TCP1201 OOPDS 1 Class & Template Lecture 2

TCP1201 OOPDS 2 Learning Objectives: To understand object behaviors To understand constructors To understand destructors and deep copy To understand update methods and query methods To understand static attributes and static methods To understand template

TCP1201 OOPDS 3 Revisit Pointers Why pointer is important? 1.Reference/Point to existing data without cloning the data. 2.Dynamic memory allocation (create the amount of data based on runtime need). 3.Dynamic polymorphism (Lecture 4) A pointer always stores the address of a data. Before we can use a pointer, it must point to a valid address that is achieved through: –Pointing it to an existing data, or –Using it to create a new data (use new operator).

TCP1201 OOPDS Is pointer that does not point to an valid address. Use of dangling pointer generates runtime error easily. 4 Dangling Pointer int* p; // 'p' usually does not point to // a valid address. *p = 10; // Runtime error usually. p [ int* ] ?

TCP1201 OOPDS 5 Point to Existing Data Without Cloning the Data a [ int ] int a = 3; int *p = &a; cout << *p; // 3 cout << p; // p [ int* ] The ampersand '&' is called address operator which returns the address of variable 'a'. 'a' stores an int. 'p' stores the address of variable 'a'. We say 'p' points to 'a'. 'p' is not a clone of 'a'.

TCP1201 OOPDS To create new data dynamically. We use new operator to create new data, and later use delete operator to release the memory used by the data. The new operator allocates the memory needed for the new data, and returns the address of the new data. 6 Dynamic Memory Allocation (DMA) int a = 3; // Automatic memory allocation. int* p; // 'p' does not point to a valid address. p = new int; // 'p' points to the new data.... delete p; // Release the memory used by *p.

TCP1201 OOPDS Use delete [] to release the memory used by a dynamic array. 7 Dynamic Memory Allocation (DMA) int* p1 = new int; // Create 1 int. delete p1; // Release 1 int. int* p2 = new int[4]; // Create a dynamic // array of 4 int. delete [] p2; // Release dynamic array p1 [ int* ] ? ? ? ? ? 1100 p2 [int* ] [int] [int]

TCP1201 OOPDS 8 4 Types of Object Behaviors 1.Constructors – Specify how a new object is created. 2.Queries – Return or find out, but not modify, the value of attributes of an object, e.g. get methods. 3.Updates – Modify the value of attributes of an object, e.g. set methods. 4.Destructors – Specify how an object is destroyed.

TCP1201 OOPDS 9 Constructors Constructors are special types of methods that are used to initialize object attributes. They are invoked/called automatically whenever an instance/object of a class is created. class Student {... }; int main() { Student s1; // Create an instance, call a constructor. Student *ps = new Student; // Create an instance, call a constructor.... delete ps; // Release the memory used by *ps. }

TCP1201 OOPDS 10 Constructors Always have the same name as the class name. Have no return value. 3 types of constructors: default, overloaded, copy. A class can have more than one constructor. class Student { int id; string name; public: Student(); // Default constructor Student(int id); // Overloaded constructor Student(string name); // Overloaded constructor Student(int id, string name); // Overloaded constructor Student(const Student& existingStudent); // Copy constructor... }; Constructor name is the same as the class name

TCP1201 OOPDS Default Constructors Enable us to initialize attributes to preferred default values when no argument is not provided during object creation. A class can have only one default constructor. class Student { int id; string name; public: Student() { // Default constructor has no parameter. name = "Unknown"; // Set default name value to "Unknown". id = 0; // Set default id value to 0. } }; int main() { Student s1; // Call default constructor, id=0, name="Unknown". Student *ps = new Student; // Call default constructor, id=0, name="Unknown".... delete ps;...

