11

Introduction to

Object Oriented Programming(Continued)

Cats II

22

Objectives

You will be able to: Create objects dynamically. Use dynamically created objects

in a C++ program. Get keyboard input from a user in

a C++ program.

33

Get the Cats Program

If you don’t have the Cats program from last class on your computer, you can download it from the class web site:

http://www.cse.usf.edu/~turnerr/Object_Oriented_Design/Downloads/2011_01_19_Cats/

44

Dynamic Allocation

We can dynamically create objects of any class using new.

Similar to malloc in C. Allocates space on the heap. Invokes constructor to initialize the

object. Returns a pointer to the object.

55

Dynamic Allocation

#include <iostream>

#include "Cat.h"

using namespace std;

int main()

{

cout << "This is program Cats\n";

Date dob = {12,1,2008};

Cat* Fluffy = new Cat("Fluffy", dob, 8.4);

Fluffy->Display();

cout << endl;

cin.get(); // Hold window open.

return 0;

}

Note arrow.

Fluffy is now a pointer .

66

Object Lifetime

Objects allocated by declaration are deallocated when the function exits. Like all local variables.

Objects allocated with new continue to exist until explicitly deleted.

delete c1;

c1 is a pointer to the object that is to be deleted.

77

Explicitly Deleting an Object

#include <iostream>

#include "Cat.h"

using namespace std;

int main()

{

cout << "This is program Cats\n";

Date dob = {12,1,2008};

Cat* Fluffy = new Cat("Fluffy", dob, 8.4);

Fluffy->Display();

cout << endl;

delete Fluffy;

Fluffy = 0;

cin.get(); // Hold window open.

return 0;

}

88

Getting User Input

What if we want the user to specify the attributes of a Cat. Could overload the constructor and provide

a version that asks for input. Better to provide a separate function

outside the class definition. Separate UI from class logic.

Let’s write a function that asks the user for information about a Cat, creates a Cat object, and returns a pointer to the object.

99

main.cppCat* Create_Cat()

{

char name[21];

Date date_of_birth;

double weight;

cout << "Please enter information for new Cat\n";

cout << "Name (up to 20 characters): ";

cin.getline(name, 20);

cout << "Date of Birth:\n";

cout << " Month: ";

cin >> date_of_birth.Month;

cout << " Day: ";

cin >> date_of_birth.Day;

cout << " Year: ";

cin >> date_of_birth.Year;

cout << "Weight: ";

cin >> weight;

Cat* cat = new Cat(name, date_of_birth, weight);

return cat;

}http://www.cse.usf.edu/~turnerr/Object_Oriented_Design/Downloads/Create_Cat.cpp.txt

1010

Getting User Input

int main()

{

cout << "This is program Cats\n";

Cat* cat1 = Create_Cat();

cat1->Display();

cout << endl;

delete cat1;

cat1 = 0;

cin.get(); // Hold window open.

cin.get();

return 0;

}

In file main.cpp

1111

Running Program Cats

1212

Running Program Cats

13

Problems with String Input

C++ String input is tricky!

What happens if the user inputs more than 20 characters for the cat's name?

Try it! Use Debug > Start without Debugging Or Ctrl+F5

14

Avoiding the Problem

Read into a large buffer array. Operating system will limit input to some

reasonable amount. (~255 characters)

Copy up the the max number of characters into the real input array.

If the user enters too many characters? Output an error message and ask the user

to try again. or

Just ignore the excess input.

15

Function Get_String()

void Get_String(char* prompt, char* input_array, size_t max_chars)

{

char input_buffer[1000];

while (true)

{

cout << prompt;

cin.getline(input_buffer, 1000);

if (strlen(input_buffer) <= max_chars)

{

break;

}

cout << "Please enter no more than " << max_chars

<< " characters\n";

}

strcpy(input_array, input_buffer);

}

http://www.cse.usf.edu/~turnerr/Object_Oriented_Design/Downloads/Get_String.cpp.txt

16

Using Get_String()Cat* Create_Cat()

{

char name[21];

Date date_of_birth;

double weight;

cout << "Please enter information for new Cat\n";

Get_String("Name (up to 20 characters): ", name, 20);

cout << "Date of Birth:\n";

...

17

Program Running

18

Long Input String

Exactly 20 characters

19

Multiple Cats

int main()

{

cout << "This is program Cats\n";

while (true)

{

Cat* cat1 = Create_Cat();

cat1->Display();

cout << endl;

delete cat1;

cat1 = 0;

}

...

20

Program Running

Whoa!

What happened here?

21

A Problem with cin >>

cin >> weight;

User enters 8.4 (followed by "Enter")

The >> operator extracts "8.4" from the keyboard input buffer.

Leaves the newline character.

Next input sees the newline character as the first thing in the buffer.

No problem when next input is for a numeric value. Big problem when next input is for a string.

22

A Related Problem

The final "." in input "8.4." was left in the keyboard input buffer and read as name of the next cat.

23

Solution

After numeric input, clear the newline character (and potential garbage) from the keyboard input buffer before doing the next real input.

while (cin.get() != '\n') ;

Do nothing

24

Clear_Keyboard_Input_Buffer()

void Clear_Keyboard_Input_Buffer()

{

while (cin.get() != '\n') ;

}

Call this function after getting a numeric value to clear the newline character following the numeric text. Also handles the problem when the user

enters nonnumeric text after the numeric value.

25

Using Clear_Keyboard_Input_Buffer

Cat* Create_Cat()

{

...

cout << "Date of Birth:\n";

cout << " Month: ";

cin >> date_of_birth.Month;

cout << " Day: ";

cin >> date_of_birth.Day;

cout << " Year: ";

cin >> date_of_birth.Year;

cout << "Weight: ";

cin >> weight;

Clear_Keyboard_Input_Buffer();

Cat* cat = new Cat(name, date_of_birth, weight);

return cat;

}

26

Program in Action

Garbage at end of numeric inputs was silently ignored.

2727

Assignment

Do today's examples for yourself if you have not done them in class.

Read Chapter 10.

Project 1

End of Presentation