Destructors Math 130 Lecture # xx Mo/Da/Yr B Smith: New lecture for 05. Use this for evolving to Java B Smith: New lecture for 05. Use this for evolving to Java B Smith: Save this for later or use in Fa06, or is this redundant? B Smith: Save this for later or use in Fa06, or is this redundant? B Smith: IS this too long? Should it be shortened? B Smith: IS this too long? Should it be shortened? B Smith: Covering derived classes more important B Smith: Covering derived classes more important

2 Overview

3 Destructors When a class object goes out of scope, Destructors “clean house”  When a function is called, it may work with temporary variables  A linked-list class might use a constructor to build nodes (malloc() ), and a destructor to free nodes (free() ) It’s good practice to clean up after your objects and free any allocated memory Use the name of the class, preceded by a tilde (~) Destructors receive no arguments and return no values

4 Destructors The class destructor neither takes an argument nor returns a value For the class Date, the prototype of the class destructor is  ~Date(); For the class Cat, the prototype of the class destructor is  ~Cat(); It's an error to specify a return value or parameters for a destructor. B Smith: redundant here? IS there a better place? B Smith: redundant here? IS there a better place?

5 Default Destructor All objects have been destroyed automatically by the default destructor In the examples where we’ve not explicitly defined a destructor, the compiler creates one Note that the default destructor does not automatically delete/free objects that have been allocated on the system heap (free store)!

6 int main() { Box Boxes[5]; // Array of Box objects declared Box Cigar(8.0, 5.0, 1.0); // Declare Cigar box Box Match(2.2, 1.1, 0.5); // Declare Match box // Initialize pointer to Cigar object address Box* pB1 = &Cigar; Box* pB2 = 0; // Pointer initialized to null cout << endl << "Volume of Cigar is " Volume(); // Volume of obj. pointed to pB2 = Boxes; // Set to address of array Boxes[2] = Match; // Set 3rd element to Match cout << endl // Now access thru pointer Volume(); cout << endl; return 0; } B Smith: Should really be focused on using references B Smith: Should really be focused on using references

7 A Simple Destructor // Ex7_01.cpp // Class with an explicit destructor #include using namespace std; class Box // Class definition at global scope { public: // Destructor definition ~Box() { cout << "Destructor called." << endl; } // Constructor definition Box(double lv=1.0, double bv=1.0, double hv=1.0) { cout << endl << "Constructor called."; length = lv; // Set values of breadth = bv; // data members height = hv; } // Function to calculate the volume of a box double Volume() { return length * breadth * height; } // Function to compare two boxes which // returns true if the first is greater // than the second, and false otherwise int compare(Box* pBox) { return Volume() > pBox->Volume(); } private: double length; // Length of a box in inches double breadth; // Breadth of a box in inches double height; // Height of a box in inches }; int main() { Box Boxes[5]; // Array of Box objects declared Box Cigar(8.0, 5.0, 1.0); // Declare Cigar box Box Match(2.2, 1.1, 0.5); // Declare Match box // Initialize pointer to Cigar object address Box* pB1 = &Cigar; Box* pB2 = 0; // Pointer initialized to null cout << endl << "Volume of Cigar is " Volume(); // Volume of obj. pointed to pB2 = Boxes; // Set to address of array Boxes[2] = Match; // Set 3rd element to Match cout << endl // Now access thru pointer Volume(); cout << endl; return 0; } B Smith: Use colors and provide as a basic handout. Put online as a separate file. B Smith: Use colors and provide as a basic handout. Put online as a separate file. B Smith: only useful in that it shows what happens with an array. Student had a great question regarding which Constructor is called. Better explanation needed. ….. Uhhh, here, there is only one constructor B Smith: only useful in that it shows what happens with an array. Student had a great question regarding which Constructor is called. Better explanation needed. ….. Uhhh, here, there is only one constructor

