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.

Destructors

Math 130

Lecture # xxMo/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

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 " << pB1->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 of Boxes[2] is " << (pB2 + 2)->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 <iostream>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 " << pB1->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 of Boxes[2] is " << (pB2 + 2)->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 <iostream>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

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 <iostream >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 <iostream>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_01class 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 <iostream> // For stream I/O#include <string> // 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_01class 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 newMessage::~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 <iostream> // For stream I/O#include <string> // 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;}