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Date: Mon, 11 Nov 1996 17:52:37 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Fri, 08 Nov 1996 17:29:43 GMTContent-length: 10301<HTML><HEAD><TITLE>Classes</TITLE></HEAD><BODY><H3>Classes</H3><P>A class is an extension of a struct. I will introduce classes slowlyby extending the functionality of structs, which we studied last week.<P>One of the most apparent differences between a class and a structis that, in addition to having "data members" like a struct, a classcan have "member functions" or "methods". We can think about thesemember functions as being operations that all objects that belong tothe class can do. For example, imagine that we wanted to augment ourAlien struct that we declared last week so that every Alien is capableof giving a greeting. We would declare our class as so:<PRE WIDTH=80><BLOCKQUOTE><CODE>const int MAXSTR = 20;class Alien{ public: char name[MAXSTR]; char homePlanet[MAXSTR]; int numLegs; int isDeadly; void greet(char who[]);};</CODE></BLOCKQUOTE></PRE>Notice that we have added an additional member, called<EM>greet</EM> to the class Alien. Unlike the other members, thismember is not declared to be a variable or array. This member is aprototype for a function which takes an array of characters andproduces no return value. We have also added the word public which wewill talk about later.<P>Now, a prototype is only <EM>half</EM> of a function. We need togive a function definition. It could look like this:<PRE WIDTH=80><BLOCKQUOTE><CODE>void Alien::greet (char who[]){ cout << "Hello " << who << ", my name is " << name << " and I am from " << homePlanet << ".\nLet's be friends for ever!\n";}</CODE></BLOCKQUOTE></PRE>This looks a lot like a normal function definition. We have a returntype (void), we have a parameter list (char who[]) and we have somecode making up the function body. What is different is the functionname <EM>Alien::greet</EM>. In our class declaration, our prototypehas only the name <EM>greet</EM>. What's going on here? Well, it ispossible that more than one class might have a method (memberfunction) called greet. For example, there might be a Robot class andit might have a greet method too. In order to specify <EM>which</EM>greet method we are providing the body for, we need to give the classname. That is why our function is called <EM>Alien::greet</EM>. Youcan read this name as meaning "the greet method for the Alien class."The :: thing is called the "scope resolution operator." This isbecause it is used to specify that its second operand (in this case"greet") is the one specified in the "scope" of its first operand (inthis case "Alien"). The :: is used whenever we want to grab a part ofa class. We will see additional uses for the :: operator later.<P>Some of you might be saying, hold on one second, we are using thevariables "name" and "homePlanet" in the greet method without thembeing declared in the method body. This is ok. We will see what thismeans soon.<P>We have defined the greet method for the Alien class. Now wejust need to call it. Suppose we have a couple of Aliens.<PRE WIDTH=80><BLOCKQUOTE><CODE>// this appears somewhere, maybe in main...Alien yoda;Alien darth;</CODE></BLOCKQUOTE></PRE>We have not yet talked about initializing objects. For the timebeing, assume that yoda has name "Yoda" and home planet "Dagobah" anddarth has name "Lord Vader" and home planet "the Death Star". We cancall the greet functions for both by the code:<PRE WIDTH=80><BLOCKQUOTE><CODE>...// this appears somewhere, maybe in main...yoda.greet("Dad");darth.greet("Mom");...</CODE></BLOCKQUOTE></PRE>We call a method for a particular object (variable) by using "the dotoperator". We have done this before with file streams (remembercin.getline() and fin.getline()?). We have also seen this as a way ofaccessing the data members of structs. Basically, the dot operator isused whenever we want to grab a piece of an object (contrast withscope resolution operator). The first line says 'call the greetmethod for the yoda object with the argument "Dad"' the second linesays 'call the greet method for the darth object with the argument"Mom"'. You can also think of these lines as being like messages tothe objects. We first tell yoda to greet Dad and then we tell darthto greet Mom. The reason that we call variables like yoda and darth"objects" is that they are, in an abstract sense, entities that canreceive messages and interact with each other.<P>When we make the calls, the variables "name" and "homePlanet" willcorrespond to whichever object makes the call. When yoda is asked togreet Dad he will be from Dagobah. When darth is asked to greet Momhe will be from the Death Star. The above two calls generate thescreen output:<PRE WIDTH=80><BLOCKQUOTE><CODE>Hello Dad, my name is Yoda and I am from Dagobah.Let's be friends for ever!Hello Mom, my name is Lord Vader and I am from the Death Star.Let's be friends for ever!