Move a unique_ptr with custom deleter to a shared_ptr

Question

I have a function which creates a unique_ptr with a custom deleter and returns it:

auto give_unique_ptr() {
    auto deleter = [](int* pi) {
        delete pi;
    };
    int* i = new int{1234};
    return std::unique_ptr<int, decltype(deleter)>(i, deleter);
}

In the client code of that function, I'd like to move the unique_ptr into a shared_ptr, but I don't know how to do that given that I don't know the decltype of my custom deleter outside of the function.

I guess it should look something like this:

auto uniquePtr = give_unique_ptr();
auto sharedPtr = std::shared_ptr<..??..>(std::move(uniquePtr));

What do I have to write instead of ..??.. to get the correct type?

If this is possible, will the shared_ptr behave nicely and call my custom deleter created inside the give_unique_ptr() function when it's usage count reaches zero?

Answer 1

If you know (or want to explicitly type) the type of the object, then you can do this:

std::shared_ptr<int> sharedPtr(std::move(uniquePtr));

The constructor of std::shared_ptr will take care of the deletor.

---

If you, however, want the type to be inferred, then:

auto sharedPtr = make_shared_from(std::move(uniquePtr));

where make_shared_from is:

template<typename T, typename D>
std::shared_ptr<T> make_shared_from(std::unique_ptr<T,D> && p)
{
   //D is deduced but it is of no use here!
   //We need only `T` here, the rest will be taken 
   //care by the constructor of shared_ptr
   return std::shared_ptr<T>(std::move(p));
};

Hope that helps.

Answer 2

auto uniquePtr = give_unique_ptr();
auto sharedPtr = std::shared_ptr<decltype(uniquePtr)::element_type>(std::move(uniquePtr));

And yes, the shared_ptr will store - and later use - the custom deleter.