1. Classes: Part 2 

2. Static vs. Dynamic Variables
Generally speaking: ‘static’ is meant to be ‘non-dynamic’
Global and local variables
Locals are automatic variables from the ‘stack’ of the memory, therefore called ‘static’
Dynamic variables are from the ‘heap’ of the memory.
In C and C++, ‘static’ is well defined:
‘static’ variables are ‘global’ with the file scope 2

3. Static Variables in C and C++
Static variables are put somewhere ‘permanently’ or ‘globally’ in memory
The static variable ‘s’ can only be accessed within the function but it is not deleted with the function
A ‘local’ static variable is a ‘global’ variable for the function, not the others.
int f(){
static int s = 0;
return s++; }
int main() {
cout << f() << endl;
Output: 1 2
Don’t do it unless you have a good reason! 3

4. Static Class Members
like a kind of global variables but they have class scope (outside the class, they cannot be accessed)
A variable that is part of a class, yet not part of an object of that class, is called a static member.
There is exactly one copy of a static member per class, instead of one copy per object, as for non-static members.
Only one copy of a variable shared by all objects of a class
“Class-wide” information
A property of the class shared by all instances, not a property of a specific object of the class
A function that needs to access to members of a class, yet does not need to be invoked for a particular object, is called a static member function. 4

5. Static Class Members Can be declared public, private or protected
Primitive (Fundamental-type) static data members
Initialized by default to 0
If you want a different initial value, a static data member can be initialized once (and only once)
A const static data member of int or enum type can be initialized in its declaration in the class definition
Alternatively, you can also initialize it in file scope
All other static data members must be defined at file scope (i.e., outside the body of the class definition)
static data members of class types (i.e., static member objects) that have default constructors need not be initialized because their default constructors will be called 5

6. Static Class Members
To access a public static class member when no objects of the class exist:
Prefix the class name and the binary scope resolution operator (::) to the name of the data member
Employee::count
Also accessible through any object of that class
Use the object’s name, the dot operator and the name of the member
Employee_object.count
static member function
Is a service of the class, not of the object of the class
Example: SEmployee.h, SEmployee.cpp, static.cpp
Private ‘static’ members still can only be accessed by member functions! 6

7. Static Member functions
Declare a member function static
It cannot access non-static data members or non-static member functions of the class (because the object may not exist when the function is called)
A static member function does not have a this pointer
static data members and static member functions exist independently of any objects of a class, i.e., when a static member function is called, there might not be any objects of its class in memory 7

8. Static member function may be called even the object does not exist.
SEmployee.h
Employee has a static function and a static data member
class Employee {
public:
Employee(const char *const, const char *const);
~Employee();
const char* getFirstName() const;
const char* getLastName() const;
static int getCount();
private:
char* firstName;
char* lastName;
static int count; // number of objects instantiated };
Static data member keeps track of number of Employee objects that currently exist;
Static member function may be called even the object does not exist. 8

9. Even static count is private!
SEmployee.cpp (1/3)
// define and initialize static data member at file scope
int Employee::count = 0; // cannot include keyword static
int Employee::getCount() {
return count; }
Even static count is private!
static data member is defined and initialized at file scope in the .cpp file
static member function can access only static data, because the function might be called when no objects exists 9

10. SEmployee.cpp (2/3)
Non-static member function (e.g., constructor) can modify the class’s static data members
Employee::Employee(const char *const first, const char *const last) {
firstName = new char[ strlen( first ) + 1 ];
strcpy(firstName, first);
lastName = new char[ strlen( last ) + 1 ];
strcpy( lastName, last );
count++;
cout << "Employee constructor for " << firstName
<< ' ' << lastName << " called." << endl; } 10

11. SEmployee.cpp (3/3) Remember to deallocate memory reserved for arrays
Employee::~Employee() {
cout << "~Employee() called for " << firstName
<< ' ' << lastName << endl;
delete[] firstName;
delete[] lastName;
count--; } 11

