Abstract Classes & Object Slicing. Pure Virtual Functions Virtual Function : Child class may...

Abstract Classes & Object Slicing

Pure Virtual Functions

• Virtual Function : Child class may override, if working with pointer/reference, check to see if we have a better version

Pure Virtual Functions

• Pure Virtual Function : Child class MUST override this… I'm not even going to describe how it works

New Problem…

• Can't make a GeometricObject:

Abstract

• Abstract Class : Class that is not designed to be instantiated– Defines common capabilities for subclasses

GeometricObject

+getArea() : double+getPerimeter() : double

Italics = Abstract Class

Italics = Abstract Method (Pure Virtual)

A Promise

• Abstract class is an organizer / promise

Using it

• We have no idea how each object will get its area, but we know it can:

Constructors

• Can't make a Geo object:

• Still need constructors for child classes

Protected Constructors

• Protected constructorsmake sure only usablefrom inside child classes

GeometricObject

+getArea() : double+getPerimeter() : double

# = Protected

Object Slicing

• Assigning subtype to parent type variable slices the object

GeometricObject

Rectangle

r1

Object Slicing

• Assigning subtype to parent type variable slices the object

GeometricObject

Rectangle

r1GeometricObject

r2

Pointers/References

• Pointers / References don't slice– Just store memory address,

don't have to

Casting

• No cast required to store child address in parent ptr:

• Virtual dispatch still findschild version of functions

Down Casting

• Can't auto convert parent pointer back to child one:

BAD static_cast, BAD

• static_cast happily pretends to turn a pointer into another pointer type…

…weird stuff may happen

Static vs Dynamic

• Static : done at compile timeCan this possibly make sense?

• Dynamic : done at runtimeTry to see if this works…

Dynamic Cast

• dynamic_cast<Type>( value )– Type must be a pointer or reference– At runtime:• Check to see if conversion makes sense• If so, do it• If not, return nullptr

• dynamic_cast allows us to check if cast worked:

GOOD dynamic_cast

• dynamic_cast allows us to check if cast worked– Up to you to check!

GOOD dynamic_cast