TCP1201 OOPDS Without Default Constructors If you do not provide both default constructor and overloaded constructor, C++ automatically creates a default constructor with blank implementation. // Your code class Student { // No default constructor or // overloaded constructor is provided. int id; string name; }; // Compiler code class Student { int id; string name; public: Student() // Compiler's default constructor. { } // Blank implementation. name = "", id = ? }; int main() { Student s1; // Call default constrcutor. name = "", id = ? Student *ps = new Student; // name = "", id = ?...

TCP1201 OOPDS Enable us to initialize attributes before constructor body is executed. Is placed between the constructor parameter list and constructor body. ClassName ( ) : {... } 13 Constructor Initializer List Constructor name Colon Initializer list Constructor body ClassName (datatype1 param1, datatype2 param2) : attribute1 (param1), attribute2 (param2) {... } General format:

TCP1201 OOPDS Constructor Initializer List The previous Student's default constructor can be rewritten to use constructor initializer list as follow: // Use constructor initializer list. class Student { int id; string name; public: Student () // Default constructor : name("Unknown"), id(0) // Initialize name = "Unknown", id = 0. { } }; int main() { Student s1; // name = "Unknown", id = 0. Student *ps = new Student; // name = "Unknown", id = 0....

TCP1201 OOPDS Constructor Initializer List Is the only way to initialize const attributes (won't be discussed), and is also only way to call superclass' constructor (Lecture 3). Hence is the preferred way to initialize attributes. Does not work on static attributes (discuss later).

TCP1201 OOPDS 16 Example: Default Constructor class Student { int id; string name; public: Student() // Default constructor : id(0), name("Unknown") { cout << "A Student object is created!\n"; } void setName (string name) { this->name = name; } void setID (int id) { this->id = id; } string getName() { return name; } int getID() { return id; } }; int main() { Student* s1 = new Student; // Call default constructor, // id = 0, name = "Unknown" cout getID() << endl getName() << endl; s1->setID (123); // id = 123 s1->setName ("Ali"); // name = "Ali" cout getID() << endl getName() << endl; delete s1; } Output: A Student object is created! Student id = 0 Student name = "Unknown" Student id = 123 Student name = "Ali"

TCP1201 OOPDS Overloaded Constructors Constructors that have parameter(s). Initialize attributes to values passed as arguments. class Student { int id; string name; public: Student (int id, string name) // Overloaded constructor. : id(id), name(name) // attribute1(param1), attribute2(param2). { } }; int main() { Student s1 (123, "Ali"); // Call overloaded constructor, id = 123, // name = "Ali" Student *ps; ps = new Student (234, "Bob"); // Call overloaded constructor, id = 234, // name = "Bob"...

TCP1201 OOPDS Overloaded Constructors A class can have more than one overloaded constructors as long as the parameter lists follow the rules of function overloading, that is no 2 overloaded constructors should have the same number of parameters and the same type of parameters. class Student { int id; string name; public: Student () {...} // Default constructor Student (int id) {...} // Overloaded constructor 1 Student (string name) {...} // Overloaded constructor 2 Student (int id, string name) {...} // Overloaded constructor 3 Student (const Student& s) {...} // Copy constructor }; int main() { Student s1; // Call default constructor Student s2 ("Ann"); // Call overloaded constructor 2 Student s3 (234, "Bob"); // Call overloaded constructor 3 Student s4 (123); // Call overloaded constructor 1 Student s5 (s4); // Call copy constructor...

TCP1201 OOPDS Overloaded Constructors We may provide default argument to constructor parameters. If all parameters of an overloaded constructor have default argument, the overloaded constructor serves as a default constructor too. class Student { string name; int id; public: Student (int id=0, string name="Unknown") // 2-in-1: default constructor + // overloaded constructor. : id(id), name(name) {} }; int main() { Student *p1 = new Student; // Call default constructor, // id = 0, name = "Unknown". Student *p2 = new Student (234, "Bob"); // Call overloaded constructor, // id = 234, name = "Bob"....

