Object Oriented Programming Lect. Dr. Daniel POP Universitatea de Vest din Timişoara Facultatea de Matematică şi Informatică

2Programming IIObject-Oriented Programming Course #3 Agenda Classes in C++ (continued) Access control Constructors Destructor Self-reference Modifiers: static, const, mutable

3Programming IIObject-Oriented Programming Access Control To implement the data hiding principle in C++, various level of access are defined for class members (data or functions): – private – accessible only to member functions and friends; – protected – accessible to member functions and friends + member functions and friends of derived classes; – public – accessible from everywhere. Public members define the public interface of one class. Example: class MyClass { int x; // private, by default public: int y, z; // public int f(); // public member function private: int w; // private data protected: int f(int, int); // protected member function }; void main() { MyClass obj; obj.x = 10; // ERROR: Cannot access private members obj.f(); // OK! obj.f(1,1); // ERROR:Cannot access protected members }

4Programming IIObject-Oriented Programming Constructors (I) Rationale: to prevent errors caused by non-initialized objects Example: Date objects, a Stack with dangling pointer to its top element etc. DEFINITION [Constructor] A member function of a class whose role is to initialize the class instances (objects). Object creation process: –Memory allocation –Find an appropriate constructor –Call the constructor to initialize the object’s state after the data members have been previously constructed/initialized by calling their constructors Q: Which member functions are the constructors? A: The constructor name = class name Characteristics of constructors: –The name = Class name –They have NO return value (Remark: void is a return value) –No pointers can be assigned to constructors (PMF pmf = &Date::Date; is illegal) –Except for the above constraints, constructors are handled as any other member function of a class (for example, they may have 0, 1 or more arguments, etc.)

5Programming IIObject-Oriented Programming Constructors (II) Example : class Date { int _day, _month, _year; void init(int day, int month, int year); public: // Constructors Date(int day, int month, int year); Date(int day, int month); Date(int day); }; void f() { Date d0 = Date(6, 10, 2003); // correct Date d1(1, 1); // correct Date d2(15); // correct Date d3; // ERROR: No appropriate constructor is found Date* pd = new Date(7, 10, 2003); // correct } class Date { int _day, _month, _year; void init(int day, int month, int year); public: // Constructors! Date(int day = 0, int month=0, int year=0); }; void f() { Date d3; // correct } #include “Date.h” Date::Date(int day, int month, int year) { _day = day ? day : today.day; _month = month ? month : today.month; _year = year ? year ? today.year; }

6Programming IIObject-Oriented Programming Constructors (III). Default Constructor Constructor without arguments Prototype: X(); If a class has no constructors then the compiler generates a default constructor for that class that implicitly calls the default constructors for class’ members of class type and bases. Remark: If a class has const or reference members then the default is not generated automatically because consts and references must be initialized. Examples: class Date { public: // default constructor Date(int day=0, int month=0, int year=0); }; class String { public: String(); // default constructor }; class Student { Date birthday; String name; // automaticlly generated default constructor that calls Date and String // defaults constructors }; class Date { public: // no default constructor is generated Date(int day); }; class Test { const int a; int& r; // no default constructor is generated };

7Programming IIObject-Oriented Programming Constructors (IV). Copy Constructor Constructor with one argument of type reference to its own class Prototype: X(const X&); const – indicates that the source object is not modified It is called: –Declaration of an object like X obj = obj_source; –Passing an object as an argument to a function call func(X); WHY? –When a temporary object is created during expression evaluation If it is not defined for one class, then the compiler automatically generates one that copies bit-by-bit the content of source object to destination object. To prevent copying of objects, the copy constructor can be declared private (no implementation is needed). To ‘deep’ copy complex objects, the copy-constructor is mandatory! (Examples in lab) Examples: class Date { public: Date(int day=0, int month=0, int year=0); // default constructor Date(const Date& ref); // user-defined copy constructor void setDay(int day); }; void g(Date d) { d.setDay(); } void f() { Date d(7, 3, 2007); // user-defined constructor Date d; // default constructor Date d2 = d; // copy constructor d2 = d; // NO copy constructor g(d); // copy constructor is called }

8Programming IIObject-Oriented Programming Constructors (V). Type Conversion Constructors Scope: convert from one data type to a class (built-in type | user-defined-type  class) Prototype: X(Datatype); Called in –declarations like X x = value; where value is a Datatype –function calls to cast the actual argument to required type class Date { public: // … Date(char* str) ; // type-conversion constructor }; void g(Date d) { d.setDay(7); } void f() { Date d = “6/3/2007”; g(“This is tricky ;-)”); } class Double { double val; public: Double(double v) { value = v; } } ; vod f() { // three equivalent declarations Double obiect = 2.5; Double obiect = Double(2.5); Double obiect(2.5); }

9Programming IIObject-Oriented Programming Destructor Rationale: to prevent errors caused by un-released objects (unreleased resources). Example: Stack without freeing the memory used, File without closing the handle etc. DEFINITION [Destructor] A member function of a class whose role is to release the class instances (objects) from memory. Object destruction process: –Call the destructor function –Call destructors of data member –Free the memory Q: Which member function is the destructor? A: The destructor name = ~class name (i.e. ~X) Characteristics of constructors: –Prototype: X::~X(); –They have NO return value (Remark: void is a return value) and they take no arguments. –No pointers can be assigned to destructor (PMF pmf = &Date::~Date; is illegal) –Except for the above constraints, destructor is handled as any other member function of a class.

