Post on 22-Dec-2015
transcript
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Classes and Data Abstraction
Chapter 15
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What a Class ! !
Specificationand
implementation
Privateand
public elements
Declaring classesdata andfunctions
What is aconstructor?
Classes wehave already
seen
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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
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Data Type
set of values(domain)
allowable operationson those values
FOR EXAMPLE, data type int has
domain
-32768 . . . 32767
operations
+, -, *, /, %, >>, <<
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Example of Abstract Data Type
Type:complex numbers
Domain:numbers of the form a + bi
Operations– add, subtract– multiply– divide
(4 + 2i) + (6 - 3i)(4 + 2i) + (6 - 3i)
(7 - 2i) * (5 + 4)(7 - 2i) * (5 + 4)
i
i
5
)23(
i
i
5
)23(
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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;// constructor initializes a + bi
complex c1, c2;c1.assign (3, -4);// stores new values in c1
complex c1, c2;c1.assign (3, -4);// stores new values in c1
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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;cout << c1.real_part();
complex c1, c2;c1.double();
complex c1, c2;c1.double();
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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; } ;
class Complex{ public : void assign (float a, float b); void print_complex (); float real_part (); . . . private: float a, b; } ;
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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; } ;
class Complex{ public : void assign (float a, float b); void print_complex (); float real_part (); . . . private: float a, b; } ;
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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();
complex c1, c2;cout << c1.real_part();
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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);void do_whatever (Complex z);
Complex new_value ( … );Complex new_value ( … );
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Built In Class Operations
Arrays of class objects
Can be automatic or staticAssign operator and dot . operator
both work
complex c1, c2;Complex c_list [20];
complex c1, c2;Complex c_list [20];
complex c1, c2;. . .c1 = c2;cout << c1.real_part();
complex c1, c2;. . .c1 = c2;cout << c1.real_part();
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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
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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
complex c1, c2;int a, b, c;. . .c1 = c2;cout << c1.real_part();// but NOT // cout << real_part
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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
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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.
clientcode
specification
abstraction barrierimplementation
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Information Hiding
EncapsulationHiding of implementation detailsKeeps client/user of the ADT from…
– depending on details– incorrectly manipulating the details
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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> ExampleExample
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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
ExampleExample
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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. . . .
21Familiar 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’) ;cin.get (someChar) ;cin.ignore (100, ‘\n’) ;
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Familiar Class Instances and Function MembersDeclare myInfile as an instance of
class ifstream.
Invoke function member open.
ifstream myInfile ;myInfile.open ( “A:\\mydata.dat” ) ;
ifstream myInfile ;myInfile.open ( “A:\\mydata.dat” ) ;
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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; }
Complex::Complex ( ) { a = 0; b = 0;Complex::Complex (int real_part, int Imag_part) { a = real_part; b = imag_part; }
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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);Complex c1, c2;Complex c3 (5.3, -6);
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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
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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
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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 ::
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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