OBJECT-ORIENTED PROGRAMMING CONCEPTS-CLASSES III

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1 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 1 OBJECT-ORIENTED PROGRAMMING CONCEPTS-CLASSES III KOM3191 Object-Oriented Programming

2 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 2 Destructors A destructor is another type of special member function. It is the opposite of constructors tilde character (~) followed by the class name MyString::~MyString() A class's destructor is called implicitly when an object is destroyed The order in which these function calls occur 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 Function exit forces a program to terminate immediately and does not execute the destructors of automatic objects. The function often is used to terminate a program when an error is detected in the input or if a file to be processed by the program cannot be opened. Function abort performs similarly to function exit but forces the program to terminate immediately, without allowing the destructors of any objects to be called.

3 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 3 Destructors // MyString.h class MyString public: MyString( const string str, int number); int getid(); string getliteral(); void setliteral(const string str); void setid(int number); int& badsetid(int number); void display(); ~MyString(); private: string literal; int id; ;

4 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 4 Destructors //MyString.cpp MyString::MyString(const string str, int number) literal=str; id=number; cout<<"constructor is active for "; display();... void MyString::display() cout<<literal<< " object:"<<id<<endl; MyString::~MyString() cout<<"~ Destructor is active for "; display();

5 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 5 Destructors MyString str1("global object",1); void helperfunction(); Con 1 Des 5 int main() 2 6 MyString str2("local object in main", 2); 3 2 static MyString str3("static local object in main",3); helperfunction(); 4 fnc 4 cout<<endl; MyString str6("local object in main",6); return 0; 5 fnc void helperfunction() static MyString str4("static local object in helperfunction",4); MyString str5("local object in helperfunction",5);

6 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 6 Destructors Constructor is active for global object object:1 Constructor is active for local object in main object:2 Constructor is active for static local object in main object:3 Constructor is active for static local object in helperfunction object:4 Constructor is active for local object in helperfunction object:5 ~ Destructor is active for local object in helperfunction object:5 Constructor is active for local object in main object:6 ~ Destructor is active for local object in main object:6 ~ Destructor is active for local object in main object:2 ~ Destructor is active for static local object in helperfunction object:4 ~ Destructor is active for static local object in main object:3 ~ Destructor is active for global object object:1 Con Des fnc 4 5 fnc Objects are created when they are declared. if an object is static, it exists until program termination

7 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 7 Returning a reference #include <iostream> using namespace std; int& addition(int number1, int number2); int result; int main() int &a=addition(22, 23); addition(2, 5)=38; cout<<&a<<endl; cout<<a<<endl; // 0x804a0d4 38 a=46; cout<<&result<<endl; cout<<result<<endl; // 0x804a0d4 46 return 0; int& addition(int number1, int number2) result=number1+number2; return result;

8 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 8 Class example - a trap A reference to an object is an alias for the name of the object and, hence, may be used on the left side of an assignment statement. In this context, the reference makes a perfectly acceptable lvalue that can receive a value. One way to use this capability (unfortunately!) is to have a public member function of a class return a reference to a private data member of that class. Note that if a function returns a const reference, that reference cannot be used as a modifiable lvalue. Int& MyString ::badsetid( int number) // MyString.h id=number; class MyString return id; //Returning a reference to a private data member public:... int& badsetid(int number);... private: string literal; int id; ;

9 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 9 Class example - a trap MyString str1("c++ Programming", 2); str1.display(); int &number1=str1.badsetid(34); cout<<number1<<endl; str1.display(); number1=-13; str1.display(); Constructor is active for C++ Programming object:2 C++ Programming object:2 34 C++ Programming object:34 C++ Programming object:-13 C++ Programming object:35 ~ Destructor is active for C++ Programming object:35 str1.badsetid(17)=35; str1.display(); int &Time::badSetId( int number) id=number; return id; //Returning a reference to a private data member

10 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 10 Class example - a trap Returning a reference or a pointer to a private data member breaks the encapsulation of the class and makes the client code dependent on the representation of the class's data. So, returning pointers or references to private data is a dangerous practice that should be avoided. a public member function of a class return a reference to a private data member of that class. If the function returns a const reference, then the reference cannot be used as a modifiable lvalue. The assignment operator (=) can be used to assign an object to another object of the same type. By default, such assignment is performed by member-wise assignment MyString str2=str1; // OK

11 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 11 const Objects - const Member Functions the principle of least privilege as one of the most fundamental principles of good software engineering const Time noon( 12, 0, 0 ); // const object noon of class Time C++ compilers disallow member function calls for const objects unless the member functions themselves are also declared const. the compiler does not allow member functions declared const to modify the object. A function is specified as const both in its prototype and in its definition by inserting the keyword const after the function's parameter list and, in the case of the function definition, before the left brace that begins the function body.

12 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 12 const Objects - const Member Functions //test_time.cpp const Time noon(12, 0, 0); noon.sethour(9); // not OK cout<<noon.gethour()<<endl; //OK //Time/cpp int Time::getMinute() const return minute;

13 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 13 const Objects - const Member Functions Defining as const a member function that modifies a data member of an object is a compilation error. Defining as const a member function that calls a non-const member function of the class on the same instance of the class is a compilation error. A const member function can be overloaded with a non-const version. The compiler chooses which overloaded member function to use based on the object on which the function is invoked The const declaration is not allowed for constructors and destructors. Attempting to declare a constructor or destructor const is a compilation error. Declare as const all of a class's member functions that do not modify the object in which they operate.

14 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 14 Initializing a const Data Member with a Member Initializer All data members can be initialized using member initializer syntax, but const data members and data members that are references must be initialized using member initializers

15 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 15 Initializing a const Data Member with a Member Initializer

16 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 16 Initializing a const Data Member with a Member Initializer Before incrementing: count = 10, increment = 5 After increment 1: count = 15, increment = 5 After increment 2: count = 20, increment = 5 After increment 3: count = 25, increment = 5

17 KOM3191 Object Oriented Programming Dr Muharrem Mercimek 17 Initializing a const Data Member with a Member Initializer Increment::Increment( int c, int i ) count = c; // allowed because count is not constant increment= i; // ERROR: Cannot modify a const object Not providing a member initializer for a const data member is a compilation error. Constant data members (const objects and const variables) and data members declared as references must be initialized with member initializer syntax; assignments for these types of data in the constructor body are not allowed.

KOM3191 Object Oriented Programming Dr Muharrem Mercimek OPERATOR OVERLOADING. KOM3191 Object-Oriented Programming

KOM3191 Object Oriented Programming Dr Muharrem Mercimek 1 OPERATOR OVERLOADING KOM3191 Object-Oriented Programming KOM3191 Object Oriented Programming Dr Muharrem Mercimek 2 Dynamic Memory Management