C++ files and streams

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Learning Objectives
C++ I/O streams.
Reading and writing sequential files.
Reading and writing random access files.

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C++ Files and Streams
C++ views each files as a sequence of bytes.
Each file ends with an end-of-file marker.
When a file is opened, an object is created
and a stream is associated with the object.
To perform file processing in C++, the
header files <iostream.h> and <fstream.h>
must be included.
<fstream.> includes <ifstream> and
<ofstream>

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Creating a sequential file
// Fig. 14.4: fig14_04.cpp D&D p.708
// Create a sequential file
#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>
int main()
{
// ofstream constructor opens file
ofstream outClientFile( "clients.dat", ios::out );
if ( !outClientFile ) { // overloaded ! operator
cerr << "File could not be opened" << endl;
exit( 1 ); // prototype in stdlib.h
}

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Sequential file
cout << "Enter the account, name, and balance.n"
<< "Enter end-of-file to end input.n? ";
int account;
char name[ 30 ];
float balance;
while ( cin >> account >> name >> balance ) {
outClientFile << account << ' ' << name
<< ' ' << balance << 'n';
cout << "? ";
}
return 0; // ofstream destructor closes file
}

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Question.
What does the above program do?

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How to open a file in C++ ?
Ofstream outClientFile(“clients.dat”, ios:out)
OR
Ofstream outClientFile;
outClientFile.open(“clients.dat”, ios:out)

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File Open Modes
ios:: app - (append) write all output to the end of
file
ios:: ate - data can be written anywhere in the file
ios:: binary - read/write data in binary format
ios:: in - (input) open a file for input
ios::out - (output) open afile for output
ios: trunc -(truncate) discard the files’ contents if
it exists

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File Open Modes cont.
ios:nocreate - if the file does NOT exists, the
open operation fails
ios:noreplace - if the file exists, the open
operation fails

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How to close a file in C++?
The file is closed implicitly when a
destructor for the corresponding object is
called
OR
by using member function close:
outClientFile.close();

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Reading and printing a
sequential file
// Reading and printing a sequential file
#include <iomanip.h>
#include <stdlib.h>
void outputLine( int, const char *, double );
int main()
{
// ifstream constructor opens the file
ifstream inClientFile( "clients.dat", ios::in );
if ( !inClientFile ) {
cerr << "File could not be openedn";
exit( 1 );
}

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int account;
char name[ 30 ];
double balance;
cout << setiosflags( ios::left ) << setw( 10 ) << "Account"
<< setw( 13 ) << "Name" << "Balancen";
while ( inClientFile >> account >> name >> balance )
outputLine( account, name, balance );
return 0; // ifstream destructor closes the file
}
void outputLine( int acct, const char *name, double bal )
{
cout << setiosflags( ios::left ) << setw( 10 ) << acct
<< setw( 13 ) << name << setw( 7 ) << setprecision( 2 )
<< resetiosflags( ios::left )
<< setiosflags( ios::fixed | ios::showpoint )
<< bal << 'n';
}

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File position pointer
<istream> and <ostream> classes provide
member functions for repositioning the file
pointer (the byte number of the next byte in the
file to be read or to be written.)
These member functions are:
seekg (seek get) for istream class
seekp (seek put) for ostream class

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Examples of moving a file
pointer
inClientFile.seekg(0) - repositions the file get pointer to
the beginning of the file
inClientFile.seekg(n, ios:beg) - repositions the file get
pointer to the n-th byte of the file
inClientFile.seekg(m, ios:end) -repositions the file get
pointer to the m-th byte from the end
of file
nClientFile.seekg(0, ios:end) - repositions the file get
pointer to the end of the file
The same operations can be performed with <ostream>
function member seekp.

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Member functions tellg() and
tellp().
Member functions tellg and tellp are provided
to return the current locations of the get and put
pointers, respectively.
long location = inClientFile.tellg();
To move the pointer relative to the current
location use ios:cur
inClientFile.seekg(n, ios:cur) - moves the file
get pointer n bytes forward.