8 // Ex6_03.cpp // Using a constructor #include using namespace std; class Box // Class definition at global scope { private: double length; // Length of a box in inches double breadth; // Breadth of a box in inches double height; // Height of a box in inches public: // Constructor definition Box(double lv, double bv, double hv) { cout << endl << "Constructor called."; length = lv; // Set values of breadth = bv; // data members height = hv; } // Function to calculate the volume of a box double Volume() { return length * breadth * height; } }; Box Class

9 int main(void) { Box Box1(78.0,24.0,18.0); // Declare and initialize Box1 Box CigarBox(8.0,5.0,1.0); // Declare and initialize CigarBox double volume = 0.0; // Store the volume of a box here volume = Box1.Volume(); // Calculate volume of Box1 cout << endl << "Volume of Box1 = " << volume; cout << endl << "Volume of CigarBox = " << CigarBox.Volume(); cout << endl; return 0; } Box Class Client

10 How It Works The constructor, Box(), has been written with three parameters of type double, corresponding to the initial values for the length, breadth and height members of a Box object. The first statement in the constructor outputs a message so that we can tell when it's been called. You wouldn't do this in production programs, but, since it's very helpful in showing when a constructor is called, it's often used when testing a program. We'll use it regularly for the purposes of illustration. The code in the body of the constructor is very simple. It just assigns the arguments passed to the corresponding data members. If necessary, we could also include checks that valid, non-negative arguments are supplied and, in a real context, you probably would want to do this, but our primary interest here is in seeing how the mechanism works. Within main(), we declare the object Box1 with initializing values for the data members length, breadth, and height, in sequence. These are in parentheses following the object name. This uses the functional notation for initialization, which can also be applied to initializing ordinary variables of basic types. We also declare a second object of type Box, called CigarBox, which also has initializing values. The volume of Box1 is calculated using the member function Volume() as in the previous example and is then displayed on the screen. We also display the value of the volume of CigarBox. The output from the example is: The first two lines are output from the two calls of the constructor, Box(), once for each object declared. The constructor that we've supplied in the class definition is automatically called when a Box object is declared, so both Box objects are initialized with the initializing values appearing in the declaration. These are passed to the constructor as arguments, in the sequence that they are written in the declaration. As you can see, the volume of Box1 is the same as before and CigarBox has a volume looking suspiciously like the product of its dimensions, which is quite a relief.

11 The Default Constructor Try modifying the last example by adding the declaration for Box2 that we had previously:  Box Box2; // Declare Box2 of type Box Here, we've left Box2 without initializing values. When you rebuild this version of the program, you'll get the error message:  error C2512: 'Box': no appropriate default constructor available

12 Supply a Default Constructor // Ex6_04.cpp // Supplying and using a default constructor #include using namespace std; class Box // Class definition at global scope { public: double length; // Length of a box in inches double breadth; // Breadth of a box in inches double height; // Height of a box in inches // Constructor definition Box(double lv, double bv, double hv) { cout << endl << "Constructor called."; length = lv; // Set values of breadth = bv; // data members height = hv; } // Default constructor definition Box() { cout << endl << "Default constructor called."; } // Function to calculate the volume of a box double Volume() { return length * breadth * height; } };

13 int main(void) { Box Box1(78.0,24.0,18.0); // Declare and initialize Box1 Box Box2; // Declare Box2 - no initial values Box CigarBox(8.0,5.0,1.0); // Declare and initialize CigarBox double volume = 0.0; // Store the volume of a box here volume = Box1.Volume(); // Calculate volume of Box1 cout << endl << "Volume of Box1 = " << volume; Box2.height = Box1.height - 10; // Define Box2 Box2.length = Box1.length/2.0; // members in Box2.breadth = 0.25*Box1.length; // terms of Box1 cout << endl << "Volume of Box2 = " << Box2.Volume(); cout << endl << "Volume of CigarBox = " << CigarBox.Volume(); cout << endl; return 0; } Box Class Client

