(Previous): C++ Classes [ 05.html ]

Preparation

Study this page and Chapter 8 but not 8.6 (pointers to member functions) and write down the questions, doubts, and surprises that you have on a piece of paper. Include your printed name!

Assigned Work Due

Hand in your questions, doubts and surprises.

Project 2 is due in.

You can resubmit Project 1 if the previous grade was B or less.

Topics

const Objects and member functions

1. In UML mark the operation with the stereotype "<<query>>" or (Dia) "const"

   this

   Friend functions and classes

2. In UML mark the friend functions with stereotype "<<friend>>"

   Do it yourself operators

   Binary operators

   Member functions

   Assignment

   Friend functions

   Comparison operators and <utility>

   Unary operators

   Simple

   Prefix

   Postfix

   Indexing[]

   Function call()

   Type Conversion operations

   Classwide or static members

TBA

Demo a class with const, friend, operators

Complete a given simple efficient 8 character buffer class for secure input/output.

Previously you met a very insecure 8 character buffer in [ lab01.html ] because it allowed stupid programmers to do stupid things ... like over-running the end of the buffer.

One way to fix this is to hide the dangerous data in a class and provide just those functions that do what is needed and can't be abused.

UML:

Here is the test program: [ testBuffy.cpp ]

Here is the "Header file": [ Buffy.h ]

Demo Splitting a header file from a body file and using make

The above was done is the simplest way -- very amateurish. Now to redo it using professional techniques: we separate the detailed code of the functions from the header file and precompile them. Then we use a program called make to assemble the program for use.

[ testBuff2.cpp ] [ Buff2.h ] [ Buff2.cpp ] [ Makefile ] [ Make in resources ]

Glossary

1. stereotype::=`a way to attach a specialized meaning to something in the UML by using guillemots or French quotation marks".

2. guillemots::ASCII= "<<" | ">>", a good UML tool will insert the real symbols for you.

   Questions

   Can you explain type_id and dynamic_cast

   We will ignore type_id in CS202 except as a way of recognizing exceptions in meeting 09 and lab/04. It turns out that our C has a pretty weird conception of what the name of a type is. Try [ type_ids.cpp ] to see how it works.

   We will do a little dynamic_cases later.

   How does & work in function headers

   It is placed before the name of a parameter to set up call by reference. Without it the actual parameter is copied into the function as the initial value of the parameter -- and never comes out again!

   Call by reference means that the address of the variable in the call is passed to the function and all actions done to the parameter actually are done to the object. This is mainly used to allow data to flow in and out of the function through a parameter.

   Notice it is faster to use call be reference (&) with objects -- the object will have more bytes in it than an address. Copying bytes takes time.

   Also -- we used to get the same effect by using '*' and '&' but this needs a great deal of knowledge and care to work without bugs.

   When to use friends

   Any class can declare friend classes and friend functions.

   Use them as little as you can. They expose the object to abuse by the friends.

   The input and output functions (operator<< and operator>>) are the main functions that you MUST define as friends in a class in the course.

   Can you point to a data member in an object

   If the data is public you can. But it is wiser not too.

   What is * this

   When you apply a function to an object:

    		object.example(data)

   then, invisibly, the program does this

    		this = & object;

   before the function starts to execute. Thus the function knows where the object is.... and can use the information in several ways.

   A common way is to tell another object about 'this' object:

    			user_interface.addButtonListener(this);

   which means that when the user clicks a button 'this' will be called with a description of the event.

   There are other uses for this to be covered later. Most of these use

    		*this

   to refer to the current object -- say to send a message to itself!

   Can you have many classes in one program or file

   Yes. AN lots of functions can be used in one program.

   Examples of static data members

   First -- static members can change. A static variable can change its value at any time. The word "static" in programming is overloaded and abused. Variables that don't vary are said to be "const".

   Static data belongs to its class, not to the individual objects. It is shared by all the objects. Its value doesn't change when you change objects.

   Use it when the data is about the whole class not about one instance/object of the class.

   The Wikipedia [ Static_variable#For_class_variables ] has a couple of examples -- one in C# and a simpler one in C++ They define a class of Requests. Each Request is for a particular URL (place on the web), but the class, itself maintains a count of created Requests. This would be used to keep track of how many outstanding requests for web pages have been made, and whether there is a problem or not.

   Other examples is a class recording the enrollment in a section of a course. The size of the enrollment is a shard "static" number, each enrollment is for a specific student who is enrolled in a particular section.

   In fact, most academic examples of static class members are of counters!

   What is a binary operator

   A binary operator is something like "+" and "/" which is written between two operands:

    		1 + 2

   How to create random letters

    		int(rand()*26)+'a'

   When does const really matter

   when it stops a program changing my wages.

   Seriously: const protects data for abuse -- use it to control bugs.

   Exercises

   Based on chapter 7 and the Input above. Examples: take UML and code in C++, take given C++ and draw UML, ...

   Lab

   Same as Monday: [ lab03.html ] on Dia and the UML.

. . . . . . . . . ( end of section CSci202 Computer Science II, Session 06, More about C++ classes) <<Contents | End>>
(Next): Deriving classes and Inheritance [ 07.html ]

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