bài giảng chú giải trình hướng đối tượng bài 2

 It is a function used to initialize the data of an object of a class ◦ Same name as class itself ◦ Cannot return anything, not even void ◦ A class may define more than one constructor

UNIT-II

 It is a function used to initialize the data

of an object of a class

◦ Same name as class itself

◦ Cannot return anything, not even void

◦ A class may define more than one constructor

 With different parameter lists

 Default constructor has no parameters

 Called automatically

◦ When class object is declared as automatic variable

◦ By new operator

 It is a function used to clean up an

object of a class prior to deleting that object

 Class name preceeded by '~'

Example

Cont…

Dynamically created objects

Cont…

When do we make copies of an object?

CMSC 202, Version 3/02

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1) When passing them to a function by value 2) When returning them from a function by value

3) When creating a new object that is initialized with a copy of an existing object 4) When assigning objects (x = y)

1.Items 1, 2 and 3 are handled by the

copy constructor.

2.Item 4 is handled by overloading the

assignment ( = ) operator.

What is a copy constructor?

Why haven’t we seen this

before?

CMSC 202, Version 3/02

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 The compiler provides a default copy

constructor which up until now has been

sufficient

 The default copy constructor simply copies each of the data members from the

existing object into the new object

 This is not sufficient if one or more of the

data members points to dynamically

allocated memory

Copy Constructor

 A copy constructor is called whenever a new

variable is created from an object

 There are 3 important places where a copy

 When an object is returned from a function

 Copy constructor is a constructor function with the same name as the class.

 It is used to make deep copy of objects

  

Copy constructor syntax

 The copy constructor takes a reference to a const parameter

 It is const to guarantee that the copy constructor doesn't

change it, and it is a reference because a value parameter would require making a copy, which would invoke the copy constructor, which would make a copy of its parameter,

which would invoke the copy constructor.

 By default, the compiler provides a copy

constructor that performs a member-by-member copy from the original object to the one being

created.

 This is called a member wise, or shallow copy.

 Overloading Assigment ( = ) Operators work

almost exactly like Copy Constructors, with a few subtle differences

 They need to check for self assignment

 They return a reference to *this

 Depending on your code, they may be different (more optimized) than your copy constructor

Virtual Functions

Allow print in both employee and manager, with different

definitions C++ will “do the right thing”, based on the actual object class.

Virtual Functions (cont’d)

Understanding the Benefits

of Overloading

 Having more than one function with the

same name is beneficial because you can

use one easy-to-understand function name without paying attention to the data types involved

 Polymorphism allows the same operation to

be carried out differently, depending on the object

 Some reserve the term polymorphism (or pure polymorphism) for situations in which one function body is used with a variety of arguments

Using the + Operator

Polymorphically

 Separate actions can result from what seems

to be the same operation or command

 The + operator has a variety of meanings,

which include:

 Alone before a value (called unary form), + indicates

a positive values, as in the expression +7

 Between two integers (called binary form), +

indicates integer addition, as in the expression 5+ 9

 Between two floating-point numbers (also called

binary form), + indicates floating-point addition, as

in the expression 6.4 + 2.1

Overloading Operators—

The Rules

 Operator overloading is the process by which you apply operators to your own abstract data types

 The +, -, *, and / symbols make it easy to work with built-in data types such as int and double

 Classes, however, contain a variety of data

members

 As a result, if you want the compiler to perform arithmetic with two class objects, you must tell the compiler what you mean

 Good programming style dictates that you

endow the operator with a reasonable meaning

Overloading Operators—

The Rules

 You overload an operator by making it a function;

subsequently, you can use it just like any other function

 C++ operators are classified as unary or binary, depending

on whether they take one or two arguments, respectively

Binary Operators that Can

Be Overloaded

Overloading Operators— The Rules

 You also cannot

change the normal

precedence of any

operator

Overloading Operators— The Rules

Overloading Output

 The << operator also is overloaded by C++

 It is both a bitwise left-shift operator and

an output operator; it is called the insertion operator when used for output

 The << operator acts as an output operator only when cout (or another output stream object) appears on the left side

 When you use cout in a program, you must include #include<iostream.h>

 The preceding function, called

operator<<(), returns a reference to

ostream

Overloading Output

ostream (locally named out in this

example) and an integer (locally named in

in this example)

with the built-in data types; you also may overload the << operator to work with

your own classes

with a Sale object, you must add the

overloaded operator <<() function to the Sale class

Overloading Output

 The operator <<() function is a friend to the class of the object it wants to print out, e.g Sale here.