12. static.cpp (1/2)
Calling static member function using class name and binary scope resolution operator
Calling a static member function through a pointer to an object returns the value of the static variable
Same as getting the value of Employee::count or calling Employee::getCount()
cout << "Number of employees before instantiation of any
objects is " << Employee::getCount() << endl;
Employee* e1Ptr = new Employee( "Susan", "Baker" );
Employee* e2Ptr = new Employee( "Robert", "Jones" );
cout << "Number of employees after objects are instantiated is “
<< e1Ptr->getCount(); 12

13. static.cpp (2/2)
Even when no object exists, we can still call static member function getCount()
cout << "\n\nEmployee 1: " << e1Ptr->getFirstName() << " "
<< e1Ptr->getLastName() << "\nEmployee 2: " <<
e2Ptr->getFirstName() << " " << e2Ptr->getLastName() << "\n\n";
delete e1Ptr;
e1Ptr = 0; // e1Ptr = NULL;
delete e2Ptr;
e2Ptr = 0;
cout << "Number of employees after objects are deleted is "
<< Employee::getCount() << endl; 13

14. static.cpp Sample Output
Number of employees before instantiation of any objects is 0
Employee constructor for Susan Baker called.
Employee constructor for Robert Jones called.
Number of employees after objects are instantiated is 2 (same as calling Employee::getCount() = 2)
Employee 1: Susan Baker
Employee 2: Robert Jones
~Employee() called for Susan Baker
~Employee() called for Robert Jones
Number of employees after objects are deleted is 0 14

15. Constant Static Variable
#include <iostream>
using namespace std;
class F {
public:
static int getcount();
// static member function cannot have `const' method qualifier
private:
const static int count; };
// initialization of constant static variable: must be here; not in main()
const int F::count = 2;
int F::getcount() {
cout << count; }
int main() {
F::getcount(); // print out 2
cout << F::count; // wrong as 'const int F::count' is private
return 0; 15

16. Constructors with Member Initializers

17. How to initialize a ‘const’ private member?
class Increment {
public:
Increment(int c=0, int i=1);
void addIncrement(){
count += increment; }
void print() const; // prints count and increment
private:
int count;
const int increment; // const data member };
const data member increment must be initialized using a member initializer 17

18. Colon (:) marks the start of a member initializer list
Increment::Increment(int c, int i)
: count(c), // initializer for non-const member
increment(i) // required initializer for const member {
// empty body }
Colon (:) marks the start of a member initializer list
c is the initial count, increment is the increment step
Member initializer for non-const member count
Required member initializer for const member increment
Not providing a member initializer for a const data member is a compilation error
See Increment.h, Increment.cpp and const2.cpp 18

19. Constructors with Member Initializer
X::X(parameter_list) : member_initializer_list { // body of constructor definition }
Required for initializing
‘reference’ data members
‘const’ data members
Member initializer list
Appears between a constructor’s parameter list and the left brace that begins the constructor’s body
Separated from the parameter list with a colon (:)
Each member initializer consists of the data member name followed by parentheses containing the member’s construction and its initial value
Multiple member initializers are separated by commas
Executes before the body of the constructor executes 19

20. Examples of member initializationOK OK
class F {
public:
F() : i(j), m(3), k(m), j(4) {
cout << i << j << k << m << endl; }
private:
const int& i;
const int j; // ANSI C++ cannot have const int j = 4;
int& k;
int m; // ANSI C++ cannot have int m = 3; };
class F {
public:
F() : i(j), k(m), j(4) {
m=3;
cout << i << j << k << m << endl; }
private:
const int& i;
const int j;
int& k;
int m; };
4433
class F {
public:
F() : i(j), k(m) {
m=3;
j = 4; // compiler complains: assignment of read-only member `F::j'
cout << i << j << k << m << endl; }
private:
const int& i;
const int j;
int& k;
int m; };
NOT OK 20

21. Example: Time Class Information hiding (Time.h and Time.cpp)
Two types of constructors
class Time {
public:
Time();
Time(unsigned initHours, unsigned initMinutes, char initAMPM);
void set(unsigned hours, unsigned minutes, char am_pm);
void display(ostream& out) const;
...
private:
unsigned myHours, myMinutes;
char myAMorPM; // 'A' or 'P'
unsigned myMilTime; // military time equivalent }; 21

