C++ - I/O Streams as an Introduction to Objects and Classes

Slide 6- 5
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
I/O Streams

I/O refers to program input and output

Input is delivered to your program via a stream object

Input can be from

The keyboard

A file

Output is delivered to the output device via a stream
object

Output can be to

The screen

A file
Slide 6- 6
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Objects

Objects are special variables that

Have their own special-purpose functions

Set C++ apart from earlier programming
languages
Slide 6- 7
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Streams and Basic File I/O

Files for I/O are the same type of files used to
store programs

A stream is a flow of data.

Input stream: Data flows into the program

If input stream flows from keyboard, the program will
accept data from the keyboard

If input stream flows from a file, the program will accept
data from the file

Output stream: Data flows out of the program

To the screen

To a file
Slide 6- 8
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
cin And cout Streams

cin

Input stream connected to the keyboard

cout

Output stream connected to the screen

cin and cout defined in the iostream library

Use include directive: #include <iostream>

You can declare your own streams to use with
files.
Slide 6- 9
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Why Use Files?

Files allow you to store data permanently!

Data output to a file lasts after the program ends

An input file can be used over and over

No typing of data again and again for testing

Create a data file or read an output file at your
convenience

Files allow you to deal with larger data sets
Slide 6- 10
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
File I/O

Reading from a file

Taking input from a file

Done from beginning to the end (for now)

No backing up to read something again (OK to start over)

Just as done from the keyboard

Writing to a file

Sending output to a file

Done from beginning to end (for now)

No backing up to write something again( OK to start over)

Just as done to the screen
Slide 6- 11
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Stream Variables

Like other variables, a stream variable…

Must be declared before it can be used

Must be initialized before it contains valid data

Initializing a stream means connecting it to a file

The value of the stream variable can be thought of
as the file it is connected to

Can have its value changed

Changing a stream value means disconnecting from
one file and connecting to another
Slide 6- 12
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Streams and Assignment

A stream is a special kind of variable called
an object

Objects can use special functions to complete tasks

Streams use special functions instead of the
assignment operator to change values
Slide 6- 13
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Declaring An
Input-file Stream Variable

Input-file streams are of type ifstream

Type ifstream is defined in the fstream library

You must use the include and using directives
#include <fstream>
using namespace std;

Declare an input-file stream variable using
ifstream in_stream;
Slide 6- 14
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Declaring An
Output-file Stream Variable

Ouput-file streams of are type ofstream

Type ofstream is defined in the fstream library

You must use these include and using directives
#include <fstream>
using namespace std;

Declare an input-file stream variable using
ofstream out_stream;

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