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C++ Stream Input/Output

Chapter 11

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What You Will Learn

Using C++ object oriented stream input and output

Formatting I/O, creating manipulators

I/O Class hierarchies Determine stream

success or failure

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Introduction

Many I/O features are object oriented Uses function and operation

overloading We will see type safe I/O

operations performed sensitive to the data type

if no I/O function exists for a given type, then compiler notes as error

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Streams

Defn => a sequence of bytes I/O moves them from/to secondary

devices Applications associate meaning to

the bytes I/O is extremely slow compared to

processing speeds C++ provides both formatted and

unformatted I/O

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IOstream Library Header files

We use the #include <iostream.h> Contains basic infor required for all

stream-I/O operations cin, cout, cerr, clog

#include <iomanip.h> formatted I/O with parameterized

stream manipulators #include <fstream.h>

file processing operations

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File Class Hierarchy

ios

istream ostream

iostream

ifstream fstream ofstream

iostream.h

fstream.h

InputOutput

File I/O

Multiple inheritanceBase Class

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Stream I/O Classes & Objects

>> and << are right and left bit shift operators

Have been overloaded for input and output

cin is an object of class istream tied to standard input, keyboard

cout is an object of class ostream tied to standard output, screen

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Stream-Insertion Operator

<< operator overloaded to output data items of built in types strings pointer values

When outputting expressions, use parentheses avoids confusion of operator

precedence with <<

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Cascading Insertion/Extraction Operators

<< associates from left to right Overloaded << operator returns a

reference to its left-operand object (cout)

Next cascaded operand sent to the result of the first function call (which points to cout)

cout << "total = " << a + b;cout << "total = " << a + b;

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Output of char * Variables

Consider data object of type char* a character stringcout << name;//contents of array printed

What if we wish to print the value of the pointer … the address to which it points?

Cast the pointer to void *cout << static_cast<void *>(name);

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Char Output with put

put is a member function of ostream Sends a single character value to

the output stream The function returns reference to

object which called it (cout) thus can be cascadedcout.put('W').put('O').put('W');

Integer value parameter taken in as ASCII character number

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Stream Input

Performed with extraction operator >> Normally skips whitespace characters

blanks, tabs, newlines Operator returns zero (false) when eof

occurs otherwise returns reference to

stream object (thus can be cascaded)

Stream has state bits used to control state of stream

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Stream-Extraction Operator

Stream extraction can be cascaded Warnings

don't try extraction >> with cout don't try insertion << with cin possible need of parentheses due to

precedence of << and >> while (cin) …

specify end of I/O with ctrl-z (ctrl-d on Mac or UNIX)

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get and getline Member Functions

ch = cin.get( );// inputs & returns one character even if it is whitespace -- returns

the whitespace character returns the EOF when end of file

reached cin.eof( )returns true (1) when

user enters ctrl-z

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get and getline Member Functions cin.get(ch); // new version

reads value from stream into ch returns 0 when eof encountered otherwise returns reference to stream

object (for cascading) cin.get (name, 30,'\n'); // 3rd version

30 specifies max number of characters '\n' terminates input '\n' not consumed

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get and getline Member Functions

cin.getline(name,30,'\n'); Similar to cin.get( … ) Difference is that the getline

version will "consume" the '\n' (or whatever delimiter is specified)

Need to be aware of using cin.ignore if we know '\n' will still be in the stream

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istream Member Functions

ignore -- skips over designated number of characters

putback -- places character previously obtained with get back onto input stream good for scanning a stream looking

for specific character peek -- returns next character in

stream without getting it off the stream

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Type-Safe I/O

<< and >> overloaded to accept data items of specific types that is, there are separate

overloaded operators for different types of operands (function parameters)

Error flags set if wrong type of data comes through the stream

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Unformatted I/O

Performed with read and write member functions

They take two parameters an array of char or a buffer a number which tells how many

characterscin.read(buffer,20);cin.write (buffer,15);

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Stream Manipulators

Provided for formatting I/O Capabilities included

setting field widths setting decimal precision set/unset format flags setting fill in char fields flushing streams

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Integral Stream Base

Integers normally interpred as base 10 (decimal) values

Make sure to #include <iomanip.h> Specify other bases with

hex for base 16 (hexadecimal) oct for base 8 (octal)

cout << n << " = " << oct << n << " in octal";

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Floating-Point Precision

Specify number of decimal digits shown (rounded to that # digits)

Make sure to #include <iomanip.h> And to cout.setf (ios::fixed); // orcout << setiosflags (ios::fixed);

Use cout << setprecision (places) ;or cout.precision (places);

Author notes -- setting precision to 0 may result in default precision setting

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Field Width

Two versionscout.width(places); // andcout << setw (places);

If values take up less space, padding added to left => right justify

If values take up more space, width specification is overridden thus may push succeeding

columns of output to the right

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User-Defined Manipulators// bell manipulator (using escape sequence \a)ostream& bell( ostream& output ) { return output << '\a'; } // ret manipulator (using escape sequence \r)ostream& ret( ostream& output ) { return output << '\r'; } // tab manipulator (using escape sequence \t )ostream& tab( ostream& output ) { return output << '\t'; }

Note use of

- return type is ostream&

- parameter type is ostream&

Usage : cout << 'a' << tab << 'c' << bell;

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Format State Flags

Note table on pg 621 (Fig. 11.20) These flags controlled by

flags ( … ) cout.setf ( … ) member cout.unsetf ( … ) functions setiosflags ( … )

// in the << sequence

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Trailing Zeros, Decimals

Use ios::showpoint floating point values print as

integers if that is what is stored showpoint forces decimal points to

be printed for floats

(Fig 11.21)

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Justification Use ios::left ios::right

ios::internal Right justification is default with

setw( ) padded characters added on

right when ios::left is specified ios::internal => a number's sign

left justified, magnitude right justified

(Fig. 11.21)

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Padding

The cout.fill('x'); specifies that padding be x's instead of spaces

Alternative version cout << setfill('x') << setw(4) << 5;

What would output would be?xxx5 (Fig. 11-24)

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Integral Stream Base

Use showbase flag

cout << setiosflags (ios::showbase) << x << endl << oct << x << endl << hex << x << endl;

cout << setiosflags (ios::showbase) << x << endl << oct << x << endl << hex << x << endl;

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Floating-Point Numbers

We have used in previous course cout.setf (ios::fixed, ios::floatfield);

Also possible to specify scientific notation cout.setf (ios::scientific, ios::floatfield);

Can be turned off with cout.unsetf (ios::scientific);

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Forms of setf( )

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Uppercase/Lowercase Control

Forces uppercase letters for scientific notation hex notation

Usecout << setiosflags (ios::uppercase) << 1.23e6;

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Setting/Resetting Format Flags

flags ( ) member function without parameters returns long value with current settings of format flags store in a long variable long setting = cout.flags( );

then later on use with argumentcout.flags (setting);

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Stream Error States

Base class ios has bits which tell state of I/O eofbit set when end-of-file

use cin.eof( ) failbit set when format error

with no loss of charuse cin.fail( )

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Stream Error States

badbit set for error with lost datause cin.bad( )

goodbit set if none of eofbit, failbitor badbit are set

use cin.good( ) cin.rdstate( ) returns errorstate of

stream cin.clear( ) restores stream state

to good

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Tying an Output Stream to an Input Stream

Interactive programs use istream for input, ostream for output Prompt must first appear Only when output buffer fills or is

flushed can input proceed To explicitly tie these cout and cin

togethercin.tie (&cout);