Unit_4: C++ Inherit, Polymorphism

Question 1

Compare Stack vs Heap-based

Answer 1

1. Heap:
   - where all dynamic memory from
   - call “new” -> object goes on heap
   - memory on heap can be reclaimed (C++: delete, Java: garbage collected)
   - heap is unorganized -> random memory allocation
2. Stack:
   - when variables declared within a scope
   - storage in stack frame (activation record) -> allocate when function is called, removed when function ended
   - Separate activation record each call => routine can call multi time (recursion), each frame contains its own variable and param values
   - object themselves not on the stack

Question 2

What is an activation record?

Answer 2

• pointer: stack based variable and the memory it points to is on heap

Question 3

Why No virtual method => No polymorphism

Answer 3

because without virtual -> base class would be called, breaking polymorphic behavior

Question 4

Why No pointer (as in a stack-based object) => No
polymorphism

Answer 4

because when you use stack base, the derived class object get sliced down to base class. without pointer => object lost its derived class identity

Question 5

compare virtual method in C++ vs JAVA

Answer 5

• C++: can implement pure virtual method in the class defines it
• JAVA: not possible

Question 6

What’s consequence of polymorphism?

Answer 6

• We use superclass variables to hold instances of subclasses -> issue using the instance as their actual type.
  eg declare ListItem x, then put CharAtom into x, later we want to work with this directly => need downcasting into a CharAtom explicitly.

Question 7

How to safely do downcasting?

Answer 7

Add our own error checking method
Animal* aPet; aPet = new Dog;
Dog* d = dynamic_cast<Dog*>(aPet);
if (d == nullptr) {
cout &laquo_space;“aPet was not a Dog!” &laquo_space;endl;
}