How to get the object type from a collection (vector) of parent objects using RTTI

Question

I have a base class which has two child classes derived from it.

class A {};
class B : public A {};
class C : public A {};

I have another class that has a pointer to collection of class A members using a vector, something like this:

vector<A*> *m_collection;

And what I do is to create objects of class B or C and add them to the collection using push_back:

B *myb = new B();
m_collection->push_back(myb);

Then I loop through the collection and try to check using 'typeid', but it always returns the base class (A). Is it not possible to know the exact type?

Thank you!

Answer 1

Firstly, there is unlikely to be a reason to create your vector dynamically using new. Simply say:

vector<A*> m_collection;

Then you need to give your base class a virtual function or two. A virtual destructor would be a good start:

class A {
   public:
     virtual ~A() {}
};

without it you cannot safely write code like:

m_collection.push_back( new B );
delete m_collection[0];

Doing this will also enable run-time type information. Howver, switching on typeid is not how C++ likes you to use RTTI - you should use dynamic_cast:

m_collection.push_back( new B );    // or new A or new C
if ( C * c = dynamic_cast<C *>( m_collection[0] ) ) {
   c->CFunc():  // function in C
}
else if ( B * b = dynamic_cast<B *>( m_collection[0] ) ) {
   b->BFunc():  // function in B
}
else if ( A * a = dynamic_cast<A *>( m_collection[0] ) ) {
   a->AFunc():  // function in A
}
else {
  throw "unknown type";
}

In general however, it is better to use the virtual function mechanism for dispatch, rather than RTTI.

Answer 2

W.r.t. typeid, you need to have a virtual function in your base class (A) (the vtable usually sits near the std::type_info structure), then you must dereference the object, i.e. typeid(*b).