1. Recitation Course 0520
Speaker: Liu Yu-Jiun

2. Reference: C++ How to Program 6e
Destructors
A special member function
Name is the tilde character (~) followed by the class name, e.g., ~Time
Called implicitly when an object is destroyed
For example, this occurs as an automatic object is destroyed when program execution leaves the scope in which that object was instantiated
Does not actually release the object’s memory
It performs termination housekeeping
Then the system reclaims the object’s memory
So the memory may be reused to hold new objects
波浪符號表示解構子與建構子是相對的
離開物件的使用域時，系統會自動呼叫物件的解構子。不代表釋放了物件的記憶體空間，要到結束之後，系統才會回收物件佔用的空間，給其他物件使用。
Reference: C++ How to Program 6e

3. Reference: C++ How to Program 6e
9.7 Destructors (Cont.)
Receives no parameters and returns no value
May not specify a return type—not even void
A class may have only one destructor
Destructor overloading is not allowed
If the programmer does not explicitly provide a destructor, the compiler creates an “empty” destructor
解構子沒有參數也沒有傳回值，而且一個類別只能有一個解構子，也不能overloading。跟建構子相同，使用者沒寫解構子，系統也會預設一個空的。
Reference: C++ How to Program 6e

4. 9.8 When Constructors and Destructors Are Called
Called implicitly by the compiler
Order of these function calls depends on the order in which execution enters and leaves the scopes where the objects are instantiated
Generally,
Destructor calls are made in the reverse order of the corresponding constructor calls
However,
Storage classes of objects can alter the order in which destructors are called
物件的儲存類別會影響解構子的呼叫順序
Reference: C++ How to Program 6e

5. 9.8 When Constructors and Destructors Are Called (Cont.)
For an automatic local object
Constructor is called when that object is defined
Corresponding destructor is called when execution leaves the object’s scope
For automatic objects
Constructors and destructors are called each time execution enters and leaves the scope of the object
Automatic object destructors are not called if the program terminates with an exit or abort function
Reference: C++ How to Program 6e

6. 9.8 When Constructors and Destructors Are Called (Cont.)
For a static local object
Constructor is called only once
When execution first reaches where the object is defined
Destructor is called when main terminates or the program calls function exit
Destructor is not called if the program terminates with a call to function abort
Global and static objects are destroyed in the reverse order of their creation
Reference: C++ How to Program 6e

7. Prototype for destructor
Reference: C++ How to Program 6e

8. Defining the class’s destructor
Reference: C++ How to Program 6e

9. Reference: C++ How to Program 6e
#include <iostream>
#include "CreagAndDestroy.h"
using namespace std;
void create(void);
CreateAndDestroy first(1, "(global before main)");
int main(){
cout << "\nMAIN FUNCTION: EXECUTION BEGINS" << endl;
CreateAndDestroy second(2, "(local automatic in main)");
static CreateAndDestroy third(3, "(local static in main)");
create();
cout << "\nMAIN FUNCTION: EXECTION RESUMES" << endl;
CreateAndDestroy fourth(4, "(local automatic in main)");
cout << "\nMAIN FUNCTION: EXECUTION ENDS" << endl;
return 0; }
void create(void){
cout << "\nCREATE FUNCTION: EXECUTION BEGINS" << endl;
CreateAndDestroy fifth(5, "(local automatic in create)");
static CreateAndDestroy sixth(6, "(local static in create)");
CreateAndDestroy seventh(7, "(local automatic in create)");
cout << "\nCREATE FUNCTION: EXECUTION ENDS" << endl;
Reference: C++ How to Program 6e

10. Reference: C++ How to Program 6e

11. 9.8 When Constructors and Destructors Are Called (Cont.)
For objects defined in global scope
Constructors are called before any other function (including main) in that file begins execution
The corresponding destructors are called when main terminates
Function exit
Forces a program to terminate immediately
Does not execute the destructors of automatic objects
Often used to terminate a program when an error is detected
Function abort
Performs similarly to function exit
But forces the program to terminate immediately without allowing the destructors of any objects to be called
Usually used to indicate an abnormal termination of the program
全域物件的建構子呼叫時間甚至比main function還早。
Main function終止的時候會呼叫相對應的解構子。
Exit:不會呼叫自動物件的解構子，程式發生執行錯誤時，利用exit function來終止程式。
Abort:不會呼叫任何物件的解構子，程式不正常結束執行。
Reference: C++ How to Program 6e