22. Default Constructor Time mealTime = Time();
Time::Time() : myHours(12), myMinutes(0), myAMorPM('A'), myMilTime(0) {
// void }
mealTime
myHours 12
myMinutes
myAMorPM A
myMilTime
Time mealTime = Time(); 22

23. Explicit-Value Constructor
Time::Time(unsigned initHours, unsigned initMinutes, char initAMPM) {
set(initHours, initMinutes, initAMPM); //a member function }
bedTime
myHours 11
myMinutes 30
myAMorPM P
myMilTime
2330
Time bedTime = Time(11,30,’P’);
(a random value if myMilTime is not set in set()) 23 23

24. Constructors with Default Arguments
Constructors can specify default arguments
Can initialize data members to a consistent state
Even if no values are provided in a constructor call
Constructor that defaults all its arguments is also a default constructor
Can be invoked with no arguments
Maximum of one default constructor per class 24

25. Possible to specify default values for constructor arguments
Time(unsigned initHours = 12, unsigned initMinutes = 0, char initAMPM = 'A');
Time t1, t2(5), t3(6,30), t4(8,15,'P'); t1
myHours 12
myMinutes
myAMorPM A
myMilTime t2
myHours 5
myMinutes
myAMorPM A
myMilTime
500 t3
myHours 6
myMinutes 30
myAMorPM A
myMilTime
630 t4
myHours 8
myMinutes 15
myAMorPM P
myMilTime
2015 25

26. Same as: Time t(bedTime); and calls ‘copy constructor’.
Copy Operations
During initialization
Time t = bedTime;
During assignment t = midnight; t
myHours 11
myMinutes 30
myAMorPM P
myMilTime
2330
bedTime
myHours 11
myMinutes 30
myAMorPM P
myMilTime
2330
Same as: Time t(bedTime); and calls ‘copy constructor’. t
myHours 12
myMinutes
myAMorPM A
myMilTime
midnight
myHours 12
myMinutes
myAMorPM A
myMilTime
‘assignment’, by default, memberwise copy of the left into the right object. 26

27. Other Class Operations
Accessors: "get" functions
unsigned Time::getMinutes() const {
return myMinutes; }
unsigned Time::getHours() const {
return myHours; }
unsigned Time::getAMPM() const {
return myAMorPM;
unsigned Time::getMilTime() const {
return myMilTime; 27

28. Mutators: "set" functions
void Time::set(unsigned hours, unsigned minutes, char am_pm) { // Check class invariant
if (hours >= 1 && hours <= 12 &&
minutes >= 0 && minutes <= 59 &&
(am_pm == 'A' || am_pm == 'P')) {
myHours = hours;
myMinutes = minutes;
myAMorPM = am_pm;
myMilTime = toMilitary(hours, minutes, am_pm); }
else
cerr << "*** Can't set time with these values
***\n"; } 28

29. Display and Operator Overloading: do it later …29

30. Explicit constructors (not explicit-value constructor 

31. Initialization T t = v; // assignment syntax
Where v is of type V, the t is initilized by calling the constructor T::T(V v).
T t(v); // explicit syntax
If v is not of type T, convert v to a temporary T object temp_t;
Initialize t using the copy constructor T::T(const T&) with temp_t as argument.
In most cases, the compiler may optimize such that the effect of the two is the same. This is the case, e.g. if there exisit a (non-explicity) constructor T::T(V), where v has type V. 31

32. Avoid implicit type conversion
class IntCell {
public:
explicit IntCell(int initialValue = 0)
: storedValue(initialValue) {}
int read( ) const
{return storedValue;}
void write(int x)
{storedValue = x;}
private:
int storedValue; }
An explicit constructor will be invoked only explicitly.
Where a copy constructor is needed, an explicit constructor will not be implicitly invoked. 32

