1	"Brad Rippe" Brad Rippe
2	Assignment 11 Gradebook.h (no need to modify) Gradebook.cpp (no need to modify) GradebookRunner.cpp (no need to modify) Student.h (no need to modify) Student.cpp (this is where you'll do your work) You need to implement the Student class. We’ll go into this a little more in a bit.
3	Gradebook Interface typedef Student* StudentPtr; class Gradebook { public: Gradebook(CString aName); ~Gradebook(); CString getName() const; void addStudent(const Student& aStudent); void addGrade(CString aStudentId, const string& aGrade); void passByValue(Student aStudent) const; private: CString mName; StudentPtr mStudents; int mStudentSize; map<CString, string*> mGrades; map<CString, int> mNumOfGrades; };
4	Student Interface typedef char* CString; #include <cstddef> // NULL class Student { public: Student(); Student(const CString aName, const CString aId); ~Student(); CString getName() const; void setName(const CString aName); CString getStudentId() const; void setId(const CString aId); Student& operator=(const Student aStudent); private: CString mName; CString mId; };
5	GradebookRunner – main() cout << "************Student Declaration***************\n"; Student student1; student1.setName("John Doe"); student1.setId("00002222"); cout << "**********Gradebook Declaration***************\n"; Gradebook csci123GB("CSCI 123 Gradebook"); cout << "**********Add a Student to Gradebook***************\n"; csci123GB.addStudent(student1); cout << "**********Add a Grade to Gradebook***************\n"; csci123GB.addGrade(student1.getStudentId(), "A"); cout << "**********Add a Grade to Gradebook***************\n"; csci123GB.addGrade(student1.getStudentId(), "B"); cout << "**********Pass a Student by value***************\n"; csci123GB.passByValue(student1); cout << "**********End Pass a Student by value*****************\n";
6	What you need to do That explains what you need to do. Again you only need to modify the Student.cpp file. Here's the modifications: add any namespaces and/or includes Implement the default constructor add code so that the student name is set to "No Name" and the student id is set to "No Id" by default Implement the two parameter constructor set the object's name and the id to the parameters passed to the constructor Implement the code for the copy constructor. Set the object's members to the Student passed to the copy constructor Implement the destructor for the Student class. It should remove any allocated memory from the heap. Implement the assignment operator. It should assign the left hand Student to the right hand student. Don't remove my return statement. Implement the setName member function. Implement the setId member function.
7	Student.cpp Each of the functions that you need to implement has the following comment above the function. The function also has couts in the function bodies, you want to leave these in the file or you won’t get the proper output The assignment should show you how the constructors, destructors, and assignment operators are called
8	Gradebook Implementation Gradebook::Gradebook(const CString aName) : mStudentSize(0) { cout << "Gradebook() - Default Constructor\n"; cout << "\tAllocating memory for a name of length " << strlen(aName) << endl; mName = new char[strlen(aName)+1]; strcpy(mName, aName); cout << "\tAllocating memory for 10 students\n"; mStudents = new Student[10]; }
9	Gradebook Implementation Gradebook::~Gradebook() { cout << "~Gradebook() - Destructor\n"; cout << "\tDeleting Gradebook Name\n"; delete [] mName; cout << "\tDeleting Gradebook Students\n"; delete [] mStudents; int i = 0; for (map<CString, string*>::iterator p = mGrades.begin( ); p != mGrades.end(); ++p ) { cout << "Deleting the collection of grades "; cout << "for student " << p->first << endl; cout << endl; // p-> returns a string pointer delete [] p->second; // delete the collection of grades // for this student } }
10	Gradebook Implementation CString Gradebook::getName() const { return mName; }
11	Gradebook Implementation void Gradebook::addStudent(const Student& aStudent) { cout << "Adding a student\n"; if(!atoi(aStudent.getStudentId())) { cout << "Invalid student id!\n"; cout << "You can only add a student with a valid student id\n\n"; } else if(mStudentSize >= 0 && mStudentSize <= 9) { mStudents[mStudentSize++] = aStudent; cout << "Allocating memory for ten grades for this student\n"; mGrades[aStudent.getStudentId()] = new string[10]; // set the number of grades for this student to // zero mNumOfGrades[aStudent.getStudentId()] = 0; } else { // only allow 10 grades for each student cout << "Gradebook is full of students\n"; } }
12	Gradebook Implementation void Gradebook::addGrade(CString aStudentId, const string& aGrade) { if(!atoi(aStudentId)) { cout << "Invalid student id!\n"; cout << "You can only add a grade with a valid student id\n\n"; } else if(mNumOfGrades[aStudentId] >= 0 && mNumOfGrades[aStudentId] <= 9) { cout << "Adding a grade for student with id " << aStudentId << endl; mGrades[aStudentId][mNumOfGrades[aStudentId]] = aGrade; mNumOfGrades[aStudentId] = mNumOfGrades[aStudentId] + 1; } else { cout << "Gradebook is full of grades\n"; } }
13	Gradebook Implementation void Gradebook::passByValue(Student aStudent) const { cout << "PassByValue aStudent\n"; // what gets called here? }