12. Reference: C++ How to Program 6e
9.9 Time Class Case Study: A Subtle Trap—Returning a Reference to a private Data Member
Returning a reference to an object
Alias for the name of an object
An acceptable lvalue that can receive a value
May be used on the left side of an assignment statement
If a function returns a const reference
That reference cannot be used as a modifiable lvalue
One (dangerous) way to use this capability
A public member function of a class returns a reference to a private data member of that class
Client code could alter private data
Same problem would occur if a pointer to private data were returned
Reference: C++ How to Program 6e

13. Reference: C++ How to Program 6e
#include <iostream>
using namespace std;
class Time{
public:
Time(int = 0 , int = 0, int = 0);
void setTime(int, int, int);
int getHour();
int &badSetHour(int);
private:
int hour;
int minute;
int second; };
Time::Time(int hr, int min, int sec){
setTime(hr, min, sec); }
void Time::setTime(int h, int m, int s){
hour = (h >= 0 && h < 24) ? h : 0;
minute = (m >= 0 && m < 60) ? m : 0;
second = (s >= 0 && s < 60) ? s : 0;
int Time::getHour(){
return hour;
Figure 9.14~9.15
Reference: C++ How to Program 6e

14. Reference: C++ How to Program 6e
int &Time::badSetHour(int hh){
hour = (hh >= 0 && hh < 24) ? hh : 0;
cout << "the address of hour is " << &hour << endl;
return hour; }
int main(){
Time t;
int &hourRef = t.badSetHour(20);
cout << "hourRef's content: " << hourRef << endl;
cout << "hourRef's address: " << &hourRef << endl;
hourRef = 30;
cout << "\nInvalid hour after modification: " << t.getHour() << endl;
t.badSetHour(12) = 74;
cout << t.getHour() << endl;
system("pause");
return 0;
Figure 9.15~9.16
hourRef
hour 74 12 20 30
Reference: C++ How to Program 6e

15. Result of Previous Code
the address of hour is 0x23ff60
the content of hourRef: 20
the address of hourRef: 0x23ff60
Invalid hour after modification: 30
Another invalid hour seeting: 74
Reference: C++ How to Program 6e

16. 9.10 Default Memberwise Assignment
Assignment operator (=)
Can be used to assign an object to another object of the same type
Each data member of the right object is assigned to the same data member in the left object
Can cause serious problems when data members contain pointers to dynamically allocated memory
Reference: C++ How to Program 6e

17. Reference: C++ How to Program 6e
Outline
Date.h
(1 of 1)
Reference: C++ How to Program 6e

18. Reference: C++ How to Program 6e
Outline
Date.cpp
(1 of 1)
Reference: C++ How to Program 6e

19. Outline
fig09_19.cpp
(1 of 1)
Memberwise assignment assigns data members of date1 to date2
date2 now stores the same date as date1
Reference: C++ How to Program 6e

20. 9.10 Default Memberwise Assignment (Cont.)
Copy constructor
Enables pass-by-value for objects
Used to copy original object’s values into new object to be passed to a function or returned from a function
Compiler provides a default copy constructor
Copies each member of the original object into the corresponding member of the new object (i.e., memberwise assignment)
Also can cause serious problems when data members contain pointers to dynamically allocated memory
Reference: C++ How to Program 6e

21. Chap 10

22. const object and const member function
object declaring
const class_name object_name;
Function prototype
return_type function_name ( p1, p2, ...) ;
const 3 1 2
const
const
void help (const char * const p1);
Reference: C++ How to Program 6e

23. const object and const member function (cont.)
const member functions
const objects are only allowed to call const member function
Member functions declared const are not allowed to modify the object
A function is specified as const both in its prototype and in its definition
const declarations are not allowed for constructors and destructors
Reference: C++ How to Program 6e

24. const object and const member function (cont.)
const and reference data member不能使用賦值運算子(=)，所以設定const資料成員的初始值得用成員初始值列表(member initializer list)。
Time::Time(int h, int m, int s)
: hour(h),
minute(m),
second(s) {}
const成員函式不可以更動任何成員資料的值。所以，如果成員函式不會更動資料成員的值，可以將成員函式設定為const。
const成員函式只能呼叫其它const成員函式。
Reference data member的話，得給一個變數名稱。
Reference: C++ How to Program 6e

