shared_ptr and slicing

Question

Someone I worked with once said that shared_ptr was unsafe and would slice when casting from a derived class to a base class (i.e. upcasting). For example if there were 2 classes A and B where B derived from A, then

shared_ptr<A> a(new B)

would slice. I pointed him to http://www.boost.org/doc/libs/1_43_0/libs/smart_ptr/shared_ptr.htm where it says

shared_ptr<T> can be implicitly converted to shared_ptr<U> whenever T* can be implicitly converted to U*.

implied that it's safe to use in those contexts but he didn't seem to think so.

Answer 1

That someone is wrong, object slicing doesn't apply to pointers. That the pointer usage is wrapped away in a shared_ptr doesn't change that - it doesn't do any particular magic here, it initializes an internal pointer with the value passed to its constructor.

Simplified it could look e.g. like this for the purpose of this question:

template<class T> struct ptr {
    T* t;
    ptr(T* t) : t(t) {}
    // ...
};

You lose the static type-information of B, yes, but nothing changes regarding the object pointed to.