TCP1201 OOPDS 20 Copy Constructors Initializes a new object by copying the value of attributes from an existing object of the same class. Is invoked when an instance is created with one of the following ways: Student x(123, "Michael"); // x invokes overloaded constructor. Student y = x; // y invokes copy constructor, copy from x. Student z(y); // z invokes copy constructor, copy from y. Student* s = new Student(y); // *s invokes copy constructor, // copy from y. // All 4 objects x, y, z and *s have id=123 and name="Michael".

TCP1201 OOPDS 21 Copy Constructors A class can have one copy constructor only. Copy constructor must have the following header/interface/signature: const prevents existing instance from being modified by the copy constructor ClassName (const ClassName& existingInstance); Parameter name refers to the source instance that is being “copied”Parameter is of the same type as the class Pass by reference to avoid cloning the parameter Student (const Student& existingStudent);

TCP1201 OOPDS 22 Copy Constructors If we do not provide a copy constructor, C++ automatically provides a default copy constructor that copies the value of all attributes from current instance to the new instance. This type of copying is called shallow copy. class Student { int id; // 2 attributes, no dynamic memory allocation. string name;... // C++ provides the following default copy constructor. Student (const Student& s) : id(s.id), name(s.name) // shallow copy 2 attributes. { } }; For our Student class, C++ automatically provides the following default copy constructor.

TCP1201 OOPDS 23 Problem of Shallow Copy We usually do not have to provide a copy constructor, unless we want to perform additional initialization that is not provided by the default copy constructor. For example, deep copy a pointer attribute that is created using dynamic memory allocation (DMA). If a pointer attribute of a source object uses DMA to create its data, shallow copy won't create that data in the target object but just makes the source object shares the data with target object.

TCP1201 OOPDS 24 Deep Copy Example Consider a scenario whereby a student have many marks. We declare a pointer named marks in Student class, and use it to create a dynamic array to store the marks. Each student should have their own dynamic array to store their marks, and they should not share the same dynamic array. Shallow copying the marks attribute would result in both the source student and target student sharing the same dynamic array, which is considered wrong. Deep copying the marks attribute would ensure that each student would have their own dynamic array for storing the marks.

TCP1201 OOPDS 25 Problem: Shallow copy a pointer attribute in Copy Constructor class Student { int size; double *marks; // Pointer attribute. public: Student (int size); // Overloaded constructor. // No copy constructor is written, C++ provides one. void setMark (int index, int mark); void print(); }; Student::Student (int size) : size(size) { this->marks = new double[size]; // Create dynamic array. for (int i = 0; i < size; i++) marks[i] = 0; } void Student::setMark (int index, int mark) { marks[index] = mark; } void Student::print() { cout << "Marks = "; for (int i = 0; i < size; i++) cout << marks[i] << " "; cout << endl; } int main() { Student *s1, *s2; s1 = new Student (3); s2 = new Student (*s1); s1->print(); // s2->print(); // s1->setMark (0, 70); s1->setMark (1, 80); s1->setMark (2, 90); s1->print(); // s2->print(); // delete s1; delete s2; } Output: Marks = Marks = s2 points to s1's marks array, hence sharing one marks array

TCP1201 OOPDS 26 Solution: Deep copy in Copy Constructor class Student { int size; double *marks; // Pointer attribute. public: Student (int size); // Overloaded constructor. Student (const Student& s); // Copy constructor. void setMark (int index, int mark); void print(); }; Student::Student (int size) : size(size) { this->marks = new double[size]; // Create dynamic array. for (int i = 0; i < size; i++) marks[i] = 0; } Student::Student (const Student& s) // Copy constructor. : size(s.size) { this->marks = new double[size]; // Create dynamic array. for (int i = 0; i < size; i++) marks[i] = s.marks[i]; // Copy array. } void Student::setMark (int index, int mark) { marks[index] = mark; } void Student::print() { cout << "Marks = "; for (int i = 0; i < size; i++) cout << marks[i] << " "; cout << endl; } int main() { Student *s1, *s2; s1 = new Student (3); s2 = new Student (*s1); s1->print(); // s2->print(); // s1->setMark (0, 70); s1->setMark (1, 80); s1->setMark (2, 90); s1->print(); // s2->print(); // delete s1; delete s2; } Output: Marks = Marks = Marks = s1 and s2 each have a different marks array

