A few c++ questions

I need reasonably brief, but complete answers for the following few questions that I am stuck on.

1.a) Suppose class Y is derived from class X.  Each class declares

    a constructor and a destructor.  Class Y has a string member 

    object that has a constructor and destructor. In what order do 

    the three constructors and destructors run when instances of 

    class Y are created and destroyed?

    b) Should X's destructor be declared virtual? Why or why not, 

    and what difference would it make?

2. What is a pure virtual function?  Why would you define a pure

    virtual function? Give an example of a pure virtual function.

3. What is an abstract base class?  What does it do? Why would you

    make a class abstract?

4. What are the advantages of using an stl::string to hold character

    data vs. a dynamic char array?