25. 10.3 Composition: Objects as Members of Classes
Sometimes referred to as a has-a relationship
A class can have objects of other classes as members
Example
class 機翼{
public:
機翼(int, int);
void 轉彎();
int 修正飛行姿勢(int);
private:
int 寬度;
int 長度; };
class 飛機{
public:
飛機(機翼, 機翼, int);
private:
機翼 左機翼;
機翼 右機翼;
int 載客數; };
int main(){
機翼 big(3, 5);
飛機 747(big, big, 600); }
三次的機翼建構子呼叫，第一次是呼叫通用建構子，第二、三次是呼叫預設複製建構子；反之，也有三次的機翼解構子呼叫。
Reference: C++ How to Program 6e

26. Reference: C++ How to Program 6e
Constructor
Difference between default and general constructor
Default
Time (int = 0, int = 0, int = 0);
Time () {hour = 0, minute = 0, second = 0}
General
Time (int, int, int);
Reference: C++ How to Program 6e

27. 10.4 friend Functions and friend Classes
friend function of a class
Defined outside that class’s scope
Not a member function of that class
Yet has the right to access the non-public (and public) members of that class
Standalone functions or entire classes may be declared to be friends of a class
Reference: C++ How to Program 6e

28. 10.4 friend Functions and friend Classes (Cont.)
To declare a function as a friend of a class:
Provide the function prototype in the class definition preceded by keyword friend
To declare a class as a friend of a class:
Place a declaration of the form
friend class ClassTwo; in the definition of class ClassOne
All member functions of class ClassTwo are friends of class ClassOne
所有在classTwo的成員函式都是classOne的夥伴函式
Reference: C++ How to Program 6e

29. 10.4 friend Functions and friend Classes (Cont.)
Friendship is granted, not taken
For class B to be a friend of class A, class A must explicitly declare that class B is its friend
Friendship relation is neither symmetric nor transitive
If class A is a friend of class B, and class B is a friend of class C, you cannot infer that class B is a friend of class A, that class C is a friend of class B, or that class A is a friend of class C
It is possible to specify overloaded functions as friends of a class
Each overloaded function intended to be a friend must be explicitly declared as a friend of the class
Reference: C++ How to Program 6e

30. Outline
fig10_15.cpp
(1 of 2)
friend function declaration (can appear anywhere in the class)
Reference: C++ How to Program 6e

31. Outline
fig10_15.cpp
(2 of 2)
friend function can modify Count’s private data
Calling a friend function; note that we pass the Count object to the function
Reference: C++ How to Program 6e

32. Reference: C++ How to Program 6e
Using the this Pointer
Outline
fig10_17.cpp
(1 of 2)
Reference: C++ How to Program 6e

33. Outline
fig10_17.cpp
(2 of 2)
Implicitly using the this pointer to access member x
Explicitly using the this pointer to access member x
Using the dereferenced this pointer and the dot operator
Reference: C++ How to Program 6e

34. Cascaded member-function calls
Outline
Time.h
(1 of 2)
set functions return Time & to enable cascading
Reference: C++ How to Program 6e

35. Reference: C++ How to Program 6e
Outline
Time.h
(2 of 2)
Reference: C++ How to Program 6e

36. Outline
Time.cpp
(1 of 3)
Returning dereferenced this pointer enables cascading
Reference: C++ How to Program 6e

37. Reference: C++ How to Program 6e
Outline
Time.cpp
(2 of 3)
Reference: C++ How to Program 6e

38. Reference: C++ How to Program 6e
Outline
Time.cpp
(3 of 3)
Reference: C++ How to Program 6e

39. Reference: C++ How to Program 6e
Outline
fig10_20.cpp
(1 of 2)
Cascaded function calls using the reference returned by one function call to invoke the next
((t.setHour(18)).setMinute(30)).setSecond(22);
((t).setMinute(30)).setSecond(22);
((t)).setSecond(22);
Note that these calls must appear in the order shown, because printStandard does not return a reference to t
Reference: C++ How to Program 6e

40. Reference: C++ How to Program 6e
Outline
fig10_20.cpp
(2 of 2)
Reference: C++ How to Program 6e