1. EGR 2261 Unit 13 Classes Read Malik, Chapter 10.
Homework #13 and Lab #13 due next week.
Final Exam next week.
-Handouts: Quiz 12
-Preview final exam.

2. Review: What is a struct?
Recall that a struct is a collection of a fixed number of components (called members), accessed by name.
The members may be of different types.
structs are very useful because they let us design complex data structures that represent real-world objects.
Recall example (on next slide) of houseType.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

3. Review: Example of a struct
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

4. Data Members versus Function Members
In our examples so far, structs have contained data members but not function members.
In other words, the structs in our examples have contained variables, but not functions.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

5. Limitations of structs
In the C language, structs could only contain data members.
C programmers came to see this as a limitation, and realized that it would be nice to be able to define structs that contain function members as well as data members.
This realization, along with other ideas about how to improve structs, led to the development of C++.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

6. Classes and Object-Oriented Programming
The main new idea in C++ is the class, which is a generalization of the struct.
Hand-in-hand with the introduction of classes is a whole new way of thinking about a program—as a collection of interacting objects. This new way of thinking is called object-oriented programming (OOP), in contrast to the procedural programming used in C and other older languages.
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7. Defining a Class
A class definition looks a lot like a struct definition.
Syntax:
Just as with struct definitions, a class definition defines a new data type, but no memory is allocated until we declare a variable of this type.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

8. Defining a Class (cont’d.)
Each class member in classMemberList can be a variable (data member) or a function (function member).
Data members are like the members we’ve used in structs.
For a function member, usually just the function prototype is listed is listed in the class definition. The function definition is given elsewhere in the program.
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9. Defining a Class: Example
Seven function members
Three data members
They'll build upon this class in Lab 13.
Note the new keywords public and private, which are called member access specifiers.

10. A New Idea: Code Within a Class
The ability to create functions inside a class gives rise to a new distinction that we haven’t had to think about until now: some code (such as the code inside setTime) “lives” inside a class while other code (such as the code in a program’s main function) lives outside of any class.

11. Member Access Specifiers
Each class member falls into one of the following categories, as designated by the access specifiers:
public
The member can be accessed by code inside or outside the class.
private (This is the default.)
The member be accessed by code inside the class, but not by code outside the class.
protected
"In between" public and private. Only needed if you use inheritance (discussed in Chapter 11).
Do practice question 1.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

12. Member Access Specifiers: Example
Since hr is a private member of clockType, code in clockType’s function members (such as setTime) can access hr, but code outside of the class (such as in main) cannot.

13. Member Access Specifiers: Example (cont'd.)
On the other hand, since setTime() is a public member, any code can call it.
In this example, all of the function members are public and all of the data members are private. This is fairly common, but it's not always the case.

14. Implementation of Member Functions
As seen above, usually only the prototypes of a class's function members are included in the class definition itself.
Your program must also contain the code for these functions. This code is called the function's implementation.
Note the syntax on the next slide, which uses the scope resolution operator ::
We previously saw the scope resolution operator when we discussed global variables.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

15. Implementation of Member Functions: Example
Class name followed by scope resolution operator
This code is outside of the class definition, so you must precede the function's name with clockType:: to indicate that this is a member function of the clockType class, rather than just a regular function.

16. Separating the Definition from the Implementation
Though not required, in programs that use classes we usually separate the code into the following files:
A header file that holds the class definition.
An implementation file that holds the implementations of the class’s member functions.
A file that holds the client program, which uses the class defined in the other two files. Here is where you’ll put your program’s main() function.
For a program that uses more than one class, files 1 and 2 will be repeated for each class.

17. Separating the Definition from the Implementation: Example

18. Including the Header File in the Other Files
On the previous slide, note that the implementation file (clockType.cpp) and the client program (myClockProgram.cpp) include the header file (clockType.h).
To do this, we use the same #include preprocessor directive that we use for library files, but we enclose the header file’s name in double quotes instead of angle brackets: #include <iostream> #include "clockType.h"
Including a library file
Including our own header file

19. Getting Started with a Program that Uses clockType
Create a new project named clockPractice and start a new source-code file named clockPractice.cpp.
Download the files named clockType.h and clockType.cpp from the website and place them in your project’s folder that contains clockPractice.cpp.
In your project’s Solution Explorer, right-click Source Files and select Add > Existing Item…. Then select clockType.cpp and click Add.
In clockPractice.cpp, type the code shown on the next slide.

20. Getting Started with a Program that Uses clockType (cont’d.)
Run the program.
Modify main to call myClock.incrementSeconds() and then call myClock.printTime() again.