TCP1201 OOPDS 27 Destructors A destructor is a special type of method that is invoked whenever an instance is destroyed (de-allocating an instance). Destructors are automatically called when an automatically allocated instance goes out of scope, or when a dynamically allocated instance is explicitly deleted (using delete operator). Destructors have no return value and no parameter There is only one destructor for each class. Destructor has the same name as the class, except that it is preceded by a tilde ‘~’. class Student {... ~Student(); };

TCP1201 OOPDS 28 Destructors Destructors are generally used to perform any cleanup necessary prior to an instance being destroyed. For example deleting dynamically allocated objects: // Continue from Deep Copy example class Student { double *marks; // Pointer attribute. public:... ~Student(); }; Student::~Student() { delete [] marks; // Destroy dynamic attribute. }

TCP1201 OOPDS 29 Destructors Same as the default constructor and copy constructor, if you do not provide a destructor, C++ provides a default destructor with blank implementation. // Your code class Student { // No destructor is provided. }; // Compiler code class Student { public: ~Student() { // Compiler's default destructor, // blank implementation. } };

TCP1201 OOPDS 30 Example: Destructor class Student { int id; public: Student(int id); // constructor ~Student(); // destructor }; Student::Student(int id) : id(id) { cout << "Object " << id << " created.\n"; } Student::~Student() { cout << "Object " << id << " destroyed.\n"; } int main() { Student* s0 = new Student(000); // constructor for 000 delete s0; // destructor for 000 Student s1(111); // constructor for 111 Student s2(222); // constructor for 222 Student* s3 = new Student(333); // constructor for 333 } // End of main() auto-calls destructor for 222 and 111. // Why destructor for 333 not called? No "delete s3;" Output: Object 000 created. Object 000 destroyed. Object 111 created. Object 222 created. Object 333 created. Object 222 destroyed. Object 111 destroyed.

TCP1201 OOPDS 31 Query Methods Query methods (also called accessors) are methods that are used to inquire (find out) about the value of an instance’s attributes. Query method does not modify the value of any of the object’s attributes. class Student { int id; string name;... int getId() { // Returns the value of attribute id. return id; } string getName() { // Returns the value of attribute name. return name; } };

TCP1201 OOPDS 32 Update Methods Update methods (also called mutators) are methods that modify the value of attribute(s) in an object. class Student { int id; string name;... void setId (int id) { // Modify attribute 'id'. this->id = id; } void setName (string name) { // Modify attribute 'name'. this->name = name; } };

TCP1201 OOPDS 33 Update Methods Recall that one of the reasons to have encapsulation is to ensure data integrity. Update methods allow us to achieve data integrity. In the following example, the setGPA update method ensures that the new GPA value is within 0-4. int Student::setGPA (double newGPA) { if ((newGPA >= 0.0) && (newGPA <= 4.0)) { gpa = newGPA; return 0; // Return 0 to indicate success } else { return -1; // Return -1 to indicate failure }

TCP1201 OOPDS 34 Update & Query Behaviors Identify the type of behaviors of the Student class on the right. Query behaviors: –getId, show_subjects. Update behaviors: –register_subject (modify attribute 'subjects'). –withdraw_subject (modify attribute 'subjects'). –setId (modify attribute 'id'). -id: int -subjects:string[*] +getId():int +setId(id:int):void +show_subjects():void +register_subject():void +withdraw_subject():void Student

TCP1201 OOPDS 35 static Attributes Are attributes that are shared across all instances of a class. The program creates only a single instance of a static attribute, regardless of how many instances of the class are created. Cannot be initialized within the class or using constructor initializer list. Must be initialized outside the class declaration using scope resolution operator "::". class Student { string name; static int count; // Use keyword "static" to declare // static attribute. }; // end of class int Student::count = 0; // Initialize count to 0.