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Updating a sequential file
Data that is formatted and written to a
sequential file cannot be modified
easily without the risk of destroying
other data in the file.
If we want to modify a record of data,
the new data may be longer than the
old one and it could overwrite parts of
the record following it.

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Problems with sequential files
Sequential files are inappropriate for so-
called “instant access” applications in
which a particular record of information
must be located immediately.
These applications include banking
systems, point-of-sale systems, airline
reservation systems, (or any data-base
system.)

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Random access files
Instant access is possible with random
access files.
Individual records of a random access file
can be accessed directly (and quickly)
without searching many other records.

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Example of a Program that
Creates a Random Access File
// Fig. 14.11: clntdata.h
// Definition of struct clientData used in
// Figs. 14.11, 14.12, 14.14 and 14.15.
#ifndef CLNTDATA_H
#define CLNTDATA_H
struct clientData {
int accountNumber;
char lastName[ 15 ];
char firstName[ 10 ];
float balance;
};
#endif

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Creating a random access file
// Creating a randomly accessed file sequentially
#include <stdlib.h>
#include "clntdata.h"
int main()
{
ofstream outCredit( "credit1.dat", ios::out);
if ( !outCredit ) {
cerr << "File could not be opened." << endl;
exit( 1 );
}

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clientData blankClient = { 0, "", "", 0.0 };
for ( int i = 0; i < 100; i++ )
outCredit.write
(reinterpret_cast<const char *>( &blankClient ),
sizeof( clientData ) );
return 0;
}

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<ostream> memebr function
write
The <ostream> member function
write outputs a fixed number of bytes
beginning at a specific location in
memory to the specific stream. When
the stream is associated with a file,
the data is written beginning at the
location in the file specified by the
“put” file pointer.

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The write function expects a first
argument of type const char *, hence
we used the reinterpret_cast <const
char *> to convert the address of the
blankClient to a const char *.
The second argument of write is an
integer of type size_t specifying the
number of bytes to written. Thus the
sizeof( clientData ).

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Writing data randomly to a
random file
#include <stdlib.h>
int main()
{
ofstream outCredit( "credit.dat", ios::ate );
if ( !outCredit ) {
exit( 1 );
}

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cout << "Enter account number "
<< "(1 to 100, 0 to end input)n? ";
clientData client;
cin >> client.accountNumber;
while ( client.accountNumber > 0 &&
client.accountNumber <= 100 ) {
cout << "Enter lastname, firstname, balancen? ";
cin >> client.lastName >> client.firstName
>> client.balance;

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outCredit.seekp( ( client.accountNumber - 1 ) *
outCredit.write(
reinterpret_cast<const char *>( &client ),
cout << "Enter account numbern? ";
cin >> client.accountNumber;
}
return 0;
}

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Reading data from a random file
#include <iomanip.h>
#include <stdlib.h>
void outputLine( ostream&, const clientData & );
int main()
{
ifstream inCredit( "credit.dat", ios::in );
if ( !inCredit ) {
exit( 1 );
}

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cout << setiosflags( ios::left ) << setw( 10 ) << "Account"
<< setw( 16 ) << "Last Name" << setw( 11 )
<< "First Name" << resetiosflags( ios::left )
<< setw( 10 ) << "Balance" << endl;
clientData client;
inCredit.read( reinterpret_cast<char *>( &client ),

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while ( inCredit && !inCredit.eof() ) {
if ( client.accountNumber != 0 )
outputLine( cout, client );
inCredit.read( reinterpret_cast<char *>( &client ),
}
return 0;
}

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void outputLine( ostream &output, const clientData &c )
{
output << setiosflags( ios::left ) << setw( 10 )
<< c.accountNumber << setw( 16 ) << c.lastName
<< setw( 11 ) << c.firstName << setw( 10 )
<< setprecision( 2 ) << resetiosflags( ios::left )
<< setiosflags( ios::fixed | ios::showpoint )
<< c.balance << 'n';
}

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The <istream> function read
inCredit.read (reinterpret_cast<char *>(&client),
sizeof(clientData));
The <istream> function inputs a specified
(by sizeof(clientData)) number of bytes
from the current position of the specified
stream into an object.

C++ files and streams

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