Overloading Input

output, it makes sense that the >>

operator also can be overloaded for input

such as >> is that the resulting programs look cleaner and are easier to read

operator>>() function, that uses istream (which is defined in iostream.h, along with ostream) by using a prototype as follows:

friend istream& operator>>(istream &in, Sale

&Sale);

Overloaded Operator>>() Function for the Sale Class

Overloading Input

Ex8-6

Overloading ++ and

and - - to decrement variables

an expression, the mathematical operation

takes place before the expression is evaluated

expression is evaluated before the

mathematical operation takes place

Inventory class, you can write the statement that increases numSold in several different

ways

Using the Prefix and Postfix ++ Operators with an Integer

The Inventory Class

Using Postfix Increment and

Decrement Operators

postfix ++ operator as well as a prefix ++ operator with a class

must supply different argument lists; for the postfix ++ operator, you use an

integer argument

function prototype is:

Inventory& operator++(int); Ex8-8

Overloading the = = Operator

be an easy task

equality in class members

choose whether equivalency means that every data field must be equivalent, or

only specific data members

either an integer or a boolean variable

representing true or false

8

Overloading the = = Operator

 A variable of type bool can hold one of two values: true or false

 Some older C++ compilers do not support the bool type; with those compilers you would use the first version of operator = =() that returns an integer

EX8-9

Overloading the = Operator

 The = operator can be overloaded for use

with your own classes

 Unlike other operators, if you don’t define the = operator, C++ provides a definition for you

 If you want the = operator to do something other than assign each member, then you

must create a customer operator=()function

 In addition, if the class contains data fields that are pointers, you should create a

custom function

EX8-9

Overloading [ ] and ( )

is declared like any other function, but called in a manner similar to

accessing an array element

want within an operator [ ] function

 Typically, you use this function to

perform a task that both requires an argument and does not quite fit into another operator’s usual meaning

Understanding the Benefits

of Overloading

 Having more than one function with the

same name is beneficial because you can

use one easy-to-understand function name without paying attention to the data types involved

 Polymorphism allows the same operation to

be carried out differently, depending on the object

 Some reserve the term polymorphism (or pure polymorphism) for situations in which one function body is used with a variety of arguments

Using the + Operator

Polymorphically

 Separate actions can result from what seems

to be the same operation or command

 The + operator has a variety of meanings,

which include:

 Alone before a value (called unary form), + indicates

a positive values, as in the expression +7

 Between two integers (called binary form), +

indicates integer addition, as in the expression 5+ 9

 Between two floating-point numbers (also called

binary form), + indicates floating-point addition, as

in the expression 6.4 + 2.1

Overloading Operators—

The Rules

 Operator overloading is the process by which you apply operators to your own abstract data types

 The +, -, *, and / symbols make it easy to work with built-in data types such as int and double

 Classes, however, contain a variety of data

members

 As a result, if you want the compiler to perform arithmetic with two class objects, you must tell the compiler what you mean

 Good programming style dictates that you

endow the operator with a reasonable meaning

Overloading Operators—

The Rules

 You overload an operator by making it a function;

subsequently, you can use it just like any other function

 C++ operators are classified as unary or binary, depending

on whether they take one or two arguments, respectively

Binary Operators that Can

Be Overloaded

Overloading Operators— The Rules

 You also cannot

change the normal

precedence of any

operator

Overloading Operators— The Rules

Type conversion

A reference conversion can be

performed wherever a reference initialization occurs, including reference initialization done

in argument passing and function return

values A reference to a class can be

converted to a reference to an accessible base class of that class as long as the conversion is not ambiguous The result of the conversion is

a reference to the base class subobject of the derived class object.

 Reference conversion is allowed if the

corresponding pointer conversion is allowed.

Arithmetic conversions and promotions

 Integral conversions

 Boolean conversion

 Floating point conversion

 Integral and floating point promotions

Pointer Conversion

 Qualification conversions

An type-qualified rvalue of any

type, containing zero or more const or

volatile qualifications, can be converted to

an rvalue of type-qualified type where the second rvalue contains more const or

volatile qualifications than the first rvalue

 An rvalue of type pointer to member of a class can be converted to an rvalue of type pointer to member of a class if the second rvalue contains more const or volatile

qualifications than the first rvalue

 Function argument conversions

When a function is called, if a function declaration

is present and includes declared argument types, the

compiler performs type checking The compiler

compares the data types provided by the calling function with the data types that the called function expects and performs necessary type conversions For example,

when function funct is called, argument f is converted to

a double, and argument c is converted to an int:

 The automatic conversions consist of the following:

1.Integral and floating-point values are promoted 2.Arrays or functions are converted to pointers

3.Non-static class member functions are converted

to pointers to members.

Explicit initialization with

constructors

 A class object with a constructor must be explicitly initialized or have a default constructor Except for aggregate initialization, explicit initialization using a constructor is the only way to

initialize nonstatic constant and reference class members.

 A class object that has no constructors, no virtual functions, no private or protected members, and no base classes is called an aggregate Examples of aggregates are C-style structures and unions.

 You explicitly initialize a class object when you create that object There are two ways to initialize a class object:

 Using a parenthesized expression list The compiler calls the

constructor of the class using this list as the constructor's

argument list

 Using a single initialization value and the = operator Because this type of expression is an initialization, not an assignment, the assignment operator function, if one exists, is not called The

type of the single argument must match the type of the first

argument to the constructor If the constructor has remaining

arguments, these arguments must have default values.

example illustrates explicit initialization

{ re = c.re; im = c.im; } // constructor with default trailing argument complx( double r, double i = 0.0)

{ re = r; im = i; } void display()

{

cout << "re = "<< re << " im = " << im << endl;

};

// initialize with a copy of one

// using complx::complx(const complx&)

complx two = one;

// construct complx(3,4)

// directly into three

complx three = complx(3,4);

// initialize with default constructor complx four; // complx(double, double) and construct

// directly into five