21. Variable (Object) Declaration
As we’ve seen, once a class has been defined and implemented, you can declare variables of that class type:
clockType myClock;
A class variable is called a class object or class instance, or simply an object.
Each object has its own copy of the data members (hr, min, and sec).
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22. Accessing Class Members From Outside
Code outside of the member functions (such as in the main() function of the client program) can access an object's public members, using the syntax:
The dot (.) is the member access operator (same as for structs).
But code outside of the member functions cannot access an object's private members.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

23. Accessing Class Members From Outside: Example
Accessing myClock's public members: okay!
Trying to access myClock's private members: no good!
Demo with week13Class.cpp.
This code is in the client program's main() function, so it is outside of the class's member functions.

24. Accessing Class Members Within the Class Code
Code within a member function of the class can access the object's public and private members directly by name (no dot operator needed).
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

25. Accessing Class Members Within the Class Code: Example
Accessing an object's private members: okay!
Demo with week13Class.cpp.
Do practice question 2 and use week13practiceCircle.cpp to see error messages.
This code is in the member function printTime(), so it can access public and private members.
Note that we don’t need to precede the member name with the object's name.

26. Constructors
A constructor is a special member function that runs automatically when a new class object is created. Its purpose is to initialize the object’s data members.
A constructor’s name must be the same as the name of the class.
A constructor has no type (not even void).
Unlike other functions, constructors cannot be explicitly called in code.
Recall that most variables are uninitialized when you declare them—this can lead to problems. Show that you can call printTime() on a clockType object as soon as you declare it, even if you haven’t called setTime() on it..
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

27. Constructors: Example
In the class’s header file:
In the class’s implementation file:
On previous slides I omitted this constructor to avoid confusion.
-Demo by playing around with the values that hr, min, sec are initialized to.

28. Constructors (cont’d.)
Constructors are functions, but they serve a very special purpose. They run automatically whenever you declare a class object. You do not call them directly in your code using the usual function-call syntax.
This main() code is illegal because it tries to call a constructor.
Constructor is called here.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

29. Constructors (cont’d.)
A class can have more than one constructor.
This is an example of function overloading, which we looked at briefly in Unit 7.
Recall from Unit 7 that each overloaded function must have a different formal parameter list.
Example of overloaded constructor functions:
clockType(); clockType(int hours, int minutes, int seconds);
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

30. Multiple Constructors: Example
In the class’s header file:
In the class’s implementation file:
Demo using week13Constructors.cpp. (But the file they’re using from the website just has the default constructor.)
Do practice question 3.

31. Invoking a Constructor
A constructor is automatically executed when you declare a class object.
Which of the constructors is executed depends on whether you specify parameters in parentheses after the class object’s name when you declare it.
Examples: clockType yourClock; clockType myClock(4, 3, 16);
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32. Invoking the Default Constructor
To invoke the default constructor, don’t specify any parameters after the class object’s name when you declare it:
Example:
clockType yourClock;
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

33. Invoking a Constructor with Parameters
Syntax:
Example: clockType myClock(4, 3, 16);
The number and type of arguments should match the formal parameters (in the order given) of one of the constructors.
Do practice question 4.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

34. Constructors and Default Parameters
A constructor can have default parameters.
The rules for declaring formal parameters are the same as for declaring default formal parameters in any function. (Recall Unit 7.)
Actual parameters are passed according to the same rules used for any function.
Default constructor: a constructor with no parameters or with all default parameters.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

35. Built-in Operations on Classes
As with structs, aggregate arithmetic, relational, and input/output operations are not allowed on class objects, but aggregate assignment is allowed: clockType myClock, yourClock; myClock = yourClock + 1; //Error if (myClock >= yourClock) //Error cout << myClock; //Error cin >> myClock; //Error myClock = yourClock; //Okay!!
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36. Arrays, structs, and Class Objects: A Comparison
On this slide I’ve erased the comment that says “(except strings)” in the Array column of the Input/output row because we skipped C-strings.
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

37. Class Objects and Functions
Objects can be passed as parameters to functions by value or by reference.
If an object is passed by value, a copy must be made of the object’s data members. For large objects, this may require a lot of time and memory, degrading performance.
If an object is passed by reference, no copy is made—may result in better performance.
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38. Class Objects and Functions (cont’d.)
Passing by reference is an efficient way to pass an object as a parameter.
Potential Problem: when passing by reference, the actual parameter is changed if the formal parameter is changed.
Solution: use const in the formal parameter list. This prevents the function from changing the parameter’s value. Example from clockType:
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39. Another Use of const
The previous slide showed how to use const to prevent a function from changing the value of a reference parameter.
Another use of const is to prevent a function from changing the object on which the function is invoked. Example from clockType:
C++ Programming: From Problem Analysis to Program Design, Seventh Edition

40. Two Uses of const: Example
This is an error because of this const.
Demo using week13Const.cpp.
This is an error because of this const.

41. Arrays of Class Objects
Similar to the arrays of structs that we examined at the end of Unit 11 and used in the third program on Lab 11.
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