10Programming IIObject-Oriented Programming Destructor (II) A class can have at most one destructor. The compiler generates an empty default destructor. Example: class String { char* str; public: // Constructors String(char* psz=NULL); ~String(); }; void f() { String s1; String s2=“We like C++;)”; String s3=s2; String* s4 = new String(“Do we?”); delete s4; // destructor is called } // destructors for s3, s2, s1 are called in this order String::String(char* psz) { if(psz==NULL) str=NULL; else { str = new char [strlen(psz)+1]; strcpy(str, psz); } String::~String() { if(str!=NULL) delete [] str; } Try this & Explain the result!

11Programming IIObject-Oriented Programming Constructors & Destructor Both, constructor and destructor can be explicitly called. NOT RECOMMENDED TO DO THIS! Example void f() { String s1; s1.String::String(“Explicit call”); s1.~String(); } RECOMMENDATION: If a class has a pointer member, it needs a copy constructor, a destructor and an assignment operator (see operator overloading).

12Programming IIObject-Oriented Programming Self-Reference Example: void f() { Date d; d.init(25,12,2007).getMonth(); } => The prototype of init function should be: Date& init(int, int, int); Each non-static member function knows what object it was invoked for and can explicitly refers to it using this keyword. this is a pointer to the object for which the function was called –For non-const function its type is: X* const this; –For const function its type is: const X* const this; Similar in Java (this), Smalltalk (self) or Simula (THIS). Date& Date::init(int day, int month, int year) { this->_day = day; //  _day = day _month = month; // implicit use of this this->year = year; // year – makes possible // to use the same name // for members and args this++; // ILLEGAL: this is a const pointer return *this; }

13Programming IIObject-Oriented Programming Modifiers: static (I) DEFINITON [static data member] A variable that is part of a class, yet is not part of an object of that class is called a static member. There is exactly one copy of a static variable ‘shared’ by all objects of that class. Static data member = global variable defined and accessible only in the scope of that class. DEFINITION [static member function] A function that needs access to members of a class, yet doesn’t need to be invoked for a particular object, is called a static member function. Static member function = global function defined and accessible only in the scope of that class. Declaration of static members: static MemberDefinition; Accessing static members: X::memberName; obj.memberName; In order to clearly differentiate between static and non-static members + enhance code readability, the first (bold) syntax is recommended to be used.

14Programming IIObject-Oriented Programming Modifiers: static (II) class Date { int day, month, year; static Date today; // declare the static data member public: // … static void initToday(); }; Date Date::today; // create the static data member void Date::initToday() { time_t t; time(&t); tm* now = localtime(&t); day = now->tm_mday; // ERROR: whose day is this day? today.day = now->tm_mday; today.month = 1 + now->tm_mon; today.year = now->tm_year; } int main(int, char*[]) { Date::initToday(); return 0; } REMARKS: static data members have to be defined/initialized somewhere (in the class definition they are only declared) static member function DOES NOT receive this pointer => the access to non-static members (data/function) is not possible from within static functions Creation, initialization and access to static members are independent of objects existence. Example: default date (today) in Date class Memory representation of static data.

15Programming IIObject-Oriented Programming Modifiers: const class Date { int day, month, year; static Date today; // declare the static data member public: // const, inline member function int getDay() const { day = 0 ; // ERROR: we’re in const function return day; } int getMonth() const { return month; } int getYear() const { return year; } }; int main(int, char*[]) { Date d; cout << d.getDay() << d.getMonth() << d.getYear(); return 0; } Data members ―Cannot be modified! ―Useful to declare constants at class scope ―Syntax: Member functions: ―Cannot modify the state of the object (i.e. data members) ―Enhances the code’s clarity ―Prevents accidental updates ―When the function is defined outside its class, const suffix is required. ―Syntax: ftype fname(arg_list) const; const data_type name;

16Programming IIObject-Oriented Programming Modifiers: mutable class Date { int day, month, year; mutable string cache; // always changeable! mutable boolean validCache; // always changeable! public: // const member function string toString() const; }; string Date::toString() const { if(!validCache) { cache = compute_new_string_representation(); validCache = true; } return cache; } Applies only to data members ―Can always be modified, even from within const functions! ―Useful for members that need to be changed in const functions and don’t represent the actual internal state of the object ―Syntax: mutable data_type name;

17Programming IIObject-Oriented Programming Further Reading [Pop, 2003] Daniel Pop – C++ Programming Language Lecture Notes [Stroustrup, 1997] Bjarne Stroustrup – The C++ Programming Language 3rd Edition, Addison Wesley, 1997 [Chapter 10][Stroustrup, 1997] Bjarne Stroustrup – The C++ Programming Language 3rd Edition, Addison Wesley, 1997 [Chapter 10]