Reading from or writing to a file in C requires 3 basic steps: Open the file. Do all the reading or writing. Close the file. Following are described the functions needed to accomplish each step. Opening a file: In order to open a file, use the member function open. Use it as: inFile.open(filename, mode); outFile.open(filename, mode); where. Input and Output operations can also be performed in C using the C Standard Input and Output Library (cstdio, known as stdio.h in the C language). This library uses what are called streams to operate with physical devices such as keyboards, printers, terminals or with any other type of files supported by the system. I want to open a file for reading, the C way. I need to be able to do it for: text files, which would involve some sort of read line function. Binary files, which would provide a way to read raw. Why my dev-c “can't open output file”? Ask Question Asked 6 years, 4 months ago. Active 4 years, 5 months ago. Viewed 9k times 0. When I re-complied a.cpp file.

• C++ User Input File Name
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• Dev C How To Open An Input File Online

In C++, you open a file, you must first obtain a stream. There are the following three types of streams:

• input
• output
• input/output

Create an Input Stream

To create an input stream, you must declare the stream to be of class ifstream. Here is the syntax:

Create an Output Stream

To create an output stream, you must declare it as class ofstream. Here is an example:

Create both Input/Output Streams

Streams that will be performing both input and output operations must be declared as class fstream. Here is an example:

Opening a File in C++

Once a stream has been created, next step is to associate a file with it. And thereafter the file is available (opened) for processing.

Opening of files can be achieved in the following two ways :

1. Using the constructor function of the stream class.
2. Using the function open().

The first method is preferred when a single file is used with a stream. However, for managing multiple files with the same stream, the second method is preferred. Let's discuss each of these methods one by one.

Opening File Using Constructors

We know that a constructor of class initializes an object of its class when it (the object) is being created. Same way, the constructors of stream classes (ifstream, ofstream, or fstream) are used to initialize file stream objects with the filenames passed to them. This is carried out as explained here:
To open a file named myfile as an input file (i.e., data will be need from it and no other operation like writing or modifying would take place on the file), we shall create a file stream object of input type i.e., ifstream type. Here is an example:

The above given statement creates an object, fin, of input file stream. The object name is a user-defined name (i.e., any valid identifier name can be given). After creating the ifstream object fin, the file myfile is opened and attached to the input stream, fin. Now, both the data being read from myfile has been channelised through the input stream object.

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Now to read from this file, this stream object will be used using the getfrom operator ('>>'). Here is an example:

Similarly, when you want a program to write a file i.e., to open an output file (on which no operation can take place except writing only). This will be accomplish by

1. creating ofstream object to manage the output stream
2. associating that object with a particular file

Here is an example,

This would create an output stream, object named as fout and attach the file secret with it.

Now, to write something to it, you can use << (put to operator) in familiar way. Here is an example,

The connections with a file are closed automatically when the input and the output stream objects expires i.e., when they go out of scope. (For example, a global object expires when the program terminates). Also, you can close a connection with a file explicitly by using the close() method :

Closing such a connection does not eliminate the stream; it just disconnects it from the file. The stream still remains there. For example, after the above statements, the streams fin and fout still exist along with the buffers they manage. You can reconnect the stream to the same file or to another file, if required. Closing a file flushes the buffer which means the data remaining in the buffer (input or output stream) is moved out of it in the direction it is ought to be. For example, when an input file's connection is closed, the data is moved from the input buffer to the program and when an output file's connection is closed, the data is moved from the output buffer to the disk file.

Opening Files Using Open() Function

There may be situations requiring a program to open more than one file. The strategy for opening multiple files depends upon how they will be used. If the situation requires simultaneous processing of two files, then you need to create a separate stream for each file. However, if the situation demands sequential processing of files (i.e., processing them one by one), then you can open a single stream and associate it with each file in turn. To use this approach, declare a stream object without initializing it, then use a second statement to associate the stream with a file. For example,

The above code lets you handle reading two files in succession. Note that the first file is closed before opening the second one. This is necessary because a stream can be connected to only one file at a time.

