CPSC 427a: Object-Oriented Programming

Michael J. Fischer

Lecture 13
October 18, 2011

Privacy Revisited (again)

Visibility rules Every class member has one of four privacy attributes: public, protected, private, or hidden.

These attributes determine the locations from which a class member can be seen.

public members can be seen from any location.
protected members can be seen from inside the class or its children.
private members can only be seen from inside the class.
hidden members cannot be seen at all.

Explicit privacy attributes

The privacy attributes for declared class members are given explicitly by the privacy keywords public, protected, and private.

There is no way to explicitly declare a hidden member.

Example:

class A {
private: int x;
protected: int y;
public: int z;
};

Implicit privacy attributes

Inherited class members are assigned implicit privacy attributes based on their attributes in the parent class and by the kind of derivation, whether public, protected, or private.

If the member is public in the parent class, then its attribute in the child class is given by the kind of derivation.
If the member is protected in the parent class, then its attribute in the child class is protected for public and protected derivation, and private for private derivation.
If the member is private or hidden in the parent class, then it is hidden in the child class.

Implicit privacy chart

Below is a revision of the chart presented in lecture 10.

Attribute in base class (
|||
{
|
||(

	Kind of Derivation

	public	protected	private

public	public	protected	private
protected	protected	protected	private
private	hidden	hidden	hidden
hidden	hidden	hidden	hidden

	Attribute in derived class.

Summary

All members of the base class are inherited by the derived class and appear in every instantiation of that class.
All inherited members receive implicitly defined privacy attributes.
Visibility of all data members is determined solely by their privacy attributes.
Public and protected base class variables are always visible within a derived class.
Private and hidden base class variables are never visible in the derived class.
The kind of derivation never affects the visibility of inherited members in the derived class; only their implicit attributes.

Problem Set 2 Code Review

A retrospective look at PS2 Class dependency structure.

class RandBit is a biased random bit generator. It depends on nothing else and can be tested alone.
class Coin is a coin flipper with dependency on the previous coin flip. It uses a RandBit object as its source of randomness to tell it whether to turn over the previous coin or leave it alone.
It should not depend on any other class.
class Experiment runs an experiment on a coin, where an experiment consists of some predetermined number of runs. It clearly needs a Coin but shouldn’t know about the details of how the coin is built.
class Simulator is in charge of controlling the experiments.

Question: Where should the parameter-processing go?

Testing

There should be unit tests for each of the classes. The code contains an incomplete set of unit tests.

(code demo)