file i/o in c++. using input/output files a computer file is stored on a secondary storage device...
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File I/O in C++
Using Input/Output Files
A computer file• is stored on a secondary storage device
(e.g., disk);• is permanent;• can be used to provide input data to a
program or receive output data from a program, or both;
• should reside in Project directory for easy access;
• must be opened before it is used.
General File I/O Steps
• Declare a file name variable
• Associate the file name variable with the disk file name
• Open the file
• Use the file
• Close the file
Using Input/Output Files
stream - a sequence of characters• interactive (iostream)
cin - input stream associated with keyboard.
cout - output stream associated with display.• file (fstream)
ifstream - defines new input stream (normally associated with a file).
ofstream - defines new output stream (normally associated with a file).
Stream I/O Library Header Files Note: There is no “.h” on standard header files : <fstream> iostream -- contains basic information required for all
stream I/O operations iomanip -- contains information useful for performing
formatted I/O with parameterized stream manipulators fstream -- contains information for performing file I/O
operations strstream -- contains information for performing in-memory
I/O operations (i.e., into or from strings in memory)
ios is the base class. istream and ostream inherit from ios ifstream inherits from istream (and ios) ofstream inherits from ostream (and ios) iostream inherits from istream and ostream (&
ios) fstream inherits from ifstream, iostream, and
ofstream
Classes for Stream I/O in C++
C++ streams
#include <fstream>int main (void){ //Local declarations ifstream fsIn; ofstream fsOut; . . . return 0;}
input_stream.open("numbers.dat")
Stream handle
Name
Calling
Object
Dot
Operator
Member Function
Name
File
Name
Object and Member Functions
File I/O Example: Writing
#include <fstream>using namespace std;int main(void){
ofstream outFile(“fout.txt");outFile << "Hello World!";outFile.close();return 0;
}
File I/O Example: Writing#include <fstream>using namespace std;
int main(void){
ofstream outFile; outFile.open(“fout.txt”);
outFile << “First line”; //behave just like coutoutFile.close();
outFile<<“Another line”<<endl; //??return 0;
}
File I/O Example: Reading#include <iostream>#include <fstream>
int main(void){
ifstream openFile(“data.txt"); //open a text file data.txtchar ch;while(!OpenFile.eof()){
OpenFile.get(ch);cout << ch;
}OpenFile.close();
return 0;}
File I/O Example: Reading#include <iostream>#include <fstream>#include <string>
int main(void){
ifstream openFile(“data.txt"); //Declare and open a text filestring line;
while(!openFile.eof()){getline(openFile,line);//fetch line from data.txt and put it in a stringcout << line;}openFile.close();
return 0;}
File I/O Example: Reading#include <iostream>#include <fstream>#include <string>
int main(void){
ifstream openFile(“data.txt"); //open a text file data.txtstring line;
if(openFile.is_open()){ //while(!openFile.eof()){getline(openFile,line);//read a line from data.txt and put it in a string
cout << line;}
else{ cout<<“File does not exist!”<<endl; exit(1);} }
openFile.close(); return 0;}
More Input File-Related Functions
ifstream fsin; fsIn.open(const char[] fname)
• connects stream fsIn to the external file fname.
fsIn.get(char& character)• extracts next character from the input stream
fsIn and places it in the character variable character.
fsIn.eof()• tests for the end-of-file condition.
More Output File-Related Functions
ofstream fsOut; fsOut.open(const char[] fname)
• connects stream fsOut to the external file fname.
fsOut.put(char character)• inserts character character to the output
stream fsOut. fsOut.eof()
• tests for the end-of-file condition.
File Open ModeName Description
ios::in Open file to read
ios::out Open file to write
ios::app All the date you write, is put at the end of the file. It calls ios::out
ios::ate All the date you write, is put at the end of the file. It does not call ios::out
ios::trunc Deletes all previous content in the file. (empties the file)
ios::nocreate If the file does not exists, opening it with the open() function gets impossible.
ios::noreplace If the file exists, trying to open it with the open() function, returns an error.
ios::binary Opens the file in binary mode.
File Open Mode#include <fstream>int main(void){
ofstream outFile("file1.txt", ios::out);outFile << "That's new!\n";outFile.close();
Return 0;}
If you want to set more than one open mode, just use the OR operator- |. This way: ios::ate | ios::binary
Dealing with Binary files
Functions for binary file handling get(): read a byte and point to the next byte to read
put(): write a byte and point to the next location for write
read(): block reading
write(): block writing
Dealing with Binary files
Some useful functions seekg():Go to a specific position when reading
seekp():Go to a specific position when writing
tellg(): Retunrs an int type, that shows the current position of the inside-pointer. This one works only when you read a file.
tellp(): The same as tellg() but used when we write in a file. flush():Save data from the buffer to the output file.
Binary File I/O Examples
//Example 1: Using get() and put()#include <iostream>#include <fstream>void main(){
fstream File("test_file",ios::out | ios::in | ios::binary);char ch;ch='o';File.put(ch); //put the content of ch to the fileFile.seekg(ios::beg); //go to the beginning of the fileFile.get(ch); //read one charactercout << ch << endl; //display itFile.close();
}
Binary File I/O Examples//Example 2: Using read() and write()
#include <fstream.h>#include <string.h>void main(){
fstream File("test_file.txt",ios::out | ios::in | ios::binary);char arr[13];strcpy(arr,"Hello World!"); //put Hello World! into the arrayFile.write(arr,5); //put the first 5 symbols into the file- "Hello"File.seekg(ios::beg); //go to the beginning of the filestatic char read_array[10]; //I will put the read data, hereFile.read(read_array,3); //read the first 3 symbols- "Hel"cout << read_array << endl; //display themFile.close();
}
More Binary File I/O Examples
#include <fstream>void main(){
//if we have "Hello" in test_file.txtifstream File("test_file.txt");char arr[10];File.read(arr,10);
//this should return 5, as Hello is 5 characters long
cout << File.tellg() << endl; File.close();
}
Summary of InputFile-Related Functions
#include <fstream>ifstream fsIn; fsIn.open(const char[] fname)
• connects stream fsIn to the external file fname. fsIn.get(char& c)
• extracts next character from the input stream fsIn and places it in the character variable c.
fsIn.eof()• tests for the end-of-file condition.
fsIn.close()• disconnects the stream and associated file.
fsIn >> c; //Behaves just like cin
Summary of OutputFile-Related Functions
#include <fstream>ofstream fsOut; fsOut.open(const char[] fname)
• connects stream fsOut to the external file fname. fsOut.put(char c)
• inserts character c to the output stream fsOut. fsOut.eof()
• tests for the end-of-file condition. fsOut.close()
• disconnects the stream and associated file. fsOut << c; //Behaves just like cout