The Concept of File Modes

The filemode describes how a file is to be used : to read from it, to write to it, to append it, and so on.

When you associate a stream with a file, either by initializing a file stream object with a file name or by using the open() method, you can provide a second argument specifying the file mode, as mentioned below :

The second method argument of open(), the filemode, is of type int, and you can choose one from several constants defined in the ios class.

List of File Modes in C++

Following table lists the filemodes available in C++ with their meaning :

Constant	Meaning	Stream Type
ios :: in	It opens file for reading, i.e., in input mode.	ifstream
ios :: out	It opens file for writing, i.e., in output mode. This also opens the file in ios :: trunc mode, by default. This means an existing file is truncated when opened, i.e., its previous contents are discarded.	ofstream
ios :: ate	This seeks to end-of-file upon opening of the file. I/O operations can still occur anywhere within the file.	ofstream ifstream
ios :: app	This causes all output to that file to be appended to the end. This value can be used only with files capable of output.	ofstream
ios :: trunc	This value causes the contents of a pre-existing file by the same name to be destroyed and truncates the file to zero length.	ofstream
ios :: nocreate	This cause the open() function to fail if the file does not already exist. It will not create a new file with that name.	ofstream
ios :: noreplace	This causes the open() function to fail if the file already exists. This is used when you want to create a new file and at the same time.	ofstream
ios :: binary	This causes a file to be opened in binary mode. By default, files are opened in text mode. When a file is opened in text mode, various character translations may take place, such as the conversion of carriage-return into newlines. However, no such character translations occur in file opened in binary mode.	ofstream ifstream

If the ifstream and ofstream constructors and the open() methods take two arguments each, how have we got by using just one in the previous examples ? As you probably have guessed, the prototypes for these class member functions provide default values for the second argument (the filemode argument). For example, the ifstream open() method and constructor use ios :: in (open for reading) as the default value for the mode argument, while the ofstream open() method and constructor use ios :: out (open for writing) as the default.

The fstream class does not provide a mode by default and, therefore, one must specify the mode explicitly when using an object of fstream class.

Both ios::ate and ios::app place you at the end of the file just opened. The difference between the two is that the ios::app mode allows you to add data to the end of the file only, when the ios::ate mode lets you write data anywhere in the file, even over old data.

You can combine two or more filemode constants using the C++ bitwise OR operator (symbol ). For example, the following statement :

will open a file in the append mode if the file exists and will abandon the file opening operation if the file does not exist.

To open a binary file, you need to specify ios :: binary along with the file mode, e.g.,

or,

Closing a File in C++

As already mentioned, a file is closed by disconnecting it with the stream it is associated with. The close() function accomplishes this task and it takes the following general form :

For example, if a file Master is connected with an ofstream object fout, its connections with the stream fout can be terminated by the following statement :

C++ Opening and Closing a File Example

Here is an example given, for the complete understanding on:

• how to open a file in C++ ?
• how to close a file in C++ ?

Let's look at this program.

Here is the sample run of the above C++ program:

More Examples

Here are some example programs of C++ listed, that you can go for. These programs uses file:

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 these classes: cin is an object of class istream and cout is an object of class ostream. Therefore, we have already been using classes that are related to our file streams. And in fact, we can use our file streams the same way we are already used to use cin and cout, with the only difference that we have to associate these streams with physical files. Let's see an example:
This code creates a file called example.txt and inserts a sentence into it in the same way we are used to do with cout, but using the file stream myfile instead.

