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Programming Language Concepts Lecture 19 Prepared by Manuel E. Bermúdez, Ph.D. Associate Professor University of Florida Object-Oriented Programming (Part 2)
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The this Pointer To invoke a class member function, use the ''dot'' operator (structure field selection):. The this pointer: an implicit parameter to each class member function, of type (Object *). Its value is the address of the “current” object, through which the member function has been invoked. It can be used inside member functions. Examples coming up.
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The const Qualifier Three places where the const qualifier can be used in a member function declaration: class A {... const X& f(const Y& y) const { // 1. Can't assign to object returned // 2. Can't change parameter y // 3. Can't change this. }
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The const Q ualifier (cont’d) A a;... a.f(y) // f is a member of class A. // a is an object of class A. // a.f(y) can't be assigned a value. // a.f(y) cannot change y. // a.f(y) cannot change a.
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Constructors class cell { cell(int i, cell *n) {info=i; next=n;} int info; cell *next; }; cell a(1,0); //declare a and call cell(1,0) cell b(2,&a); //declare b and call cell(2,&a) These don’t just declare a and b of type cell. They also call the constructor function cell.
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Overloading Constructors class cell { cell (int i) {info=i; next=this;} cell (int i, cell *n) { info=i; next=n; } int info; cell *next; }; cell c(7);
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Friend Classes and Information Hiding Class members are private by default. A friend declaration in a class allows functions from another class to access members of the current one. class cell { friend class circlist; cell (int i) {info=i; next=this;} cell (int i, cell *n) {info=i; next=n;} int info; cell *next; }; cell has no public interface. It’s an 'auxiliary' data type, solely for use with circlist.
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Friend Classes and Information Hiding class circlist { private: cell *rear; public: circlist() {rear=new cell(0);} ~circlist(); int empty() const {return rear==rear->next;} int top() const {return rear->next->info; } void push (int); int pop(); void enter(int); }; The class circlist has a total of seven member functions, four of which still need to be defined.
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Friend Classes and Information Hiding circlist::~circlist(){ cell *tmp1=rear, *tmp2=rear->next; while (tmp2!=rear) { delete tmp1; tmp1=tmp2; tmp2=tmp1->next; }; delete tmp1; } void circlist::push(int x) { rear->next = new cell (x, rear->next); }
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Friend Classes and Information Hiding void circlist::enter(int x) { rear->info = x; rear = rear->next = new cell(0,rear->next); } int circlist::pop() { if ( empty() ) return 0; cell *front = rear->next; rear->next = front->next; int x = front->info; delete front; return x; }
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Friend Classes and Information Hiding Class circlist hides its internal details. Objects of type circlist are manipulated exclusively through the public interface of circlist. Information hidden inside circlist : circlist based on cell, only rear is stored. Should the implementation of circlist ever change, users of circlist will be COMPLETELY UNAFFECTED. Three principles of Object-Oriented Programming: Abstraction, Encapsulation, Information Hiding.
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Derived Classes Humans tend to abstract on two dimensions: A part-of B, (a.k.a. has_a) A kind-of B, (a.k.a. is_a). ADT programming focuses on the has_a relation. OOP also includes the is_a abstraction. Support for the is_a abstraction implies: if A is_a B, and B has_a X, then A has_a X. This is inheritance !!
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Derived Classes (cont’d) In general, class : public { } All members of are also members of. The may have additional members. public clause: derived members retain their attributes (public, private, or protected). private clause: derived members will be private.
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The circlist class, revisited class circlist { public: int empty () const { return rear==rear->next;} int top() const { return rear->next->info; } // 'inspectors' protected: circlist () {rear = new cell(0);}; ~circlist(); void push(int); int pop(); void enter(int); // 'mutators' private: cell *rear; }
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The circlist class, revisited Protected members behave as if they were private, except that: –they are visible to members (and friends) of derived classes. Some of the circlist functions are now protected so they'll be inherited by the new class queue.
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Derived class queue class queue: private circlist { public: queue() { } ~queue() { } void enqueue (int x) { enter(x); } int dequeue () { return pop(); } circlist::empty; circlist::top; };
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Complete list of members of queue Public Functions: queue new constructor ~queue new destructor enqueue new dequeue new empty, top inherited, explicitly made public Private Functions: push inherited pop inherited enter inherited Private Variables (accessible to new functions of queue): none Private Variables (accessible only by inherited functions): rear inherited
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Another class derived from circlist class stack: private circlist { public: stack () { } ~stack () { } void push (int x) { circlist::push(x); } int pop() { return circlist::pop(); } circlist::empty; circlist::top; };
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Sample Use of these Classes main () { stack s; queue q; s.push(1); s.push(2); s.push(3); q.enqueue(7); q.enqueue(8); q.enqueue(9); cout << "Stack Top: " << s.top() << endl; cout << "Queue Top: " << q.top() << endl; cout << s.pop() << " "; cout << s.pop() << endl; cout << q.dequeue() << " "; cout << q.dequeue() << endl; } Results: Stack Top: 3 Queue Top: 7 3 2 1 7 8 9
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Class Hierarchies class employee { /*... */ }; class manager: public employee { /*... */ }; class director: public manager { /*... */ }; class temporary { /*... */ }; class secretary: public employee { { /*... */ }; class tsec: public temporary, //MULTIPLE INHERITANCE !! public secretary { /*... */ }; class consultant: public temporary, public manager { /*... */ };
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Class Hierarchy is a DAG
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Type Identification In our example, given a pointer of type employee*, it can point to: employee, secretary, tsec, manager, or director. Three solutions, in general, to this problem: 1.Only use pointers to classes that have no derived classes. Sure... 2.Use a variable to determine the type of the object. Very easy to forget to check the type variable. 3.Use virtual functions.
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Virtual Functions Allow print in both employee and manager, with different definitions. C++ will “do the right thing”, based on the actual object class.
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Virtual Functions (cont’d)
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Operator Overloading Allow intrinsic operators in C++ to be applied to objects of new types. Overloading is achieved by defining functions named operatorXXX, where XXX is one of: Can’t overload these: Can’t change precedence, arity or associativity. Can’t add new operators, either.
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Operator Overloading (cont’d) Example: the subscript operator, which returns a reference. First, without operator overloading:
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Operator Overloading (cont’d) Now, simply replace elem with operator[]:
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Overloading Operators > We wish to use > for user-defined objects, the same way they are (normally) used for "built-in" objects (e.g. int, char, etc.).
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Overloading Operators > Input is similar. The signature is: To use them:
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The Orthodox Canonical Form An idiom (pattern). The OCF is characterized by the presence of: A default constructor: X::X() A copy constructor: X::X(const X&) An assignment operator: X& operator=(const X&) A destructor: X::~X()
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Programming Language Concepts Lecture 19 Prepared by Manuel E. Bermúdez, Ph.D. Associate Professor University of Florida Object-Oriented Programming (Part 2)
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