14	Overview 15.1 Inheritance Basics 15.2 Inheritance Details 15.3 Polymorphism
15	"Inheritance" Inheritance
16	Inheritance Basics Inheritance is the process by which a new class, called a derived class, is created from another class, called the base class A derived class automatically has all the member variables and functions of the base class A derived class can have additional member variables and/or member functions The derived class is a child of the base or parent class
17	Shape Classes
18	A Base Class We will define a class called Shape for all Shapes (Rectangles and Circles) The Shape class will be used to define classes for Rectangles and Circles A definition of the Shape class is found in following files: Shape.h and Shape.cpp Circle.h and Circle.cpp Rectangle.h and Rectangle.cpp ShapeRunner.cpp
19	Class Rectangle & Circle Rectangle and Circle are derived from the Shape class Rectangle and Circle inherit all member functions and member variables of Shape The class definition begins class Rectangle : public Shape :public Shape shows that Rectangle is derived from Shape Rectangle declares additional member variables width and height Circle declares additional member variable radius
20	Inherited Members A derived class inherits all the members of the parent class The derived class does not re-declare or re-define members inherited from the parent, except… The derived class re-declares and re-defines member functions of the parent class that will have a different definition in the derived class The derived class can add member variables and functions
21	Implementing a Derived Class Any member functions added in the derived class are defined in the implementation file for the derived class Definitions are not given for inherited functions that are not to be changed The Rectangle and Circle implementation can be found in the Rectangle.cpp and Circle.cpp files
22	Function toString() Function toString() will have different definitions in the Shape and Circle class toString prints the color and whether the shape is filled toString is redefined in the Circle class the toString() function in the Circle class hides the function in the base class, Shape
23	Parent and Child Classes Recall that a child class automatically has all the members of the parent class The parent class is an ancestor of the child class The child class is a descendent of the parent class The parent class (Shape) contains all the code common to the child classes You do not have to re-write the code for each child
24	Derived Class Types An Circle is a Shape In C++, an object of type Circle can be used where an object of type Shape can be used An object of a class type can be used wherever any of its ancestors can be used An ancestor cannot be used wherever one of its descendents can be used
25	Derived Class Constructors A base class constructor is not inherited in a derived class The base class constructor can be invoked by the constructor of the derived class The constructor of a derived class begins by invoking the constructor of the base class in the initialization section: Circle::Circle : Shape( ), radius(1) { //no code needed }
26	Default Initialization If a derived class constructor does not invoke a base class constructor explicitly, the base class default constructor will be used If class B is derived from class A and class C is derived from class B When a object of class C is created The base class A's constructor is the first invoked Class B's constructor is invoked next C's constructor completes execution
27	Calling Base Class Constructors
28	Default Initialization Circle constructor: Circle::Circle(double radius) : Shape("black", false) { this->radius = radius; }
29	Private is Private A member variable (or function) that is private in the parent class is not accessible to the child class The parent class member functions (accessors) must be used to access the private members of the parent This code would be illegal: string Circle::toString() { return "Circle color " + this->color + " filled " + ((this->filled) ? "true" : "false"); } color and filled is a private member of Shape!
30	The protected Qualifier protected members of a class appear to be private outside the class, but are accessible by derived classes If member variables name, color, and filled are listed as protected (not private) in the Shape class, this code, illegal on the previous slide, becomes legal: string Circle::toString() { return "Circle color " + this->color + " filled " + ((this->filled) ? "true" : "false"); }
31	Programming Style Using protected members of a class is a convenience to facilitate writing the code of derived classes. Protected members are not necessary Derived classes can use the public functions of their ancestor classes to access private members Many programming authorities consider it bad style to use protected member variables
32	Redefinition of Member Functions When defining a derived class, only list the inherited functions that you wish to change for the derived class The function is declared in the class definition Circle has its own definition of toString() How does C++ know to call the derived class definition of toString()?
