Calling derived class functions from within a vector (c++) - c++

Question

I have two classes:
class Object {
public:
Object();
virtual void update();
virtual void draw();
private:
protected:
int x, y, tick;
}
and
class Unit : public Object {
public:
Unit();
void update();
private:
protected:
}
I then define the constructors and functions in sepparate .cpp files.
Here's the definitions for Object:
Object::Object() {
x = y = 0;
};
Object::update() {
tick ++;
};
Object::draw() {
// All my draw code is in here.
};
And Unit:
Unit::Unit() : Object() {
};
Unit::update() {
Object::update();
// Then there's a bunch of movement related code here.
};
Everything works fine individually, but I run into a problem when trying to call functions from within a vector.
vector<Object> objects;
I then do this in my void main():
for (int i = 0; i < objects.size(); i ++) {
objects[i].update();
objects[i].draw();
};
This draws everything fine, but it only calls the Object verson of update() not the version as defined by the derived class. Do I have to make a vector for each type that I derive from the Object class for it to work, or is there another way to call the derived functions?
Thanks in advance - Seymore

Answer 1

Yes, it calls methods of class Object, because you have a vector of class Object objects:
vector<Object> objects; // stores instances of class Object
The possible solution is to use a vector of pointers:
vector<Object*> objects;
objects.push_back( new Unit() );
and then call though pointers:
for (int i = 0; i < objects.size(); i ++) {
objects[i]->update();
objects[i]->draw();
}

Answer 2

If you want to use object oriented polymorphism in C++ (with the meaning : you have a base class and derived classes and at runtime you can come across any off those) you must use pointers.
vector<Object *> objects;
That means manual memory management (but you can use shared_pointers or http://www.boost.org/doc/libs/1_35_0/libs/ptr_container/doc/ptr_container.html), and possibly a very slight loss of performance (because of one extra dereferencing and v-table, however you shouldn't really care about it). And don't forget to make your methods virtual ;)
I think this is why I rarely use on my daily work object oriented programming as I learned it at school ;) I use heritage, but don't mix mother and daugther classes in the same containers (and I use templates for many interfaces).