Cannot dynamic cast when using dynamic_pointer_cast

Question

Why does this code not work?

std::shared_ptr<Event> e = ep->pop();
std::shared_ptr<TrackerEvent> t;

t = std::dynamic_pointer_cast<TrackerEvent>(e);

I get the following error:

/usr/include/c++/4.6/bits/shared_ptr.h:386: error: cannot dynamic_cast '(& __r)->std::shared_ptr<Event>::<anonymous>.std::__shared_ptr<_Tp, _Lp>::get [with _Tp = Event, __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]()' (of type 'class Event*') to type 'class TrackerEvent*' (source type is not polymorphic)

TrackerEvent inherits from Event so I guess the problem is that I can't cast in this direction. But ep->pop() might either return an object of type Event or TrackerEvent. And I was hoping that when I try to cast it to TrackerEvent and it returns NULL I would know whether I have an Event or TrackerEvent...

How would I do that?

Answer 1

The compiler is telling you what is going on at the end of the message:

(source type is not polymorphic)

Your Event base class needs to have at least one virtual member function (i.e. be a polymorphic type) in order to allow dynamic casts. You could make the destructor of Event virtual:

class Event
{
public:
    virtual ~Event() { /* whatever goes here, or nothing... */ }
    // ...
};

Here is a live example with polymorphic types, showing that the code compiles (removing the virtual destructor would cause a compilation error similar to the one you are seeing).

As correctly mentioned by Luc Danton in the comments, a defaulted version of a virtual destructor can be defined this way (if your compiler is C++11-compliant in this respect):

class Event
{
public:
    virtual ~Event() = default;
    // ...
};

Answer 2

To perform a dynamic_cast, the type you're casting from must be polymorphic. For this to be true, it must have or inherit some virtual members. Make sure Event has a virtual member function (at the very least, a virtual destructor).