33	Redefining or Overloading A function redefined in a derived class has the same number and type of parameters The derived class has only one function with the same name as the base class An overloaded function has a different number and/or type of parameters than the base class The derived class has a function with the same name as the base class One (function – toString() ) is defined in the base class (Shape), one in the derived class (Circle)
34	Function Signatures A function signature is the function's name with the sequence of types in the parameter list, not including any const or '&' An overloaded function has multiple signatures Some compilers allow overloading based on including const (Visual C++) or not including const
35	Access to a Redefined Base Function When a base class function is redefined in a derived class, the base class function can still be used To specify that you want to use the base class version of the redefined function: cout << "Base Class toString()\n"; cout << circle.Shape::toString() << endl;
36	Section 15.1 Conclusion Can you Explain why the declaration for getColor is not part of the definition of Circle? Give a definition for a class Triangle derived from class Shape with one additional string called title? Add two member functions getTitle and setTitle.
37	"Inheritance Details" Inheritance Details
38	Inheritance Details Some special functions are, for all practical purposes, not inherited by a derived class Some of the special functions that are not effectively inherited by a derived class include Destructors Copy constructors The assignment operator
39	Copy Constructors and Derived Classes If a copy constructor is not defined in a derived class, C++ will generate a default copy constructor This copy constructor copies only the contents of member variables and will not work with pointers and dynamic variables The base class copy constructor will not be used Why won’t the generated copy constructor work for pointers?
40	Operator = and Derived Classes If a base class has a defined assignment operator = and the derived class does not: C++ will use a default operator that will have nothing to do with the base class assignment operator
41	Destructors and Derived Classes A destructor is not inherited by a derived class The derived class should define its own destructor
42	The Assignment Operator In implementing an overloaded assignment operator in a derived class: It is normal to use the assignment operator from the base class in the definition of the derived class's assignment operator Recall that the assignment operator is written as a member function of a class
43	The Operator = Implementation This code segment shows how to begin the implementation of the = operator for a derived class: Derived& Derived::operator= (const Derived& rhs) { Base::operator=(rhs) // call the base class operator This line handles the assignment of the inherited member variables by calling the base class assignment operator The remaining code would assign the member variables introduced in the derived class
44	The Copy Constructor Implementation of the derived class copy constructor is much like that of the assignment operator: Derived::Derived(const Derived& object) :Base(object), <other initializing> {…} Invoking the base class copy constructor sets up the inherited member variables Since object is of type Derived it is also of type Base
45	Destructors in Derived Classes If the base class has a working destructor, defining the destructor for the derived class is relatively easy When the destructor for a derived class is called, the destructor for the base class is automatically called The derived class destructor need only use delete on dynamic variables added in the derived class, and data they may point to
46	Destruction Sequence If class B is derived from class A and class C is derived from class B… When the destructor of an object of class C goes out of scope The destructor of class C is called Then the destructor of class B Then the destructor of class A Notice that destructors are called in the reverse order of constructor calls
47	Section 15.2 Conclusion Can you List some special functions that are not inherited by a derived class? Write code to invoke the base class copy constructor in defining the derived class's copy constructor?
