1. Classes and Data Abstraction
Chapter 15

2. What a Class ! ! Private and public elements
Specification and implementation
Classes we have already seen
Declaring classes data and functions
What is a constructor?

3. Abstract Data Types
Defn => A type whose properties (domain and operations) are specified independent of any particular implementation
Domain => what are possible values for the type
Operations => what things can be done to/with the values in the domain

4. Data Type set of values allowable operations (domain) on those values
FOR EXAMPLE, data type int has
domain
operations
+, -, *, /, %, >>, <<

5. Example of Abstract Data Type
Type: complex numbers
Domain: numbers of the form a + bi
Operations
add, subtract
multiply
divide
(4 + 2i) + (6 - 3i)
(7 - 2i) * (5 + 4)

6. Categories of Operations
Constructor => create new instance (variable) of Abstract Data Type (ADT)
Transformer => builds a new value of the ADT, given one or more previous values of the type
complex c1, c2; // constructor initializes a + bi
complex c1, c2; c1.assign (3, -4); // stores new values in c1

7. Categories of Operations
Observer => Allows a look at the state of an instance of an ADT without changing it.
Iterator => Enables processing (one at a time) of all the components of an instance of an ADT
complex c1, c2; cout << c1.real_part();
complex c1, c2; c1.double();

8. C++ Classes Data and Operations bound into a single unit
Like a struct which includes functions to manipulate the data
class Complex { public : void assign (float a, float b); void print_complex (); float real_part (); private: float a, b; } ;

9. Classes, Objects, Members
Class => a structured type in a programming language
used to represent an abstract data type
Class Member => Component of a class
functions
data
class Complex { public : void assign (float a, float b); void print_complex (); float real_part (); private: float a, b; } ;

10. Classes, Objects, Members
Class Object <=> class instance
variable of a class type
Client => Software that declares and manipulates objects of a particular class:
complex c1, c2; cout << c1.real_part();

11. Built In Class Operations
Programmer defined classes can be like built in types
Declare as many objects of a class as you like
Pass as parameters to functions
Return as function values
void do_whatever (Complex z);
Complex new_value ( … );

12. Built In Class Operations
Arrays of class objects
Can be automatic or static
Assign operator and dot . operator both work
complex c1, c2; Complex c_list [20];
complex c1, c2; c1 = c2; cout << c1.real_part();

13. Class Operations
Recall that built in types (int, float, etc.) have some operators * / == <= <<
Classes do NOT come with these available by default
if you need them, you must program them

14. Class Scope Name of a class member is local to the class.
Same identifier declared outside the class will be unrelated;
complex c1, c2; int a, b, c; c1 = c2; cout << c1.real_part(); // but NOT // cout << real_part

15. Information Hiding Class object has an "invisible wall"
called the abstraction barrier
Protects private data and functions
client code cannot access private elements
client can access only public members
Think of a class as a "black box"
it will act on the data
but you need not know how it works

16. Information Hiding
Class implementation details are hidden from the client’s view. This is called information hiding.
Public functions of a class provide the interface between the client code and the class objects.
client
code
abstraction
barrier
specification
implementation

17. Information Hiding Encapsulation Hiding of implementation details
Keeps client/user of the ADT from…
depending on details
incorrectly manipulating the details

18. Specification File File which describes the behavior of the data type
does not reference the implementation details
This is the .h file with the prototypes, the declarations
Both the client and implementation file will have #include <xxxx.h>
Example

19. Implementation File Contains all the function definitions
includes function heading
includes body of function
Similar to function definitions below main ( )
except is in different file
Function headings in this file must match prototypes in .h file
Example

20. 2 Separate Files Generally Used for class Type
// SPECIFICATION FILE ( timetype .h )
// Specifies the data and function members.
class TimeType {
public:
private:
} ;
// IMPLEMENTATION FILE ( timetype.cpp )
// Implements the TimeType member functions.

21. Familiar Class Instances and Function Members
The member selection operator ( . ) selects either data members or function members.
Header files iostream and fstream declare the istream, ostream,and ifstream, ofstream I/O classes.
Both cin and cout are class objects
Both get and ignore are function members.
cin.get (someChar) ;
cin.ignore (100, ‘\n’) ;

22. Familiar Class Instances and Function Members
Declare myInfile as an instance of class ifstream.
Invoke function member open.
ifstream myInfile ;
myInfile.open ( “A:\\mydata.dat” ) ;

23. Class constructors Guarantee initialization of class object
Constructor function has same name as the class
Can have different versions of constructor
Complex::Complex ( ) { a = 0; b = 0; Complex::Complex (int real_part, int Imag_part) { a = real_part; b = imag_part; }

24. Invoking a Constructor
Invoked automatically whenever a class object is created (instantiated)
Example:
Which version gets used, depends on the number of parameters
Complex c1, c2; Complex c3 (5.3, -6);

25. Guidelines for Constructors
A constructor canNOT return a value
no return value type is declared
Multiple constructors are allowed
compiler chooses appropriate one
according to number & data types of parameters
Parameters passed to a constructor
place actual parameter list after name of class object being declared

26. Guidelines for Constructors
If class object declared WITHOUT a parameter list
results depend on what constructors are provided
even if NONE provided, there is a default constructor which allocates memory for private data elements
When array of class objects declared, default constructor is invoked

27. Testing and Debugging Hints
Don't forget semicolon ; at end of class type declaration
Function declaration in .h file (specification) end with semicolons
Function definitions in .cpp file (implementation) do NOT end with ;
In implementation, don't forget to prefix function name with
name of class
double colon ::

28. Testing and Debugging Hints
Only built in operations (we know about) are
the dot . for member selection
the assignment =
Functions which inspect (but do not modify) should be const member functions