33. Try to implicitly call IntCell (x), which is ‘explicit’!
main(){
int x = 4; // same as int x(4);
IntCell z(5); // now 5 (call explicit-value constructor)
IntCell t; // now 0 (call default constructor)
IntCell u = IntCell(x); // now 4 (call explicit-value constructor,
// then copy constructor)
IntCell y = x; // invalid implicit conversion: y = IntCell(x) … }
Try to implicitly call IntCell (x), which is ‘explicit’!
If no ‘explicit’ keywork, IntCell y = x; is OK. 33

34. Objects as class members

35. Objects as Members of Classes
A class can have objects of other classes as members
Sometimes referred to as a ‘has-a’ relationship
Example: Date.h, Date.cpp, Employee.h, Employee.cpp and composition.cpp
class Employee {
public:
Employee(const char *const,
const char *const,
const Date&,
const Date&);
~Employee();
void print() const;
private:
char firstName[25];
char lastName[25];
const Date birthDate;
const Date hireDate; };
const T (const only modifies a type T), so it does not see the ‘*’ if you write const T*, therefore const char* p is a constant object.
T *const is a constant pointer.
Const T *const is both constant object and pointer
const char *const: see ‘pointer’ slides. 35

36. Employee’s constructor
Initializing member objects
Member initializers pass arguments from the object’s constructor to member-object constructors
Before the enclosing class object (host object) is constructed
If a member initializer is not provided, the member object’s default constructor will be called implicitly
Employee::Employee(const char *const first,
const char *const last,
const Date& dateOfBirth,
const Date& dateOfHire)
: birthDate(dateOfBirth),
hireDate(dateOfHire) {
... }
Member initializers pass arguments to Date’s implicit copy constructor (equivalent to const Date birthDate = dateOfBirth;)
A compilation error occurs if a const member object is not initialized with a member initializer in the constructor 36

37. Object reference and self-reference: the this pointer

38. Pointers to Class Objects
Possible to declare pointers to class objects
Access with or
Call delete to free the memory
Time* timePtr = &t;
Time* timePtr = new Time(12, 0, ‘A’, 0);
timePtr t
myHours 12
myMinutes
myAMorPM A
myMilTime
timePtr->getMilTime()
(*timePtr).getMilTime()
delete timePtr; // call destructor 38

39. A Member Function Returning a Reference
We can have a member function which returns a reference.
For example, if a member function returns an integer reference, there are 4 possibilities.
int& f();
This is for non-constant objects. It returns an integer reference and hence can be subsequently changed.
E.g., for a non-constant object ncfoo, we can call ncfoo.f() = 10; or i = ncfoo.f();
const int& f();
This is for non-constant objects. It has to be a rvalue.
i = ncfoo.f(); // good
ncfoo.f() = 10; // wrong: compilation error
const int& f() const;
This is for both constant and non-constant objects (constant object can call it only). It returns a constant reference and hence can only be rvalue.
i = cfoo.f(); // good; or i= ncfoo.f();
cfoo.f() = 10; //wrong; and nor ncfoo.f() = 10;
int& f() const;
This returns a reference which can be a lvalue. However, because it can be called by a constant object (which should never be a lvalue), this should not be used.
Therefore, you can have
Either first or second for non-constant objects depending on what you want on the return value; and
The third one for constant objects
The compiler will make the call depending on whether the object is constant or not. 39

40. The this Pointer Every class has a keyword, this
a pointer whose value is the address of the object
Value of *this would be the object itself
Class Object
Function members
*this
Data members
this 40

41. Using the this Pointer when you want to return the modified object:
F& F::f(){
// …
return *this; }
Every object has access to its own address through a pointer called this (a C++ keyword)
Objects use the this pointer implicitly or explicitly
Implicitly when accessing members directly
Explicitly when using keyword this
Type of the this pointer (i.e., whether it can be modified or not) depends on the type of the object and whether the executing member function is declared const 41

42. Members of the class, not that of the objects
Summary: static and non-static members
A a,b,c,d, … ;
a.l
b.l
c.l
d.l (multiple copies)
a.g = b.g = c.g = d.g = … = A::g (one copy)
class A { …
int l;
static int g; }
A::g
Members of the class, not that of the objects
member data, non-static, so far belong to ‘objects’ or ‘instances’ of the class
member data, static, are ‘variables’ of the class, not the objects.

