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Input/output with files C provides the following classes to perform output and input of characters to/from files: ofstream: Stream class to write on files; ifstream: Stream class to read from files; fstream: Stream class to both read and write from/to files.; These classes are derived directly or indirectly from the classes istream and ostream.We have already used objects whose types were. Jul 12, 2007  ifstream from stdin. C / C Forums on Bytes. On some systems, opening '/dev/stdin' might work, but the general solution is to use a pointer: Ex. Ifstream inf. How can I use a ifstream object to input from stdin? On some systems, opening '/dev/stdin' might work, but the general solution.

When you open a file, all kinds of things can go wrong. A file lives on a physical device — a fixed disk, for example, or perhaps on a flash drive or SD card — and you can run into problems when working with physical devices.

For example, part of the disk might be damaged, causing an existing file to become corrupted. Or, less disastrous, you might run out of disk space. Or, even less disastrous, you might try to open a file in a directory that doesn’t exist.

If you try to open a file for writing by specifying a full path and filename but the directory does not exist, the computer responds differently, depending on the operating system you’re using. If you’re unsure how your particular operating system will respond, try writing a simple test application that tries to create and open something like /abc/def/ghi/jkl/abc.txt. (Of course, you’ll want to be sure to use a directory that doesn’t exist.)

Sep 07, 2005  C fstream and friends. Smart Device Development Visual Studio Smart Device Development - Native C Project. Visual Studio Smart Device Development - Native C Project https. Aug 16, 2006  My native class takes an ifstream as a method parameter and does a lot of magic with it in its implementation. Instead of converting this whole class to managed C, along with a plethra of dependencies, I would like to create a managed C / CLI class containing a. The fstream library allows us to work with files. To use the fstream library, include both the standard AND the fstream header file: Example. #include #include fstream There are three objects included in the fstream library, which are used to create, write or read files. Find answers to ifstream vs istream in c from the expert community at Experts Exchange. Iostream.h and fstream.h. There are types and functions in the library iostream.h that are used for standard I/O. Fstream.h includes the definitions for stream classes ifstream (for input from a file), ofstream (for output to a file) and fstream (for input to and output from a file). Make sure you always include that header when you use files.

Then one of two things will happen: Either the directory and the file will get created, or nothing will happen.

For example, on a Windows system, if we attempt to create a file in a directory that doesn’t exist, the system does not create the directory. That’s because deep down inside, the application ultimately calls an operating system function that does the dirty work of creating the file. And this particular operating system function (it’s called CreateFile(), if you even care) has a rule that it will not create a directory for you.

If you want to determine whether the ostream class was unable to create a file, you can call its fail() member function. This function returns true if the object couldn’t create the file. And that’s what happens when a directory doesn’t exist. The DirectoryCheck01 example shown demonstrates an example of this.

When you run this code, assuming that you don’t have a directory called /abc/def/ghi on your system, you should see the message Couldn’t open the file! Assuming that your particular operating system doesn’t create a directory in this case; if it does, your computer will open the file, write Hi to it, and move on with its happy life after closing things out.

As an alternative to calling the fail() member function, you can use an operator available in various stream classes. This is !, fondly referred to as the “bang” operator, and you would use it in place of calling fail(), as in this code:

Most people prefer to use !outfile instead of outfile.fail(), although !outfile makes confusing code. The reason is that outfile is an object, and the notion of !outfile simply doesn’t make sense.

In fact, !outfile trips up many beginning programmers. They know that outfile is not a pointer in this sample code, and they wonder how you could test it against 0 as you normally can only do with a pointer. (Remember, by saying !x, where x is some pointer, you’re testing x against 0.) And that simply doesn’t make sense! And so, to avoid confusion, just call fail(). It makes more sense.

Here are some reasons your file creation may choke:

• The directory doesn’t exist.

• You’re out of disk space and out of luck.

• Your application doesn’t have the right permissions to create a file.

• The filename was invalid — that is, it contained characters the operating system doesn’t allow in a filename, such as * or ?.

Like any good application, your application should do two things:

1. 1.Check whether a file creation succeeded.

2. 2.If the file creation failed, handle it appropriately.

   Don’t just print a horrible message like Oops!Aborting!, leaving your poor users with no choice but to toss the monitor onto the floor. Instead, do something friendlier — such as presenting a message telling them there’s a problem and suggesting that they might free more disk space.

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So far, we have been using the iostream standard library, which provides cin and cout methods for reading from standard input and writing to standard output respectively.

This tutorial will teach you how to read and write from a file. This requires another standard C++ library called fstream, which defines three new data types −

To perform file processing in C++, header files <iostream> and <fstream> must be included in your C++ source file.

Opening a File

A file must be opened before you can read from it or write to it. Either ofstream or fstream object may be used to open a file for writing. And ifstream object is used to open a file for reading purpose only.

Following is the standard syntax for open() function, which is a member of fstream, ifstream, and ofstream objects.

Here, the first argument specifies the name and location of the file to be opened and the second argument of the open() member function defines the mode in which the file should be opened.

You can combine two or more of these values by ORing them together. For example if you want to open a file in write mode and want to truncate it in case that already exists, following will be the syntax −

Similar way, you can open a file for reading and writing purpose as follows −

Closing a File

When a C++ program terminates it automatically flushes all the streams, release all the allocated memory and close all the opened files. But it is always a good practice that a programmer should close all the opened files before program termination.

Following is the standard syntax for close() function, which is a member of fstream, ifstream, and ofstream objects.

Writing to a File

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While doing C++ programming, you write information to a file from your program using the stream insertion operator (<<) just as you use that operator to output information to the screen. The only difference is that you use an ofstream or fstream object instead of the cout object.

Reading from a File

You read information from a file into your program using the stream extraction operator (>>) just as you use that operator to input information from the keyboard. The only difference is that you use an ifstream or fstream object instead of the cin object.

Read and Write Example

Following is the C++ program which opens a file in reading and writing mode. After writing information entered by the user to a file named afile.dat, the program reads information from the file and outputs it onto the screen −

C++ Ifstream Getline

When the above code is compiled and executed, it produces the following sample input and output −

Above examples make use of additional functions from cin object, like getline() function to read the line from outside and ignore() function to ignore the extra characters left by previous read statement.

File Position Pointers

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Both istream and ostream provide member functions for repositioning the file-position pointer. These member functions are seekg ('seek get') for istream and seekp ('seek put') for ostream.

The argument to seekg and seekp normally is a long integer. A second argument can be specified to indicate the seek direction. The seek direction can be ios::beg (the default) for positioning relative to the beginning of a stream, ios::cur for positioning relative to the current position in a stream or ios::end for positioning relative to the end of a stream.

The file-position pointer is an integer value that specifies the location in the file as a number of bytes from the file's starting location. Some examples of positioning the 'get' file-position pointer are −