C++ Classes How to Create and Use Them

1. C++ ClassesHow to Create and Use Them

2. Overview • Terminology • File Topology • Designing Classes • Functions in Classes (methods) • Constructor • Accessors/Modifiers • Miscellaneous • The Driver and Object instantiation

3. Terminology • Now in OO land (Object-Oriented) – Real C++ programming! • A class is just a combination of data and functions • It creates a new data type! • A class is a generic description • Create several instances of this class (objects) • Each instance has their own variables (they don’t share) • Functions are now called methods or behaviors (and rarely member functions)

4. File Topology(separate files for everything) Cat.cpp Dog.cpp Description of a cat Description of a dog Driver.cpp This is where we create instances of dog and cats and then tell them what to do. It’s the controlling algorithm Person.cpp Description of a person

5. A couple of rules • Classes usually begin with an upper-case letter • The name of the file that you put the class in should be called the • <class name>.cpp • Especially if it’s a public class (later) • Example: • If we are creating a dog, we should name the class Dog and put it into a file named Dog.cpp • Remember, C++ is case-sensitive!

6. The Smallest Class • Has template: class <class name> { }; • Example: you’re told to create class X • Solution: class X { }; • Remember, this should go in a separate file named X.cpp (not x.cpp)

7. A “real life” example • The CDog • Attributes (characteristics) • rabid or not rabid (bool) • weight (int or float) • name (char [ ]) • Behaviors • growl • eat

8. Step 1: The Skeleton • class CDog { • // attributes will go here – name, weight, rabid • // behaviors will go here – growl, eat • };

9. Step 2: The attributes(we’ll discuss public later) • class CDog { • public: • boolean rabid; • int weight; • char name[255]; • // Behaviors go here • };

10. Step 3: The Constructor • This is a special function • Used to give initial values to ALL attributes • Is activated when someone creates a new instance of the class • The name of this function MUST be the same name as the class

11. Step 3: Designing the Constructor • Constructors will vary, depending on design • Ask questions: • Are all CDogs born either rabid or non-rabid? (yes – they are all born non-rabid) • Are all CDogs born with the same weight? (no – they are born with different weights) • Are all CDogs born with the same name? (no – they all have different names) • If ever “no”, then you need information passed in as parameters.

12. Step 3: The Constructor • class CDog { • public: • boolean rabidOrNot; • int weight; • char name [255]; • // Constructor • CDog::CDog (int x, String y) { • rabid = false; • weight = x; • strcpy (name, y); • } • // Behaviors go here • }; Notice that every CDog we create will be born non-rabid. The weight and name will depend on the values of the parameters

13. The Driver • We’re talking about a whole new file! • The Driver contains main() • It’s the controlling algorithm • Steps: • Type in the skeleton • Create a couple of instances of classes • Start telling them what to do

14. Step 1: Type in the Skeleton #include <iostream.h> void main ( ) { // Create your instances here! }

15. Step 2: Declare A CDog Pointer #include <iostream.h> void main ( ) { CDog* c1; } • // null means “dead” Memory null c1

16. The new operator • new • Brings instances “to life” • Calls the class’ constructor • Opens up enough space in memory to fit an instance of that class • Format: <class>* <var_name>; <var_name> = new <class> (<parameters>);

17. Step 3: Bring c1 to Life #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); } Memory null c1 • new calls the CDog’s constructor • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • }

18. Opens Space #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); } Memory null c1 • new calls the CDog’s constructor • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • }

19. Data Passing #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); } Memory null c1 // This is over in CDog.cpp • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • }

20. Data Passing #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); } Memory null c1 // This is over in CDog.cpp • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • } rabid:false weight:14 name:”bob”

21. Declare a 2nd CDog #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); CDog c2 (7, “Ethel”); } Memory null c1 rabid:false // This is over in CDog.cpp • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • } weight:14 name:”bob”

22. Opens Space #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); CDog c2 (7, “Ethel”); } Memory null c1 c2 // This is over in CDog.cpp • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • } rabid:false weight:14 name:”bob”

23. Data Passing #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); CDog c2 (7, “Ethel”); } Memory null c1 c2 // This is over in CDog.cpp rabid:false • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • } weight:14 name:”bob”

24. Copy Values to Memory #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); CDog c2 (7, “Ethel”); } Memory null c1 c2 // This is over in CDog.cpp rabid:false rabid:false • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • } weight:14 weight:7 name:”bob” name:”Ethel”

25. Back to CDog • class CDog { • public: • boolean rabidOrNot; • int weight; • char name [255]; • // Constructor • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • } • // Behaviors we still need to eat and growl • };

26. Miscellaneous Methods • Follow the pattern void CDog::eat ( ) { cout << name << “ is now eating” << endl; weight++; } void CDog::growl ( ) { cout << “Grrrr” << endl; }

27. Add Methods • class CDog { • public: • boolean rabidOrNot; • int weight; • char name [255]; • // Constructor • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy (name, y); • } • void CDog::eat ( ) { cout << name << “ is now eating” << endl; weight++; } void CDog::growl ( ) { cout << “Grrrr” << endl; } • };

28. The “.” and “->” operators • “Dot” operator used for non-pointers to: • Get to an instances attributes • Get to an instances methods • Basically get inside the instance • Format: <instance>.<attribute or method> • Arrow operator used for pointers • Format: <instance> -> <attribute or method>