</CODE></BLOCKQUOTE></PRE><P>Exercise: extend the class declaration to include a method calledmutate which takes an integer and returns nothing. Write a methoddefinition for mutate which adds its integer argument to the aliensnumber of legs, "mutating" it. It should accept negative arguments.At no time should an Alien's numLegs drop below zero.(<!WA0><!WA0><!WA0><A HREF="http://www.cs.wisc.edu/~turnidge/cs302/classes-ex1.html">Solution</A>).<H3>Constructors</H3><P><EM>Below this point these notes are written for my lecture. Theyare very terse and will likely not make sense. I am leaving them herebecause it doesn't make sense to remove them:</EM><P>First example:<PRE WIDTH=80><BLOCKQUOTE><CODE>class Alien{ public: char name[MAXSTR]; char homePlanet[MAXSTR]; int numLegs; int isDeadly; Alien(char nm[], char home[], int legs, int deadly); void greet(char who[]); void mutate(int moreLegs);};Alien::Alien (char nm[], char home[], int legs, int deadly){ strcpy(name, nm); strcpy(homePlanet, home); numLegs = legs; isDeadly = deadly;}</CODE></BLOCKQUOTE></PRE><P>Overloaded:<PRE WIDTH=80><BLOCKQUOTE><CODE>class Alien{ public: char name[MAXSTR]; char homePlanet[MAXSTR]; int numLegs; int isDeadly; Alien(); Alien(char nm[], char home[], int legs, int deadly); void greet(char who[]); void mutate(int moreLegs);};Alien::Alien (){ //set to reasonable defaults name[0] = '\0'; homePlanet[0] = '\0'; numLegs = 0; isDeadly = 0;}Alien::Alien (char nm[], char home[], int legs, int deadly){ strcpy(name, nm); strcpy(homePlanet, home); numLegs = legs; isDeadly = deadly;}</CODE></BLOCKQUOTE></PRE>Point: why don't we call strcpy (name, "") instead?<P>Two constructors. Talk about how we decide which to call (parameters,types, of args). Typical overloaded function. Show example:<PRE WIDTH=80><BLOCKQUOTE><CODE> //in main or some other function... Alien Generra; // No init values, calls lame-o constructor. Alien Gamera ("Gamera", "Tokyo 7", 2, 1);</CODE></BLOCKQUOTE></PRE>Points to ponder: Why doesn't Generra have the ()'s? What wouldhappen if it did?<P>No args constructor is called default constructor. Good idea tohave one because you probably will want it. (E.g. when you make anarray of classes!)<P>You can use = to assign one whole class to another:<PRE WIDTH=80><BLOCKQUOTE><CODE>Generra = Gamera;</CODE></BLOCKQUOTE></PRE>You can also call the constructor explicitly to make an anaonymousvalue of a certain type and assign it:<PRE WIDTH=80><BLOCKQUOTE><CODE>Generra = Alien("ET", "Home", 2, 0);</CODE></BLOCKQUOTE></PRE><H3>Alien::</H3><P>Example where I use it to resolve name clash in constructor:<PRE WIDTH=80><BLOCKQUOTE><CODE>Alien::Alien (char name[], char homePlanet[], int numLegs, int isDeadly){ strcpy(Alien::name, name); strcpy(Alien::homePlanet, homePlanet); Alien::numLegs = numLegs; Alien::isDeadly = isDeadly;}</CODE></BLOCKQUOTE></PRE><P>Constant member: <PRE WIDTH=80><BLOCKQUOTE><CODE>class Alien{ public: // ... const int killer = 1; const int lamb = 0; // ...};</CODE></BLOCKQUOTE></PRE>Can use the killer and lamb constants in our Alien methods. Can alsouse them in calls:<PRE WIDTH=80><BLOCKQUOTE><CODE> Alien Yoda("Yoda", "Dagobah", 2, Alien::killer);</CODE></BLOCKQUOTE></PRE><P>This way we can hide the "magic number". Point: why can't we do thiswith non-constants?<H3>Friday's Class</H3><H2>Public v. Private</H2><P>Talk about the separation of implementation and interface. Itwould help to have an example of two different ways of doing the samething. Preferably something I have shown before.<P>Define impl and intf<P>Idea: provide a common interface that hides the details ofimplementation. This way we can replace code later.<P>Can also avoid things like: yoda.numLegs = -2; //semantic error.<P>Tell them how. Tell them what it means. Give some decls. Showaccessors/constructors. <PRE WIDTH=80><BLOCKQUOTE><CODE>void Alien::Read (){ char YorN; cout << "Enter the Alien's Name > "; cin >> ws; cin.getline (name, 20); cout << "Enter Home World > "; cin.getline (homePlanet, 20); cout << "Enter Number of Legs > "; cin >> numLegs; cout << "Is " << lifeForm.name << " deadly? (y/n) > "; cin >> YorN; isDeadly = (toupper(YorN) == 'Y');}</CODE></BLOCKQUOTE></PRE>Contrast with:<PRE WIDTH=80><BLOCKQUOTE><CODE>void ReadInAlien (Alien &lifeForm){ char YorN; cout << "Enter the Alien's Name > "; cin >> ws; cin.getline (lifeForm.name, 20); cout << "Enter Home World > "; cin.getline (lifeForm.homePlanet, 20); cout << "Enter Number of Legs > "; cin >> lifeForm.numLegs; cout << "Is " << lifeForm.name << " deadly? (y/n) > "; cin >> YorN; lifeForm.isDeadly = (toupper(YorN) == 'Y');}</CODE></BLOCKQUOTE></PRE><H3>Accessor Functions</H3>Tips: Make all member variables private, define accessor functions,use the assignment operator.<H3>Reworks</H3><P>Skyline. Alien catalog. Hangman.<P>(not covered yet: The "this" object)<HR>Last modified: Fri Nov 8 11:29:42 1996 by Todd Turnidge<ADDRESS> <!WA1><!WA1><!WA1><A HREF="http://www.cs.wisc.edu/~turnidge/turnidge.html">turnidge@cs.wisc.edu</A></ADDRESS></BODY></HTML>⌨️ 快捷键说明
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