43. General form of ‘constructors’
X::X(…) : …
{ … }
Have to use ‘member initializers’ for
‘reference’ data members
‘const’ data members
class A { …
int& r;
const int c; } 43

44. Separate compilation

45. Interface and Implementation
In C++ it is more common to separate the class interface from its implementation.
Abstract data type
The interface lists the class and its members (data and functions).
The implementation provides implementations of the functions.
What to do?
How to do? 45

46. Interface
Describes what services a class’s clients can use and how to request those services
But does not reveal how the class carries out the services
A class definition that lists only member function names, return types and parameter types
Function prototypes
A class’s interface consists of the class’s public member functions (services) 46

47. Separate File for Reusability
.cpp source-code files
.h header files
Separate files in which class definitions are placed
Allow compiler to recognize the classes when used elsewhere
Generally have .h filename extensions
Driver files
Program used to test software (such as classes)
Contains a main function so it can be executed
See GradeBook4.h and Gradebook4.cpp 47

48. #include preprocessor directive
#include "GradeBook.h"
include header files
Instructs C++ preprocessor to replace directive with a copy of the contents of the specified file
Quotes for user-defined header files
Preprocessor first looks in current directory
If the file is not found, looks in C++ Standard Library directory
Angle brackets for C++ Standard Library
Preprocessor looks only in C++ Standard Library directory
#include <iostream> 48

49. The interface is typically placed in a file that ends with .h.
IntCell::IntCell(int initialValue)
: storedValue(initialValue)
{ }
int IntCell::read( ) const
{return storedValue;}
void IntCell::write(int x)
{storedValue = x;}
class IntCell {
public:
explicit IntCell(int initialValue = 0 );
int read( ) const;
void write( int x );
private:
int storedValue; }
IntCell.h
IntCell.cpp
The interface is typically placed in a file that ends with .h.
The implementation file typically ends with .cpp, .cc, or .C. 49

50. Separate Class Interface from Implementation
Client code should not break if the implementation changes, as long as the interface stays the same
Define member functions outside the class definition, in a separate source-code file
In source-code file for a class
Use binary scope resolution operator (::) to “tie” each member function to the class definition
Implementation details are hidden
Client code does not need to know the implementation
In the header file for a class
Function prototypes describe the class’s public interface 50

51. Class Libraries Class declarations placed in header file
Given .h extension
Contains data items and prototypes
Implementation file
Same prefix name as header file
Given .cpp extension
Programs which use this class library called client programs 51

52. Reality: the separation is not perfect
Header files do contain some portions of the implementation and hint about others
private members are listed in the class definition in the header file 52

53. Typical C++ Development Environment
Edit
Programmer writes program (and stores source code on disk)
Preprocess
Perform certain manipulations and file I/O before compilation
Compile
Compiler translates C++ programs into machine languages in object codes
Link
Link object codes with missing functions and data
Load
Transfer executable image to memory
Execute
Execute the program one instruction at a time 53

54. The Compilation and Linking Process
Source-code file is compiled to create the class’s object code (source-code file must #include header file)
Class implementation programmer only needs to provide header file and object code to client
Client must #include header file in their own code
So compiler can ensure that the main function creates and manipulates objects of the class correctly 54

55. Translating a Library Program Source File Program Object File
C++ Compiler .o
g++ -c
Library Header File
Linker
Program Executable File
C++ Compiler .o
e.g.,
g++ foo.cpp bar.o fb.o
Library Implementation File
Library Object File
g++ -c 55

56. library.h main.cpp source.cpp Output:Hello 9900
#ifndef ABC
#define ABC
int TestInt = 99;
int functionA( int ); #endif
extern int TestInt;
main.cpp
source.cpp
#include <iostream>
#include "library.h"
using namespace std;
int TestInt=99;
int main(){
cout << "Hello"<<endl;
cout << functionA(100) << endl;
return 0; }
#include "library.h"
int functionA( int i ){
return TestInt* i; }
Output:Hello
9900
> g++ main.cpp source.cpp
ld: fatal: symbol `TestInt' is multiply-defined:
(file /var/tmp/ccvkmxE2.o type=OBJT; file /var/tmp/ccgj1SDu.o type=OBJT);
ld: fatal: File processing errors. No output written to a.out
collect2: ld returned 1 exit status 56