C++ User Input File Name

But let's go step by step:

Open a file

The first operation generally performed on an object of one of these classes is to associate it to a real file. This procedure is known as to open a file. An open file is represented within a program by a stream (i.e., an object of one of these classes; in the previous example, this was myfile) and any input or output operation performed on this stream object will be applied to the physical file associated to it.
In order to open a file with a stream object we use its member function open:
open (filename, mode);

Where filename is a string representing the name of the file to be opened, and mode is an optional parameter with a combination of the following flags:

ios::in	Open for input operations.
ios::out	Open for output operations.
ios::binary	Open in binary mode.
ios::ate	Set the initial position at the end of the file. If this flag is not set, the initial position is the beginning of the file.
ios::app	All output operations are performed at the end of the file, appending the content to the current content of the file.
ios::trunc	If the file is opened for output operations and it already existed, its previous content is deleted and replaced by the new one.

All these flags can be combined using the bitwise operator OR (). For example, if we want to open the file example.bin in binary mode to add data we could do it by the following call to member function open:

Each of the open member functions of classes ofstream, ifstream and fstream has a default mode that is used if the file is opened without a second argument:

class	default mode parameter
ofstream	ios::out
ifstream	ios::in
fstream	ios::in ios::out

For ifstream and ofstream classes, ios::in and ios::out are automatically and respectively assumed, even if a mode that does not include them is passed as second argument to the open member function (the flags are combined).
For fstream, the default value is only applied if the function is called without specifying any value for the mode parameter. If the function is called with any value in that parameter the default mode is overridden, not combined.
File streams opened in binary mode perform input and output operations independently of any format considerations. Non-binary files are known as text files, and some translations may occur due to formatting of some special characters (like newline and carriage return characters).
Since the first task that is performed on a file stream is generally to open a file, these three classes include a constructor that automatically calls the open member function and has the exact same parameters as this member. Therefore, we could also have declared the previous myfile object and conduct the same opening operation in our previous example by writing:
Combining object construction and stream opening in a single statement. Both forms to open a file are valid and equivalent.
To check if a file stream was successful opening a file, you can do it by calling to member is_open. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise:

Closing a file

When we are finished with our input and output operations on a file we shall close it so that the operating system is notified and its resources become available again. For that, we call the stream's member function close. This member function takes flushes the associated buffers and closes the file:
Once this member function is called, the stream object can be re-used to open another file, and the file is available again to be opened by other processes.
In case that an object is destroyed while still associated with an open file, the destructor automatically calls the member function close.

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Text files

Text file streams are those where the ios::binary flag is not included in their opening mode. These files are designed to store text and thus all values that are input or output from/to them can suffer some formatting transformations, which do not necessarily correspond to their literal binary value.
Writing operations on text files are performed in the same way we operated with cout:

Reading from a file can also be performed in the same way that we did with cin:
This last example reads a text file and prints out its content on the screen. We have created a while loop that reads the file line by line, using getline. The value returned by getline is a reference to the stream object itself, which when evaluated as a boolean expression (as in this while-loop) is true if the stream is ready for more operations, and false if either the end of the file has been reached or if some other error occurred.

Checking state flags

The following member functions exist to check for specific states of a stream (all of them return a bool value):

bad()

Returns true if a reading or writing operation fails. For example, in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left.

fail()

Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number.

eof()

Returns true if a file open for reading has reached the end.

good()

It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true. Note that good and bad are not exact opposites (good checks more state flags at once).

The member function clear() can be used to reset the state flags.

get and put stream positioning

All i/o streams objects keep internally -at least- one internal position:
ifstream, like istream, keeps an internal get position with the location of the element to be read in the next input operation.
ofstream, like ostream, keeps an internal put position with the location where the next element has to be written.
Finally, fstream, keeps both, the get and the put position, like iostream.
These internal stream positions point to the locations within the stream where the next reading or writing operation is performed. These positions can be observed and modified using the following member functions:

tellg() and tellp()

These two member functions with no parameters return a value of the member type streampos, which is a type representing the current get position (in the case of tellg) or the put position (in the case of tellp).

seekg() and seekp()

These functions allow to change the location of the get and put positions. Both functions are overloaded with two different prototypes. The first form is:
seekg ( position );
seekp ( position );

