c++ problem with polymorphism and vectors of pointers

Question

Consider the following example code:

class Foo
{
};

class Bar : public Foo
{
};

class FooCollection
{
protected:
    vector<shared_ptr<Foo> > d_foos;
};

class BarCollection : public FooCollection
{
public:
    vector<shared_ptr<Bar> > &getBars()
    {
        // return d_foos won't do here...
    }
};

I have a problem like this in my current project. The client code uses BarCollection, which stores pointers to Bars in d_foos which is declared in FooCollection. I'd now like to expose the collection of pointers to Bars to the client code. I could just give the client code access to the vector of pointers to Foos and cast these to pointers to Bars in the client code, but this feels wrong since the client doesn't have to know about Foo's existence.

I could also define a get() member that retrieves objects from d_foos and casts them, but this feels quite clumsy. Preferably, I'd like to just return d_foos as a vector<shared_ptr<Bar> > &, but I cannot seem to do this.

It could also be that my design is just plain wrong. It seemed to most natural solution though, as Bar is a specialization of Foo and BarCollection is a specialization of FooCollection and they share functionality.

Could you suggest nice solutions to implement getBars in BarCollection or better design alternatives?

Edit:

Turns out my design was bad indeed. BarCollection is not a FooCollection, despite of requiring all of FooCollection's functionality. My current solution based on the answers below -- which is a lot cleaner -- is now:

class Foo
{
};

class Bar : public Foo
{
};

template<class T>
class Collection
{
    vector<shared_ptr<T> > d_items;
};

typedef Collection<Foo> FooCollection;

class BarCollection : public Collection<Bar>
{
    // Additional stuff here.
};

Thanks for all the excellent suggestions and examples!

Answer 1

template<class T>
class MyContainer {
  vector<shared_ptr<T> > d_foos;
public:
  vector<shared_ptr<T> > & getVector();
};

class FooCollection : public MyContainer<Foo> {
};

class BarCollection : public MyContainer<Bar> {
};