shared_ptr magic :)

Question

Mr. Lidström and I had an argument :)

Mr. Lidström's claim is that a construct shared_ptr<Base> p(new Derived); doesn't require Base to have a virtual destructor:

Armen Tsirunyan: "Really? Will the shared_ptr clean up correctly? Could you please in this case demonstrate how that effect could be implemented?"

Daniel Lidström: "The shared_ptr uses its own destructor to delete the Concrete instance. This is known as RAII within the C++ community. My advice is that you learn all you can about RAII. It will make your C++ coding so much easier when you use RAII in all situations."

Armen Tsirunyan: "I know about RAII, and I also know that eventually the shared_ptr destructor may delete the stored px when pn reaches 0. But if px had static type pointer to Base and dynamic type pointer to Derived, then unless Base has a virtual destructor, this will result in undefined behavior. Correct me if I am wrong."

Daniel Lidström: "The shared_ptr knows the static type is Concrete. It knows this since I passed it in its constructor! Seems a bit like magic, but I can assure you it is by design and extremely nice."

So, judge us. How is it possible (if it is) to implement shared_ptr without requiring polymorphic classes to have virtual destructor? Thanks in advance

Answer 1

Yes, it is possible to implement shared_ptr that way. Boost does and the C++11 standard also requires this behaviour. As an added flexibility shared_ptr manages more than just a reference counter. A so-called deleter is usually put into the same memory block that also contains the reference counters. But the fun part is that the type of this deleter is not part of the shared_ptr type. This is called "type erasure" and is basically the same technique used for implementing the "polymorphic functions" boost::function or std::function for hiding the actual functor's type. To make your example work, we need a templated constructor:

template<class T>
class shared_ptr
{
public:
   ...
   template<class Y>
   explicit shared_ptr(Y* p);
   ...
};

So, if you use this with your classes Base and Derived ...

class Base {};
class Derived : public Base {};

int main() {
   shared_ptr<Base> sp (new Derived);
}

... the templated constructor with Y=Derived is used to construct the shared_ptr object. The constructor has thus the chance to create the appropriate deleter object and reference counters and stores a pointer to this control block as a data member. If the reference counter reaches zero, the previously created and Derived-aware deleter will be used to dispose of the object.

The C++11 standard has the following to say about this constructor (20.7.2.2.1):

Requires: p must be convertible to T*. Y shall be a complete type. The expression delete p shall be well formed, shall have well defined behaviour and shall not throw exceptions.

Effects: Constructs a shared_ptr object that owns the pointer p.

…

And for the destructor (20.7.2.2.2):

Effects: If *this is empty or shares ownership with another shared_ptr instance (use_count() > 1), there are no side effects. Otherwise, if *this owns an object p and a deleter d, d(p) is called. Otherwise, if *this owns a pointer p, and delete p is called.

(emphasis using bold font is mine).