Lab Sheet 8
Understanding the Concept of Console and File Input/Output
Console Input/Output
For console input/output the stream class library provides the following classes
istream - for input
ostream - for output
iostream - for input and output.
The classes istream and ostream are inherited from class ios and class iostream is inherited from istream and ostream. For the console input/output handling one should include <iostream> in the program.
Within the console, we can perform unformatted and formatted input/output. For unformatted input/output stream functions like put(), get(), getline(), write(), read() etc are used. For formatted input/output, the stream objects cin and cout are used along with ios functions and flags, and manipulators.
The ios functions that can be used for formatting are
width()
fill()
precision()
setf()
unsetf()
flags() etc
However to use the setf(), unsetf() and flags() functions one should know the flags available in ios class. The input/output can also be done with manipulators. The manipulators equivalent to ios functions and flags are available in the stream library. Some manipulators are non-parameterized and some are parameterized. To use parameterized manipulators we should include header <iomanip>
File Handling
Following are classes for handling files.
ifstream - for handling input files
ofstream - for handling output files
fstream - for handling input as well as output files.
In all these classes, passing a filename as the first parameter in the constructor itself can open a file.
e.g ifstream infile("test.txt") opens the file test.txt in the input mode.
The constructors for all these classes are defined as follows
ifstream( const char *path, int mode=ios::in)
ofstream( const char *path, int mode=ios::out)
fstream( const char *path, int mode=ios::in|ios::out)
where path specifies the file to be opened, mode specifies the mode in which the file is to be opened.
For file handling, we should include the header file <fstream> where classes for file handling are declared.
File opening can also be done explicitly by calling the member function open() of the file stream classes. The open() functions have similar prototypes as the constructors.
After opening, the file contents can be written or read by using the stream operators with the file objects as
ofstream ofile("test.txt");
ofile<<"C++ lab class";
This statement writes "C++ lab class" in the file "test.txt"
Reading and Writing A class Object
The binary input and output functions read() and write() are designed to handle the entire structure of an object (or variable) as a single unit, using the computer's internal representation of data. The function write() stores a class object byte-to-byte to file without conversion. Similarly, the function read() reads objects(or variables) from file without conversion.
Binary output and input functions take the following form
ipfile.read(reinterpret_cast<char*>(&obj),sizeof(obj));
opfile.write(reinterpret_cast<char*>(&obj),sizeof(obj));
Example
#include<iostream>
#include<fstream>
#include<iomanip>
using namespace std;
class demofile
{
private:
int a;
int b;
public:
demofile(){}
demofile(int x,int y){a=x;b=y;}
void display()
{cout<<"a= "<<a<<endl<<"b= "<<b<<endl;}
};
int main()
{
demofile de(10,20);
fstream file;
file.open("demo.txt",ios::in|ios::out);
file.write(reinterpret_cast<char*>(&de),sizeof(de));
file.seekg(0);
file.read(reinterpret_cast<char*>(&de),sizeof(de));
de.display();
file.close();
return 0;
}
Write a program to demonstrate the use of different ios flags and functions to format the output. Create a program to generate the bill invoice of a department store by using different formatting.
Write a program to create a user-defined manipulator that will format the output by setting the width, precision, and fill character at the same time by passing arguments.
Write a program to overload stream operators to read a complex number and display the complex number in a+ib format.
Write a program that stores the information about students (name, student id, department, and address) in a structure and then transfers the information to a file in your directory. Finally, retrieve the information from your file and print it in the proper format on your output screen.
Write a program for transaction processing that write and read object randomly to and from a random access file so that user can add, update, delete and display the account information (account-number, last-name, first-name, total-balance).