48	"Polymorphism" Polymorphism
49	Polymorphism Polymorphism refers to the ability to associate multiple meanings with one function name using a mechanism called late binding Polymorphism is a key component of the philosophy of object oriented programming
50	A Late Binding Example Imagine a graphics program with several types of figures Each figure may be an object of a different class, such as a Circle, Oval, Rectangle, etc. Each is a descendant of a class Shape Each has a function draw( ) implemented with code specific to each shape Class Shape has functions common to all shapes
51	A Problem Suppose that class Shape has a function center Function center moves a shape to the center of the screen by erasing the shape and redrawing it in the center of the screen Function center is inherited by each of the derived classes Function center uses each derived object's draw function to draw the shape The Shape class does not know about its derived classes, so it cannot know how to draw each figure
52	Virtual Functions Because the Shape class includes a method to draw shapes, but the Shape class cannot know how to draw the different shapes, virtual functions are used Making a function virtual tells the compiler that you don't know how the function is implemented and to wait until the function is used in a program, then get the implementation from the object. This is called late binding
53	No Polymorphic Behavior void displayObject(BaseClass someClass) { cout << someClass.toString().data() << endl; } // each class has its own toString() function int main() { displayObject(BaseClass()); displayObject(DerivedClass1()); displayObject(DerivedClass2()); return 0; }
54	No Polymorphic Behavior Application Output: Base Class Base Class Base Class
55	Polymorphic Behavior class BaseClass { public: virtual string toString() { return "Base Class"; } }; class DerivedClass1: public BaseClass { string toString() { return "DerivedClass 1"; } };
56	Polymorphic Behavior void displayObject(BaseClass someClass) { cout << someClass->toString().data() << endl; } int main() { BaseClass bc = new BaseClass(); DerivedClass1 dc1 = new DerivedClass1(); DerivedClass2 dc2 = new DerivedClass2(); displayObject(bc); displayObject(dc1); displayObject(dc2); return 0; }
57	Polymorphic Behavior BaseClass toString is declared as virtual, so that polymorphism is turned on. Meaning the compiler will wait till runtime to determine which function is called The parameter to displayObject is changed to a pointer
58	Define Virtual Functions To enable dynamic binding for a function, you need to do two things: The function must be declared virtual in the base class. The variable that references the object for the function must contain the address (be a pointer) of the object. The keyword virtual tells C++ to wait until displayObject is used in a program to get the implementation of displayObject from the calling object
59	Virtual Details To define a function differently in a derived class and to make it virtual Add keyword virtual to the function declaration in the base class virtual is not needed for the function declaration in the derived class, but is often included virtual is not added to the function definition Virtual functions require considerable overhead so excessive use reduces program efficiency
60	Overriding Virtual functions whose definitions are changed in a derived class are said to be overridden Non-virtual functions whose definitions are changed in a derived class are redefined
61	Type Checking C++ carefully checks for type mismatches in the use of values and variables This is referred to as strong type checking Generally the type of a value assigned to a variable must match the type of the variable Recall that some automatic type casting occurs Strong type checking interferes with the concepts of inheritance
62	Type Checking and Inheritance Consider class Pet { public: virtual void print(); string name; } and class Dog :public Pet { public: virtual void print(); string breed; }
63	A Sliced Dog is a Pet C++ allows the following assignments: vdog.name = "Tiny"; vdog.breed = "Great Dane"; vpet = vdog; However, vpet will loose the breed member of vdog since an object of class Pet has no breed member This code would be illegal: cout << vpet.breed; This is the slicing problem
64	The Slicing Problem It is legal to assign a derived class object into a base class variable This slices off data in the derived class that is not also part of the base class Member functions and member variables are lost
65	Extended Type Compatibility It is possible in C++ to avoid the slicing problem Using pointers to dynamic variables we can assign objects of a derived class to variables of a base class without loosing members of the derived class object
66	Dynamic Variables and Derived Classes Example: ppet->print( ); is legal and produces: name: Tiny breed: Great Dane
67	Use Virtual Functions The previous example: ppet->print( ); worked because print was declared as a virtual function This code would still produce an error: cout << "name: " << ppet->name << "breed: " << ppet->breed;
68	Why? ppet->breed is still illegal because ppet is a pointer to a Pet object that has no breed member Function print( ) was declared virtual by class Pet When the computer sees ppet->print( ), it checks the virtual table for classes Pet and Dog and finds that ppet points to an object of type Dog Because ppet points to a Dog object, code for Dog::print( ) is used
69	Remember Two Rules To help make sense of object oriented programming with dynamic variables, remember these rules If the domain type of the pointer pParent is a base class for the for the domain type of pointer pChild, the following assignment of pointers is allowed pParent = pChild; and no data members will be lost Although all the fields of the pChild are there, virtual functions are required to access them Think about the object that was created. What was the constructor that was called? What is at the address?
70	Virtual Compilation When using virtual functions, you will have to define each virtual function before compiling Declaration is no longer sufficient Even if you do not call the virtual function you may see error message: "undefined reference to ClassName virtual table"
71	Virtual Destructors Destructors should be made virtual? Consider Base *pBase = new Derived; … delete pBase; If the destructor in Base is virtual, the destructor for Derived is invoked as pBase points to a Derived object, returning Derived members to the freestore The Derived destructor in turn calls the Base destructor
72	Non-Virtual Destructors If the Base destructor is not virtual, only the Base destructor is invoked This leaves Derived members, not part of Base, in memory
73	static matching vs. dynamic binding
74	Section 15.3 Conclusion Can you Explain why you cannot assign a base class object to a derived class object? Describe the problem with assigning a derived class object to a base class object?