TCP1201 OOPDS 36 static Methods A static method can be invoked without creating an instance of the class. Use the scope resolution operator "::". They cannot access non-static attributes or non-static methods. The primary reason to declare static methods is to be able to access static attributes without creating an instance of the class.

TCP1201 OOPDS 37 Example: static Members class Student { string name; static int count; public: Student() { ++count; } ~Student() { --count; } static int getCount() { return count; } }; int Student::count = 0; int main() { cout << "Student count = " << Student::getCount() << endl; Student a; // create 1 instance cout << "Student count = " << a.getCount() << endl; Student* b = new Student; // create 1 instance. cout << "Student count = " getCount() << endl; { Student c; // create 1 instance. cout << "Student count = " << c.getCount() << endl; } // c is destroyed delete b; // b is destroyed. cout << "Student count = " << Student::getCount() << endl; // Why not zero? } Output: Student count = 0 Student count = 1 Student count = 2 Student count = 3 Student count = 1

TCP1201 OOPDS 38 Example: static Members class Student { string name; static int count; public: Student() { ++count; } ~Student() { --count; } static int getCount() { return count; } }; int Student::count = 0; int main() { cout << "Student count = " << Student::getCount() << endl; Student a; // create 1 instance cout << "Student count = " << a.getCount() << endl; Student* b = new Student; // create 1 instance. cout << "Student count = " getCount() << endl; { Student c; // create 1 instance. cout << "Student count = " << c.getCount() << endl; } // c is destroyed delete b; // b is destroyed. cout << "Student count = " << Student::getCount() << endl; // Why not zero? } Output: Student count = 0 Student count = 1 Student count = 2 Student count = 3 Student count = 1

TCP1201 OOPDS 39 static Members int main() { cout << Student::getCount(); // Invokes static method // without instance. Student* s1 = new Student; // 1 instance. cout getCount(); // Invokes static method // via the instance s1. Student s2[100]; // 100 instances. cout << s2[0].getCount(); // Invokes static method // via the instance s2[0]. // At this point, there are 101 instances of name, // but only 1 instance of count. delete s1; }

TCP1201 OOPDS 40 Template Template allows us to write generic functions or classes (called function template, class template) that accept any data type as parameters or attributes. During compilation, the compiler will produce a separate definition for every data type that uses the template.

TCP1201 OOPDS 41 Function Template To swap 2 integers, we may write the following function: To swap 2 strings, we need to write another function: The only different between these 2 functions are data type. Template is the best solution. void Swap (int& m, int& n) { int temp = m; m = n; n = temp; } void Swap (string& m, string& n) { string temp = m; m = n; n = temp; }

TCP1201 OOPDS 42 Function Template Example Template version of Swap function: template void Swap(T& m, T& n) { T temp = m; m = n; n = temp; } int main() { int m = 22, n = 66; Swap(m, n); // integers cout << "m = " << m << endl << "n = " << n << endl; string s1 = "Michael", s2 = "Kelly"; Swap(s1, s2); // strings cout << "s1 = " << s1 << endl << "s2 = " << s2 << endl; } Output: m = 66 n = 22 s1 = Kelly s2 = Michael Compiler produces 2 definitions of Swap() functions: one for int, one for string.

