C++ - typeid(), used on derived class doesn't return correct type

Question

Maybe I'm misunderstanding how inheritance works here, but here's my problem:

I have a class Option, and a class RoomOption that derives from it. I have another class Room which holds a vector of shared_ptrs. In main I add a RoomOption to that vector. Then, using typeid() I check the type, and it tells me its an Option. From what I've read, typeid is supposed to return derived types, and shared_ptrs dont cause slicing, so I'm not sure what I'm doing wrong.

Here's the code:

Room.h:

vector<shared_ptr<Option> > options;
void addOption(shared_ptr<Option>);
shared_ptr<Option> getOption(int);

Room.cpp:

void Room::addOption(shared_ptr<Option> option){
    options.push_back(option);
}

shared_ptr<Option> Room::getOption(int i){
    return options[i];
}

main:

shared_ptr<Room> outside(new Room(0, "", ""));
outside->addOption(shared_ptr<RoomOption>(new RoomOption(0, "Go inside", hallway)));
cout<<typeid(player->getRoom()->getOption(0)).name().get()<<endl; 
//This line prints "class std::tr1::shared_ptr<class Option>

It occurs to me that maybe when adding or getting an Option, the RoomOption is casted as an Option due to the return/argument type. If that's the case then how am I supposed to store a vector of more than one type? Or am I getting this all wrong? =\

Answer 1

First of all yor getting the typeid of the shared_ptr.

Then you should use dynamic_cast instead of typeid. E.g:

if (dynamic_cast<RoomOption*>(player->getRoom()->getOption(0).get()) != 0){
    cout << "Its a RoomOption!" << endl;
}

Answer 2

The typeid works differently for polymorphic (for classes having at least one virtual function) and non-polymorphic types :

• If the type is polymorphic, the corresponding typeinfo structure which represents it is determined at run-time (the vtable pointer is commonly used for that purpose, but this is an implementation detail)

• If the type isn't polymorphic, the corresponding typeinfo structure is determined at compile time

In your case, you actually have a polymorphic class Option, but shared_ptr<Option> itsef isn't polymorphic at all. It basically is a container holding an Option*. There is absolutely no inheritance relation between Option and shared_ptr<Option>.

If you want to get the real type, you first need to extract the real pointer from its container using Option* shared_ptr<Option>::get() :

Option * myPtr = player->getRoom()->getOption(0).get();
cout << typeid(*myPtr).name(); << endl;

Or alternatively (it is exactly the same thing) :

Option& myPtr = *player->getRoom()->getOption(0);
cout << typeid(myPtr).name(); << endl;

Answer 3

The type of object a shared_ptr<Option> points to is part of its value, not its type. So this line of code is broken:

cout<<typeid(player->getRoom()->getOption(0)).name()<<endl; 

You want this:

cout<<typeid(player->getRoom()->getOption(0).get()).name()<<endl; 

Or perhaps:

cout<<typeid(*(player->getRoom()->getOption(0))).name()<<endl; 

What typeid does is tell you the actual type of the thing you passed to it. You passed it a shared_ptr<Option>. It doesn't look inside the object to see what it contains.