Using this prototype, the stream pointer is changed to the absolute position position (counting from the beginning of the file). The type for this parameter is streampos, which is the same type as returned by functions tellg and tellp.
The other form for these functions is:
seekg ( offset, direction );
seekp ( offset, direction );

Using this prototype, the get or put position is set to an offset value relative to some specific point determined by the parameter direction. offset is of type streamoff. And direction is of type seekdir, which is an enumerated type that determines the point from where offset is counted from, and that can take any of the following values:

ios::beg	offset counted from the beginning of the stream
ios::cur	offset counted from the current position
ios::end	offset counted from the end of the stream

The following example uses the member functions we have just seen to obtain the size of a file:

Notice the type we have used for variables begin and end:

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streampos is a specific type used for buffer and file positioning and is the type returned by file.tellg(). Values of this type can safely be subtracted from other values of the same type, and can also be converted to an integer type large enough to contain the size of the file.
These stream positioning functions use two particular types: streampos and streamoff. These types are also defined as member types of the stream class:

Type	Member type	Description
streampos	ios::pos_type	Defined as fpos<mbstate_t>. It can be converted to/from streamoff and can be added or subtracted values of these types.
streamoff	ios::off_type	It is an alias of one of the fundamental integral types (such as int or long long).

Each of the member types above is an alias of its non-member equivalent (they are the exact same type). It does not matter which one is used. The member types are more generic, because they are the same on all stream objects (even on streams using exotic types of characters), but the non-member types are widely used in existing code for historical reasons.

Binary files

For binary files, reading and writing data with the extraction and insertion operators (<< and >>) and functions like getline is not efficient, since we do not need to format any data and data is likely not formatted in lines.
File streams include two member functions specifically designed to read and write binary data sequentially: write and read. The first one (write) is a member function of ostream (inherited by ofstream). And read is a member function of istream (inherited by ifstream). Objects of class fstream have both. Their prototypes are:
write ( memory_block, size );
read ( memory_block, size );

Where memory_block is of type char* (pointer to char), and represents the address of an array of bytes where the read data elements are stored or from where the data elements to be written are taken. The size parameter is an integer value that specifies the number of characters to be read or written from/to the memory block.

In this example, the entire file is read and stored in a memory block. Let's examine how this is done:
First, the file is open with the ios::ate flag, which means that the get pointer will be positioned at the end of the file. This way, when we call to member tellg(), we will directly obtain the size of the file.
Once we have obtained the size of the file, we request the allocation of a memory block large enough to hold the entire file:
Right after that, we proceed to set the get position at the beginning of the file (remember that we opened the file with this pointer at the end), then we read the entire file, and finally close it:

At this point we could operate with the data obtained from the file. But our program simply announces that the content of the file is in memory and then finishes.

Buffers and Synchronization

When we operate with file streams, these are associated to an internal buffer object of type streambuf. This buffer object may represent a memory block that acts as an intermediary between the stream and the physical file. For example, with an ofstream, each time the member function put (which writes a single character) is called, the character may be inserted in this intermediate buffer instead of being written directly to the physical file with which the stream is associated.
The operating system may also define other layers of buffering for reading and writing to files.

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When the buffer is flushed, all the data contained in it is written to the physical medium (if it is an output stream). This process is called synchronization and takes place under any of the following circumstances:

• When the file is closed: before closing a file, all buffers that have not yet been flushed are synchronized and all pending data is written or read to the physical medium.
• When the buffer is full: Buffers have a certain size. When the buffer is full it is automatically synchronized.
• Explicitly, with manipulators: When certain manipulators are used on streams, an explicit synchronization takes place. These manipulators are: flush and endl.
• Explicitly, with member function sync(): Calling the stream's member function sync() causes an immediate synchronization. This function returns an int value equal to -1 if the stream has no associated buffer or in case of failure. Otherwise (if the stream buffer was successfully synchronized) it returns 0.

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