Type cast child to grandparent class

Current code:

class A { 
    // Abstract class
    ... 
};

class B : public A { 
    // Concrete class
    ... 
};

class C : public B { 
    // Concrete class
    ... 
};

class D : public C { 
    // Abstract class
    ... 
};

class E : public A {
    // Concrete class
private:
    std::deque< std::shared_ptr<A> > m_deque;

public:
    E& operator+=( std::shared_ptr<D> temp ) {
        m_deque.push_front( temp );
        ...
    }
    ...
};

When trying to overload the += operator I want to push the parameter temp into the deque m_deque. The problem is that the types don't match up. How do I push temp into the deque m_deque?

I was trying to type cast temp, but I'm not sure how to type cast to a grandparent class.