Lec 47.48 - stream-files

Chapter: 12

Stream and Files
Lecture: 47 and 48
Date: 12.11.2012

Objectives
 Overview of stream classes

 Showing how to perform file-related activities using streams:
 How to read and write data to files in a variety of ways
 How to handle files or I/O errors
 How files and OOP are related

Streams
 Stream (flow of data)
 A transfer of information in the form of a sequence of bytes

 In C++, a stream is represented by an object of a
particular class; e.g., cin and cout are objects of
iostream class.

 I/O Operations:
 Input: A stream that flows from an input device ( i.e.: keyboard,
disk drive, network connection) to main memory
 Output: A stream that flows from main memory to an output
device ( i.e.: screen, printer, disk drive, network connection)

keyboard
standard
input stream
CPU
standard
output MEM
monitor stream
terminal
console

HDD
What does information
travel across?
Streams

keyboard
standard
input stream
CPU

standard
output MEM
monitor stream
terminal file
console input
stream
LOAD HDD
What does information READ
travel across? file
files output
Streams stream
SAVE
WRITE

C++ Stream Input/output
 iostream library has hundreds of I/O capabilities
 iostream: basic input and output
 fstream: file processing

 iostream library contains many I/O related classes:
 istream ( the extraction operator >>, and get(), getline(), read() are
members of this class)
 ostream (the insertion operator <<, put(), write() are members of
this class)
 istream and ostream are subclasses of ios base class
 cout is a predefined object of the iostream_withassign
class, whereas cin is a predefined object of
istream_withassign class

C++ Stream Input/output
 The classes used for input and output to the video
display and keyboard are declared in the header file
iostream, e.g., #include <iostream>

 The classes used specifically for disk file I/O are
declared in the fstream header file,
e.g., #include <fstream>

 All of them can be found in the include subdirectory
of the C++ compiler.

ios Class
 The granddaddy of all the stream classes, and
contains the majority of the features needed to
operate C++ streams

 Three most important features of ios class are:
1) Formatting flags
2) Error-status flags
3) File operation mode

ios Formatting Flags

 The formatting flags act as on/off switches that
specify choices for various aspects of input and
output format and operation.


 Since they are the members of the ios class, they
must follow the name ios and the scope resolution
operator;
e.g., ios::showpoint
 All the flags must be set using the setf() and usetf()
ios member functions;
e.g., cout.setf(ios::showpoint)
cout.unsetf(ios::showpoint)

#include <iostream>
#include <conio.h>
using namespace std;

int main(void)
{
float x = 18.0;
cout<< x << endl; //displays 18

cout.setf(ios::showpoint);
cout<< x << endl; //displays 18.0000

cout.setf(ios::scientific);
cout<< x << endl; //displays 1.800000e+001

cout.unsetf(ios::showpoint);
cout.unsetf(ios::scientific);

cout<<x<<endl; //displays 18
getch();
return 0; }


 Many formatting flags can be set using manipulators

 Manipulators are formatting instructions inserted
directly into a stream; e.g., endl, oct, setw() etc.

 Manipulators come in two flavors:
1) No-argument ios manipulators
2) ios manipulators with arguments

ios Manipulators with Arguments

#include <iostream>
#include <iomanip>
#include <conio.h>
int main(void)
{
int var = 11;

cout << setw(8) << 22 << "n";
cout << setw(8) << 4444 << "n";
cout << setw(8) << 666666 << endl;

cout<< var <<“in hexadecimal is ” << hex <<var;

cout<< setpercision(5) <<20.99055;
getch();
return 0; }

istream Class

 The istream class is derived from ios class and
performs input specific activities.

istream Class

 cin.get(): inputs a character from stream
(even white spaces) and returns it

 cin.get( c ): inputs a character from stream
and stores it in c

istream Class
 cin.get(array, size):
 Accepts 3 arguments: array of characters, the size limit, and a delimiter ( default
of ‘n’).
 Uses the array as a buffer
 When the delimiter is encountered, it remains in the input stream
 Null character is inserted in the array
 Unless delimiter flushed from stream, it will stay there
 cin.getline(array, size)
 Operates like cin.get(buffer, size) but it discards the delimiter from
the stream and does not store it in array
 Null character inserted into array

#include <iostream>
#include <conio.h>

int main()
{

const int SIZE = 10;
char buffer[ SIZE ];

cout << "Enter a sentence:n";
cin.getline( buffer, SIZE );

cout << "nThe sentence entered is:n" << buffer << endl;

getch();
return 0; }

ostream Class

 The ostream class is derived from ios class and
handles output or insertion activities.

Stream Errors
 Following statements are well-known to us:

cout<<“Good afternoon”
Or
cin >> var
What if the user enters “five” instead of “5” for
integer variable “var” ?

Stream Errors
 Following statements are well-known to us:

cout<<“Good afternoon”
Or
cin >> var
What if the user enters “five” instead of “5” for
integer variable “var” ?

The compiler will through an error message!

Errors-Status Bits
 The stream error status flags report errors that occur
in an input or output operation.

int i;

while(true) // cycle until input OK
{
cout << "nEnter an integer: ";
cin >> i;
if( cin.good() ) // if no errors
{
cin.ignore(10, 'n'); // remove newline
break; // exit loop
}
cin.clear(); // clear the error bits
cout << "Incorrect input";

cin.ignore(10, 'n'); // remove newline
}
cout << "integer is " << i; // error-free integer

Lec 47.48 - stream-files

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