CSE240 Lecture 17

Introduction to Programming Languages
new and delete
(202110)

Transcript

1. jgs CSE 240 Introduction to Programming Languages Lecture 17: new and delete Dr. Javier Gonzalez-Sanchez javiergs@asu.edu javiergs.engineering.asu.edu | javiergs.com PERALTA 230U Office Hours: By appointment
2. jgs Previously
3. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 3 jgs Example in One File: queue.cpp 1. #include <iostream> 2. using namespace std; 3. class Queue { 4. private: 5. int queue_size; 6. protected: 7. int *buffer; 8. int front; 9. int rear; 10. public: 11. Queue(int v) { 12. cout<<"constructor\n"; 13. } 14. void enqueue(int v) { 15. cout<<"enqueue\n"; 16. } 17. int dequeue(void){ 18. cout<<"dequeue\n"; return 5; 19. } 20. }; 21. int main(){ 22. Queue q1(5); 23. Queue *q2 = new Queue(5); 24. 25. // Access Object 26. q1.enqueue(2); 27. q1.enqueue(8); 28. 29. // Access Object Pointer 30. q2->enqueue(25); 31. int x = q2->dequeue(); 32. return 0; 33. }
4. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 4 jgs Example in Two Files: time.h 1. class Time { 2. public: 3. Time(); // constructor 4. void setTime( int, int ); // set hour, minute 5. void printMilitary(); // print military time format 6. void printStandard(); // print standard time format 7. 8. private: 9. int hour; // 0 - 23 10. int minute; // 0 - 59 11. };
5. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 5 jgs Example in Two Files: time.cpp 1. #include <iostream> 2. #include "time.h” 3. using namespace std; 4. Time::Time() { // constructor 5. hour = minute = 0; 6. } 7. void Time::setTime( int h, int m) { // Set a new mil Time 8. hour = ( h >= 0 && h < 24 ) ? h : 0; 9. minute = ( m >= 0 && m < 60 ) ? m : 0; 10.} 11.void Time::printMilitary() { // Print time in military format 12. cout << ( hour < 10 ? "0" : "" ) << hour << ":" 13. << ( minute < 10 ? "0" : "" ) << minute; // add "0" 14.}
6. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 6 jgs Example in Two Files: time.cpp 15. void Time::printStandard() { // Print in standard format 16. cout << ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 ) 17. << ":" << ( minute < 10 ? "0" : "" ) << minute 18. << ( hour < 12 ? " AM" : " PM" ) << endl; //endl is equal to “\n” 19. } 20. 21. int main() { 22. Time t; // new is not mandatory - instantiate object t of class Time 23. cout << "The initial military time is "; 24. t.printMilitary(); 25. cout << "\nThe initial standard time is "; 26. t.printStandard(); 27. t.setTime(15, 27); 28. cout << "\n\nMilitary time after setTime is "; 29. t.printMilitary(); 30. cout << "\nStandard time after setTime is "; 31. t.printStandard(); 32. return 0; 33. }
7. jgs Next: Memory Management
8. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 8 jgs Outline § static, § constructor and destructor, § new and delete
9. jgs static
10. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 10 jgs Example 1. #include <iostream> 2. using namespace std; 3. class Foo { 4. public: 5. void counting(); 6. int getCounterB(); 7. static int counterA; 8. private: 9. int counterB; 10. }; 11. 12. // initial value to static member variable 13. int Foo::counterA = 0; 14. int counter = 0; A static variable is shared by all the objects from a class. Therefore, changes made to a static variable will impact all objects from the class
11. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 11 jgs Example 1. #include <iostream> 2. using namespace std; 3. class Foo { 4. public: 5. void counting(); 6. int getCounterB(); 7. static int counterA; 8. private: 9. int counterB; 10. }; 11. 12. // initial value to static member variable 13. int Foo::counterA = 0; 14. int counter = 0; static versus global Variables static variables prevents functions outside the class to access the variable. As a global variable, all other functions can read/write it. A static variable is shared by all the objects from a class. Therefore, changes made to a static variable will impact all objects from the class
12. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 12 jgs Example 18. void Foo::counting() { // Set a new mil Time 19. counterA++; 20. counterB++; 21. } 22. int Foo::getCounterB() { 23. return counterB; 24. } 25. int main() { 26. Foo f1, f2, f3; 27. f1.counting(); 28. f2.counting(); 29. f2.counting(); 30. f3.counting(); 31. cout << Foo::counterA<<endl; 32. cout << f1.getCounterB()<<endl; 33. cout << Foo::counterA<<endl; 34. cout << f2.getCounterB()<<endl; 35. return 0; 36. } A static variable go out of scope only if the program terminated. And it can be used even without creating an object from the class.
13. jgs Constructors and destructors
14. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 14 jgs Constructor and Destructor A constructor in a class: § is a function whose name is same as the class name, and § is used to automatically initialize objects. A destructor in a class: § is a function whose name is same as the class name (with a ~ as prefix), and § is used to collect garbage.
15. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 15 jgs Constructor and Destructor #include <iostream> using namespace std; class Queue { private: int queue_size; protected: int *buffer; int front; int rear; public: Queue(); Queue(int n); ~Queue(); }; Queue::Queue() { // constructor cout << "constructor(void)"<<endl; // code... } Queue::Queue(int n) { // constructor overload cout << "constructor (int)"<<endl; // code... } Queue::~Queue(void) { cout << "destructor"<<endl; // code... } int main () { Queue myQueue1(500); Queue myQueue2; // more code... return 0; }
16. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 16 jgs Constructor and Destructor #include <iostream> using namespace std; class Queue { private: int queue_size; protected: int *buffer; int front; int rear; public: Queue(); Queue(int n); ~Queue(); }; Queue::Queue() { // constructor cout << "constructor(void)"<<endl; // code... } Queue::Queue(int n) { // constructor overload cout << "constructor (int)"<<endl; // code... } Queue::~Queue(void) { cout << "destructor"<<endl; // code... } int main () { Queue myQueue1(500); Queue myQueue2; // more code... return 0; }
17. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 17 jgs When is a destructor called? § When a local object (from stack) with block scope goes out of scope. § When a program (main function) ends and global or static objects exist (OS will collect them anyway). § When the destructor is explicitly called. § When the delete keyword is called.
18. jgs new and delete
19. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 19 jgs new and delete int main () { Queue myQueue2(500); // declare a pointer only Queue *myQueue; // reserve memory for an object myQueue = new Queue(500); // use the object myQueue->enqueue(23); myQueue2.enqueue(8); // delete will call ~Queue(); delete myQueue; ... delete myQueue2;//no needed }
20. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 20 jgs new and delete with constructors and destructors #include <iostream> using namespace std; class Queue { private: int queue_size; protected: int *buffer; int front; int rear; public: Queue(); Queue(int n); ~Queue(); }; Queue::Queue() { // constructor cout << "constructor(void)"<<endl; // code... buffer = NULL; } Queue::Queue(int n) { // constructor overload cout << "constructor (int)"<<endl; // code... buffer = new int[queue_size]; } Queue::~Queue(void) { cout << "destructor"<<endl; // code... delete [] buffer; } int main () { Queue myQueue1(500); Queue myQueue2; Queue *myQueue3 = new Queue(100); // more code... delete myQueue3; return 0; }
21. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 21 jgs Summary • If an object is on the stack, instead of on the heap, destructor will be called when the object goes out of scope. No delete operation is necessary. • All heap objects must be explicitly deleted before leaving the function, if they are no longer needed. • The function delete will implicitly call the destructor of the class, so that an object linked to a variable in the to-be-deleted object can be de- allocated too, i.e., using delete for variables created in the class (normally in the constructor).
22. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 22 jgs delete and Array of Objects #include <iostream> using namespace std; #define size 4 class arrayObject { public: int x; double y; arrayObject() { cout << "arrayObject's constructor called" << endl; } ~arrayObject() { cout << "arrayObject's destructor called" << endl; } }; int main() { arrayObject *p, *q; // declare two pointers p = new arrayObject[size]; // Create an array of objects for (q = p; q < p + size; q++) { // Initialize the objects q->x = 10; q->y = 1.5; } for (q = p; q < p + size; q++) { cout << "Element address " << q << " Element x value: " << q->x << endl; cout << "Element address " << q << " Element y value: " << q->y << endl; } delete[] p; return 0; }
23. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 23 jgs Summary § How do we delete an array of objects? We can use a loop to delete each element, § However, the language provides a library function to delete all the elements one by one without the user to explicitly use a loop: delete[] array;
24. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 24 jgs Summary Java § Primitive variables (int, float, boolean) use value type. § All other variables (string, array, user defined classes) use reference type. (Java uses automatic garbage collection). C++ § Both value and reference types exist: § if value semantics is used then memory will be allocated on stack. Memory de- allocation is done automatically. § if reference semantics is used (e.g. variable is a pointer to an object), then memory must be allocated explicitly using new and explicitly de-allocated using delete
25. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 25 jgs Summary // in C++ Report r; // an object is allocated to r Report *rp1, *rp2; // two pointers declared rp1 = &r; // rp1 points to object r rp2 = new Report(); // an object is created, linked to rp2 // .. delete rp2; // in Java Report r; // an reference is allocated r = new Report (); // an object is created and linked to r
26. Javier Gonzalez-Sanchez | CSE240 | Fall 2021 | 26 jgs Questions
27. jgs CSE 240 Introduction to Programming Languages Javier Gonzalez-Sanchez, Ph.D. javiergs@asu.edu Fall 2021 Copyright. These slides can only be used as study material for the class CSE240 at Arizona State University. They cannot be distributed or used for another purpose.