Solution: Polymorphism Trace

Output

15
I got this!
10
When are office hours?
30
I got this!
25
When are office hours?
d3
d2
d1

Why

Class behavior recap

• Question::getValue() is not virtual, so it uses the static type.
• studentResponse() is virtual, so it uses dynamic dispatch.
• Destructors are virtual, so deleting via Question* calls derived destructors.

First loop (Question* p[2])

Static type is Question*, so getValue() calls Question::getValue().

• p[0] = new NonTrivialQuestion(15)
  • val = 15
  • getValue() → 15
  • studentResponse() → “I got this!”
• p[1] = new DifficultQuestion (default Question(10))
  • val = 10
  • getValue() → 10
  • studentResponse() → “When are office hours?”

Second loop (NonTrivialQuestion* q[2])

Static type is NonTrivialQuestion*, so getValue() calls NonTrivialQuestion::getValue().

• q[0] = new NonTrivialQuestion(15)
  • getValue() → 15 + 15 = 30
  • studentResponse() → “I got this!”
• q[1] = new DifficultQuestion
  • getValue() → 15 + 10 = 25
  • studentResponse() → “When are office hours?”

Destructor output

Only delete p[1]; is called. Because the destructor is virtual:

DifficultQuestion::~DifficultQuestion() -> d3
NonTrivialQuestion::~NonTrivialQuestion() -> d2
Question::~Question() -> d1