29. Using the “.” and “->” Operators #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); CDog c2 (7, “Ethel”); c2.bark( ); c1->growl( ); }

30. Accessors and Modifiers • Accessors • A method that returns the value of an attribute • Will have non-void return type • Has return statement inside • Modifiers • A method that changes the value of an attribute • Change is based off of a parameter value • Has a void return type • Have one accessor and one modifier per attribute

31. Accessors and Modifiers • Accessor for the rabid attribute bool CDog::getRabid ( ) { return rabid; } • Modifier for the rabid attribute void CDog::setRabid (bool myBoolean) { rabid = myBoolean; } • Put these inside of the CDog class

32. Using accessors and modifiers #include <iostream.h> void main ( ) { CDog* c1; c1 = new CDog (14, “Bob”); CDog c2 (7, “Ethel”); c1->setRabid (1); // prints 1 for true cout << c1->getRabid( ) << endl; }

33. Make a Separate Header File(for the generic description) class CDog { public: int weight; bool rabid; char name [ ]; CDog (int x, char y[ ]); bool getRabid ( ); void setRabid (bool x); char [ ] getName ( ); void setName (char z[ ]); int getWeight ( ); void setWeight (int x); void bark( ); void growl( ); };

34. Our Final CDog.cpp • #include <iostream.h> • #include <CDog.h> • // Constructor • CDog::CDog (int x, char y[ ]) { • rabid = false; • weight = x; • strcpy(name, y); • } • void CDog::eat ( ) { cout << name << “ is eating”; • } • void CDog::growl ( ) { • cout << “Grrrr”; • } Cdog.cpp Cdog.h bool CDog::getRabid ( ) { return rabid; } void CDog::setRabid (bool x) { rabid = x; } int CDog::getWeight ( ) { return weight; } void CDog::setWeight (int y) { weight = y; } char[ ] CDog::getName ( ) { return name; } void setName (char z[ ]) { name = z; }

35. Summary of Class Concepts • A class is a generic description which may have many instances • When creating classes • Sketch the skeleton • Fill in the attributes • Make the constructor • Make the accessors/modifiers/miscellaneous • Classes go in separate files • The “.” and “->” operators tell the instances which method to run

36. Inheritance and Objects

37. Inheritance • Inheritance allows one class to take on the properties of another • Superclass-subclass relationship • Sometimes called parent-child relationship • Use the : <class name> to express this relationship • Subclass will “inherit” certain attributes and methods • Benefit: good design, reuse of code

38. Class Hierarchy Mammal int weight giveBirth( ) Land-Mammal int numLegs Question: how many attributes does Dog have? Dog boolean rabid Chihuahua SheepDog

39. Things to Note • Land-Mammal is a superclass to Dog, but a subclass to Mammal • Dog has three attributes • weight, numLegs and rabid • Two from inheritance, one it declared itself

40. An Example(CDog.h – just review) #include <iostream.h> #include <string.h> class CDog { public: int weight; char name [255]; bool rabid; CDog(int x); void growl( ); };

41. CDog.cpp(still review) • #include "CDog.h" • CDog::CDog(int x) { • weight = x; • rabid = false; • strcpy (name, "bob"); • cout << "A CDog was made" << endl; • } • void CDog::growl( ) { • cout << “Grrrr" << endl; • }

42. Poodle: The evil subclass • Since a poodle is a type of CDog, how can we make class Poodle without re-writing a lot of code? • Does a poodle necessarily growl the same way a CDog growls?

43. Class Poodle(both header and cpp file) #include "CDog.h“ class CPoodle:public CDog { public: CPoodle(int x); }; CPoodle::CPoodle(int x) : CDog (x){ cout << "A poodle was made" << endl; } Here’s where we inherit from CDog

44. What’s going on here? CPoodle::CPoodle(int x) : CDog (x){ cout << "A poodle was made" << endl; } The super class’s constructoris always called*; in this case, we call the constructor which takes an int. Normal Constructor Stuff *Note: if you don’t explicitly call the super constructor, it looks fora no-arguments constructor in the super class (which we don’t have)

45. What just happened? • We created a poodle that has all the attributes and all the methods that a CDog has • Problem: CPoodle* myPoodle = new CPoodle (3); myPoodle->growl ( ); // prints GRRRR • Poodles don’t go GRRRR, they “yip”

46. Overriding methods • When you override a method, you give new functionality to the class • In this case, we need to override the growl method for the poodle • Simply redefine the method

47. A Better Poodle(if there is such a thing) #include "CDog.h" class CPoodle:public CDog { public: CPoodle(int x); void growl(); }; CPoodle::CPoodle(int x) : CDog (x){ cout << "A poodle was made" << endl; } void CPoodle::growl( ) { cout << "Yip!" << endl; } Now when we tell the poodle to growl, it will print “Yip”

48. Access Keywords: public, private(and protected) • We can restrict access to attributes and methods inside a class using public and private • public – means “everyone can see it” • private – means “only that class can see it”

49. Bad Example(won’t compile) class X { private: int myInt; // Constructor… } void main ( ) { X* myX = new X ( ); myX->myInt = 42; // Not Allowed! }

50. Working Example class X { public: int myInt; // Constructor… } void main ( ) { X* myX = new X ( ); myX->myInt = 42; // Allowed! }