57. Why #ifndefine #define #endif Statement?
It is ok to have multiple declarations of a function prototype, but not for its definition
In the .h file, put the prototypes there
.h files are likely to be multiply-included
In creating the .o file, there may be nested #include statement
The nested #include statement may be recursive
In main.cpp, #include “foo.h”
In foo.h, #include “bar.h”
In bar.h, #include “foo.h”
To break the infinite “recursive” inclusion, use #ifndefine #define to define a “variable” in the compilation process of .o file
If a variable has been defined, the compiler will skip the code segment between #ifndefine and #endif. 57

58. Data Integrity
Data integrity are not automatic by putting data members as private
The programmer must provide appropriate validity checking and report the errors
Member functions that set the values of private data should verify that the intended new values are proper
They should place the private data members into an appropriate state
set functions can be used to validate data besides simply setting the value
Known as validity checking
Keeps object in a consistent state
The data member contains a valid value
Can return message indicating that attempts were made to assign invalid data 58

59. Gradebook6.h (with Validity Checking)
#ifndef GRADEBOOK_H
#define GRADEBOOK_H
class GradeBook {
public:
GradeBook( string );
void setCourseName( string );
string getCourseName();
void displayMessage();
private:
string courseName; };
#endif
Same as Gradebook4.h, but with the interface, implementation and driver separated into three files
Interface contains data members and member function prototypes only
Note the #ifndef statements to prevent multiple inclusions 59

60. Gradebook6.cpp
#include "GradeBook6.h" // include definition of class GradeBook
void GradeBook::setCourseName( string name ) {
// if name has 25 or fewer characters
if ( name.length() <= 25 )
courseName = name;
// if name has more than 25 characters
if ( name.length() > 25 )
// set courseName to first 25 characters of parameter name
courseName = name.substr( 0, 25 );
cout << "Name \"" << name << "\" exceeds maximum length
(25).\n" << "Limiting courseName to first 25 characters."; }
set functions perform validity checking to keep courseName in a consistent state
GradeBook implementation is placed in a separate file
Include the header file to access the class name GradeBook
Binary scope resolution operator :: “ties” a function to its class 60

61. driver6.cpp (1/2) Include the header file to use the class GradeBook
#include "GradeBook6.h" // include definition of class GradeBook
int main() {
// initial course name of gradeBook1 is too long
GradeBook gradeBook1( "COMP104 Introduction to Programming in C++" );
GradeBook gradeBook2( "COMP152 OOP and Data Structures" );
cout << "gradeBook1's initial course name is: "
<< gradeBook1.getCourseName()
<< "\ngradeBook2's initial course name is: "
<< gradeBook2.getCourseName() << endl;
Include the header file to use the class GradeBook 61

62. driver6.cpp (2/2)
// modify myGradeBook's courseName (with a valid-length string)
gradeBook1.setCourseName( "COMP104 C++ Programming" );
// display each GradeBook's courseName
cout << "\ngradeBook1's course name is: "
<< gradeBook1.getCourseName()
<< "\ngradeBook2's course name is: "
<< gradeBook2.getCourseName() << endl;
return 0; }
Call set function to perform validity checking directly
In Linux, compile them all together using
g++ Gradebook6.cpp driver6.cpp
Or using object files:
g++ -c Gradebook6.cpp;
g++ -c driver6.cpp;
g++ driver6.o driver6.o 62

63. driver6.cpp Sample Output
Name "COMP104 Introduction to Programming in C++" exceeds maximum length (25).
Limiting courseName to first 25 characters.
gradeBook1's initial course name is: COMP104 Introduction to P
gradeBook2's initial course name is: COMP152 OOP and Data Stru
gradeBook1's course name is: COMP104 C++ Programming
gradeBook2's course name is: COMP152 OOP and Data Stru 63