you want clients of your API from doing something similar to the code above, one possibility is to change the API function's parameter type from a shared_ptr to a parameter pack of constructor arguments. For instance

template<typename T, typename... Args>
void api_func(Args&&... args)
{
    auto p = std::make_shared<T>(std::forward<Args>(args)...);
    // Use the shared_ptr p as before
}

Then, instead of passing a shared_ptr<int>, client code would call the above function as api_func<int>(10);.