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Classes: Part 2 

Classes: Part 2 . Static vs. Dynamic Variables. Generally speaking: ‘static’ is meant to be ‘non-dynamic’. In C and C++, ‘static’ is well defined:. ‘static’ variables are ‘global’ with the file scope. Global and local variables

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Classes: Part 2 

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  1. Classes: Part 2 

  2. Static vs. Dynamic Variables Generally speaking: ‘static’ is meant to be ‘non-dynamic’ In C and C++, ‘static’ is well defined: • ‘static’ variables are ‘global’ with the file scope 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. 2

  3. Static Variables in C and C++ int f(){ static int s = 0; return s++; } int main() { cout << f() << endl; cout << f() << endl; } Don’t do it unless you have a good reason! 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. 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 Private ‘static’ members still can only be accessed by member functions! 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 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. SEmployee.h 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. Employee has a static function and a static data member 8

  9. 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) 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; } Non-static member function (e.g., constructor) can modify the class’s static data members 10

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

  12. static.cpp (1/2) 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(); 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() 12

  13. static.cpp (2/2) cout << "\n\nEmployee 1: " << e1Ptr->getFirstName() << " " << e1Ptr->getLastName() << "\nEmployee 2: " << e2Ptr->getFirstName() << " " << e2Ptr->getLastName() << "\n\n"; deletee1Ptr; e1Ptr = 0; // e1Ptr = NULL; deletee2Ptr; e2Ptr = 0; cout << "Number of employees after objects are deleted is " << Employee::getCount() << endl; Even when no object exists, we can still call static member function getCount() 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 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. 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 initialization OK OK 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; }; 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; }; 4433 NOT OK 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; }; 20

  21. Example: Time Class 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 }; Information hiding (Time.h and Time.cpp) Two types of constructors 21

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

  23. Explicit-Value Constructor Time::Time(unsigned initHours, unsigned initMinutes, char initAMPM) { set(initHours, initMinutes, initAMPM); //a member function } Time bedTime = Time(11,30,’P’); (a random value if myMilTime is not set in set()) 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. Time(unsigned initHours = 12, unsigned initMinutes = 0, char initAMPM = 'A'); Time t1, t2(5), t3(6,30), t4(8,15,'P'); Possible to specify default values for constructor arguments 25

  26. Copy Operations Same as: Time t(bedTime); and calls ‘copy constructor’. ‘assignment’, by default, memberwise copy of the left into the right object. During initialization Time t = bedTime; During assignmentt = midnight; 26

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

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

  33. 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 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 char *const: see ‘pointer’ slides. 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 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 Time* timePtr = &t; Time* timePtr = new Time(12, 0, ‘A’, 0); timePtr timePtr->getMilTime() (*timePtr).getMilTime() delete timePtr; // call destructor Possible to declare pointers to class objects Access with or Call delete to free the memory 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 Class Object Function members *this Data members this Every class has a keyword, this • a pointer whose value is the address of the object • Value of *this would be the object itself 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 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(…) : … { … } class A { … int& r; const int c; } Have to use ‘member initializers’ for • ‘reference’ data members • ‘const’ data members 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 interfacelists 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. 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

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