Transcript I/O in C++ - McGill University

I/O in C++
October 7, 2008.
Junaed Sattar
Stream I/O

a stream is a flow of bytes/characters/ints or
any type of data

input streams: to the program

output streams: from the program

note I use plural

one program can have multiple I/O streams
associated

and vice-versa
Input/Output

Console based, no GUI

standard streams:

cin: standard input

cout: standard output

cerr: standard error
Extraction/Insertion
cout << “Hello world!”;
cout << “The value of i is “ << i << endl;
//endl puts a new line
cout << “Please enter your name: “;
string name;
cin >> name;
What are cin and cout?

Stream classes

Classes have methods, as we know

so does cin and cout

some common to all I/O stream classes in C++

File I/O, binary/text mode I/O, console I/O
One example

cin inputs ints, chars, null-terminated strings,
string objects

but terminates when encounters space (ascii
character 32)

workaround?

use the “get” method
Snippet
char tData[100];

// This is a method
in C++ istream classes
for
// inputting text

//including spaces
cin.get( tData, 99 );
// or
cin.get(tData,99,'\n');

Inputting “i am oh so
cool”
cin.get gets the entire
line
just cin will get “I”

space termination
Or,

Use the getline function
getline( cin, name );
File I/O

Reading from or writing to files on disk

ifstream and ofstream classes


dedicated for input and output respectively
or, use fstream
Example Files Program(filesdemo)
ofstream myofile;
myofile.open( “sample.txt” );
myofile << “This is a sample line I'm writing\n”;
myofile.close();
...
ifstream myifile;
myifile.open( “sample.txt” );
string oneLine;
getline( myifile, oneLine );
cout << oneLine;
myifile.close();
Read/Write to files (files1/2)

Similar to how we use cin and cout


remember, these are I/O streams too
myfile is a file stream object, then:

to write an int:


int i = 10;
myfile << i;
to read an int:

int i;
myfile >> i;
Binary files


As opposed to text files, they are unformatted
as ascii.

text files stores everything as ascii text strings

even numbers

binary files do not
Example: consider outout of the program in the
previous slide
Difference?


Example program

Accepts student ID (I input 1010)

Accepts name (I input Junaed)

Accepts CGPA (I input 4.5)
Save into two files, as text and binary
Storage
TEXT FILE
BINARY FILE
Binary files

Files by default are text

Different methods to write and read


requires casting (we'll see casting soon)

different data format
If time permits, we'll revisit
Failures?


If open fails?

Check before use

if( !myifile )
{
cerr << “Cannot open file!”;
exit(1);
}
End of file?

while( myifile.fail() )
{
//do your operations here
}
Random vs Sequential

Random access files

nonsequential,

as a result faster access times,

content must be suitable for random access

for example. not on network streams!

or console input
File “heads”

Access positions

one each for read and write

hence two methods:

seekg (as in “get”) for reading

seekp (as in “put”) for writing

ifstreams have seekg

ofstreams have seekp
seeking

seekg( position, mode) //(same for seekp)

position is a long integer signed offset

mode can be

ios::beg: from the beginning

ios::end from the end

ios::cur from current position
telling

tellg and tellp

returns as long integer, the position of the get and
put positions, respectively
example seeks
file.seekg( 20L, ios::beg );
file.seekp( -100L, ios::cur );
long pPosition = file.tellp();
long gPosition = file.tellg();