How to do simple File I/O in C++

[bold]Reading and writing files[/bold]

In C, the process of opening and manipulating files requires a lot of language background to prepare you for the complexity of the operations. However, the C++ iostream library provides a simple way to manipulate files, and so this functionality can be introduced much earlier than it would be in C.


To open files for reading and writing, you must include <fstream>. Although this will automatically include <iostream>, itÆs generally prudent to explicitly include <iostream> if youÆre planning to use cin, cout, etc.


To open a file for reading, you create an ifstream object, which then behaves like cin. To open a file for writing, you create an ofstream object, which then behaves like cout. Once youÆve opened the file, you can read from it or write to it just as you would with any other iostream object. ItÆs that simple (which is, of course, the whole point).


One of the most useful functions in the iostream library is getline( ), which allows you to read one line (terminated by a newline) into a string object[28]. The first argument is the ifstream object youÆre reading from and the second argument is the string object. When the function call is finished, the string object will contain the line.


HereÆs a simple example, which copies the contents of one file into another:


//: C02:Scopy.cpp
// Copy one file to another, a line at a time
#include <string>
#include <fstream>
using namespace std;

int main() {
ifstream in("Scopy.cpp"); // Open for reading
ofstream out("Scopy2.cpp"); // Open for writing
string s;
while(getline(in, s)) // Discards newline char
out << s << "\n"; // ... must add it back
} ///:~
To open the files, you just hand the ifstream and ofstream objects the file names you want to create, as seen above.


There is a new concept introduced here, which is the while loop. Although this will be explained in detail in the next chapter, the basic idea is that the expression in parentheses following the while controls the execution of the subsequent statement (which can also be multiple statements, wrapped inside curly braces). As long as the expression in parentheses (in this case, getline(in, s)) produces a ôtrueö result, then the statement controlled by the while will continue to execute. It turns out that getline( ) will return a value that can be interpreted as ôtrueö if another line has been read successfully, and ôfalseö upon reaching the end of the input. Thus, the above while loop reads every line in the input file and sends each line to the output file.


getline( ) reads in the characters of each line until it discovers a newline (the termination character can be changed, but that wonÆt be an issue until the iostreams chapter in Volume 2). However, it discards the newline and doesnÆt store it in the resulting string object. Thus, if we want the copied file to look just like the source file, we must add the newline back in, as shown.


Another interesting example is to copy the entire file into a single string object:


//: C02:FillString.cpp
// Read an entire file into a single string
#include <string>
#include <iostream>
#include <fstream>
using namespace std;

int main() {
ifstream in("FillString.cpp");
string s, line;
while(getline(in, line))
s += line + "\n";
cout << s;
} ///:~
Because of the dynamic nature of strings, you donÆt have to worry about how much storage to allocate for a string; you can just keep adding things and the string will keep expanding to hold whatever you put into it.


One of the nice things about putting an entire file into a string is that the string class has many functions for searching and manipulation that would then allow you to modify the file as a single string. However, this has its limitations. For one thing, it is often convenient to treat a file as a collection of lines instead of just a big blob of text. For example, if you want to add line numbering itÆs much easier if you have each line as a separate string object.

-Extracted from Bruce Eckels freely downloadable "Thinking in C++"

/Srikanth