14 Problems? class Box // Class definition at global scope { public: double length; // Length of a box in inches double breadth; // Breadth of a box in inches double height; // Height of a box in inches // Constructor definition Box(double lv = 1.0, double bv = 1.0, double hv = 1.0) { cout << endl << "Constructor called."; length = lv; // Set values of breadth = bv; // data members height = hv; } // Default constructor definition Box() { cout << endl << "Default constructor called."; } // Function to calculate the volume of a box double Volume() { return length * breadth * height; } };

15 The Fix // Ex6_05.cpp // Supplying default values for constructor arguments #include using namespace std; class Box // Class definition at global scope { public: double length; // Length of a box in inches double breadth; // Breadth of a box in inches double height; // Height of a box in inches // Constructor definition Box(double lv=1.0, double bv=1.0, double hv=1.0) { cout << endl << "Constructor called."; length = lv; // Set values of breadth = bv; // data members height = hv; } // Function to calculate the volume of a box double Volume() { return length * breadth * height; } };

16 int main(void) { Box Box2; // Declare Box2 - no initial values cout << endl << "Volume of Box2 = " << Box2.Volume(); cout << endl; return 0; }

17 Destructors and malloc()/free() Allocate memory for class data members dynamically with new or malloc Memory requested is subseqently free’d with free() Destructors enable a type of “garbage collection”  they’re automatically run when an object goes out of scope With a suitable destructor, memory is automatically cleaned up B Smith: Discuss malloc/free and new/delete! Use linked-list as an example?? Or Josephus?? B Smith: Discuss malloc/free and new/delete! Use linked-list as an example?? Or Josephus??

18 delete/new

19 Destructor using delete[] //Listing 02_01 class Message { private: char* pmessage; //Pointer to object text string public: // Function to display a message void ShowIt(void) { cout << endl << pmessage; } // Constructor definition Message(const char* text = "Default message") { // Allocate space for text pmessage = new char[strlen(text)+1]; // Copy text to new memory strcpy(pmessage, text); } ~Message(); // Destructor prototype }; // Listing 02_02 // Destructor to free memory allocated by new Message::~Message() { cout << "Destructor called." // track what happens << endl; delete[] pmessage; // Free memory assigned to ptr // Ex7_02.cpp // Using a destructor to free memory #include // For stream I/O #include // For strlen() and strcpy() using namespace std; // Put the Message class definition here (Listing 02_01) // Put the destructor definition here (Listing 02_02) int main() { // Declare object Message Motto("A miss is as good as a mile."); // Dynamic object Message* pM = new Message("A cat can look at a queen."); Motto.ShowIt(); // Display 1st message pM->ShowIt(); // Display 2nd message cout << endl; // delete pM; // Manually delete object created with new return 0; }

20 //Listing 02_01 class Message { private: char* pmessage; //Pointer to object text string public: // Function to display a messageoid ShowIt(void) { cout << endl << pmessage; } // Constructor definition Message(const char* text = "Default message") { // Allocate space for text pmessage = new char[strlen(text)+1]; // Copy text to new memory strcpy(pmessage, text); } ~Message(); // Destructor prototype }; class Message

21 // Listing 02_02 // Destructor to free memory allocated by new Message::~Message() { cout << "Destructor called." // track what happens << endl; delete[] pmessage; // Free the memory assigned to ptr } Message Destructor

22 Message Class driver function // Using a destructor to free memory #include // For stream I/O #include // For strlen() and strcpy() using namespace std; // Put the Message class definition here (Listing 02_01) // Put the destructor definition here (Listing 02_02) int main() { // Declare object Message Motto("A miss is as good as a mile."); // Dynamic object Message* pM = new Message("A cat can look at a queen."); Motto.ShowIt(); // Display 1st message pM->ShowIt(); // Display 2nd message cout << endl; // delete pM; // Manually delete object created with new return 0; }