C++ shared_ptr return this from derived method

I have been using this:

struct Bar;

struct Foo 
{
   virtual Bar * GetBar() { return nullptr; }
}

struct Bar : public Foo
{
   virtual Bar * GetBar() { return this; }
}

Foo * b = new Bar();
//...
b->GetBar();

I used this instead of slow dynamic_cast. Now I have changed my code to use std::shared_ptr

std::shared_ptr<Foo> b = std::shared_ptr<Bar>(new Bar());

How can I change GetBar method to return std::shared_ptr and get the same functionality as with raw pointers (no need for dynamic_cast) ?

I have tried this:

 struct Foo : public std::enable_shared_from_this<Foo>
 {
     virtual std::shared_ptr<Bar> GetBar() { return nullptr; }
 } 


struct Bar : public Foo
{
    virtual std::shared_ptr<Bar> GetBar() { return shared_from_this(); }
}

But it wont compile