TCP1201 OOPDS 43 Declaring Function Template Function templates are declared in a similar as ordinary functions, except that it is preceded by the specification below: Type parameter 'T' is used in place of ordinary data types within the function definition. A template may have several type parameters: template // Less confusing, or template // more confusing, class? template

TCP1201 OOPDS 44 Declaring Class Template Class templates are declared in a similar way as ordinary classes, except that it is preceded by the specification below: template class Pair { T a; T b;... template class Box { T data1; U data2;... }; A class template may have several type parameters:

TCP1201 OOPDS 45 Class Template Example 1 template class Pair { T a; T b; public: void set (T a, T b) { this->a = a; this->b = b; } T getA() { return a; } T getB() { return b; } }; Define a class template Pair that have 2 attributes of the same type. int main() { Pair p1; p1.set (3,5); Pair p2; p2.set ("Peter", "Jackson"); Pair *p3 = new Pair ; p3->set ('P','J'); cout << p1.getA() << " " << p1.getB() << endl << p2.getA() << " " << p2.getB() << endl getA() << " " getB(); delete p3; }

TCP1201 OOPDS 46 Class Template Example 1 template class Pair { T a; T b; public: void set (T a, T b) { this->a = a; this->b = b; } T getA() { return a; } T getB() { return b; } }; Define a class template Pair that have 2 attributes of the same type. int main() { Pair p1; p1.set (3,5); Pair p2; p2.set ("Peter", "Jackson"); Pair *p3 = new Pair ; p3->set ('P','J'); cout << p1.getA() << " " << p1.getB() << endl << p2.getA() << " " << p2.getB() << endl getA() << " " getB(); delete p3; }

TCP1201 OOPDS 47 Class Template Example 1 template class Pair { T a; T b; public: void set (T a, T b) { this->a = a; this->b = b; } T getA() { return a; } T getB() { return b; } }; Output: 3 5 Peter Jackson P J Define a class template Pair that have 2 attributes of the same type. int main() { Pair p1; p1.set (3,5); Pair p2; p2.set ("Peter", "Jackson"); Pair *p3 = new Pair ; p3->set ('P','J'); cout << p1.getA() << " " << p1.getB() << endl << p2.getA() << " " << p2.getB() << endl getA() << " " getB(); delete p3; }

TCP1201 OOPDS 48 Class Template Example 1 template class Pair { T a; T b; public: void set (T a, T b) { this->a = a; this->b = b; } T getA() { return a; } T getB() { return b; } }; Output: 3 5 Peter Jackson P J Class template allows us to write one class that works with different data types. int main() { Pair p1; p1.set (3,5); Pair p2; p2.set ("Peter", "Jackson"); Pair *p3 = new Pair ; p3->set ('P','J'); cout << p1.getA() << " " << p1.getB() << endl << p2.getA() << " " << p2.getB() << endl getA() << " " getB(); delete p3; }

TCP1201 OOPDS 49 Class Template Example 2: Multiple Type Parameters template <typename T, typename U> class Pair { T a; U b; public: void set (T a, U b) { this->a = a; this->b = b; } T getA() { return a; } U getB() { return b; } }; Output: 1 A B Boy 99 Attributes may have different data types. int main() { Pair p1; p1.set (1, 'A'); Pair p2; p2.set ('B',"Boy"); Pair *p3 = new Pair ; p3->set (99,99); cout << p1.getA() << " " << p1.getB() << endl << p2.getA() << " " << p2.getB() << endl getA() << " " getB(); delete p3; }

TCP1201 OOPDS 50 Revisit STL vector Class The vector class provided by C++ Standard Template Library (STL) is actually a class template. vector is better than array when we need to insert and/or remove data from a collection of data, because vector can grow and shrink automatically.

TCP1201 OOPDS 51 vector Example... #include using namespace std; class Student {... }; int main() { vector s; // vector of Student. // Insert at back. s.push_back (Student(22,"Bob")); // Bob // Insert at beginning or index 0. s.insert (s.begin(), Student(11,"Ali")); // Ali Bob // Insert at index 1. s.insert (s.begin()+1, Student(33,"Cat")); for (int i = 0; i < s.size(); i++) // Ali Cat Bob cout << s[i].getName() << " "; cout << endl; // Erase item at index 1. s.erase (s.begin()+1); // Ali Bob // Erase last item. s.erase (s.end()); // Ali for (int i = 0; i < s.size(); i++) cout << s[i].getName()<< endl; }