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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 −
|Sr.No||Data Type & Description|
This data type represents the output file stream and is used to create files and to write information to files.
This data type represents the input file stream and is used to read information from files.
This data type represents the file stream generally, and has the capabilities of both ofstream and ifstream which means it can create files, write information to files, and read information from files.
To perform file processing in C++, header files <iostream> and <fstream> must be included in your C++ source 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.
|Sr.No||Mode Flag & Description|
Append mode. All output to that file to be appended to the end.
Open a file for output and move the read/write control to the end of the file.
Open a file for reading.
Open a file for writing.
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If the file already exists, its contents will be truncated before opening the file.
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 −
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.
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.
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.
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 −
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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.
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 −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.
ostream. We have already used objects whose types were these classes:
cinis an object of class
coutis 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
cout, with the only difference that we have to associate these streams with physical files. Let's see an example:
example.txtand inserts a sentence into it in the same way we are used to do with
cout, but using the file stream
myfile) and any input or output operation performed on this stream object will be applied to the physical file associated to it.
open (filename, mode);
filenameis a string representing the name of the file to be opened, and
modeis an optional parameter with a combination of the following flags:
|Open for input operations.|
|Open for output operations.|
|Open in binary mode.|
|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.
|All output operations are performed at the end of the file, appending the content to the current content of the file.|
|If the file is opened for output operations and it already existed, its previous content is deleted and replaced by the new one.|
). For example, if we want to open the file
example.binin binary mode to add data we could do it by the following call to member function
openmember functions of classes
fstreamhas a default mode that is used if the file is opened without a second argument:
|class||default mode parameter|
ios::outare automatically and respectively assumed, even if a mode that does not include them is passed as second argument to the
openmember function (the flags are combined).
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.
openmember function and has the exact same parameters as this member. Therefore, we could also have declared the previous
myfileobject and conduct the same opening operation in our previous example by writing:
is_open. This member function returns a
truein the case that indeed the stream object is associated with an open file, or
close. This member function takes flushes the associated buffers and closes the file:
ios::binaryflag 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.
trueif the stream is ready for more operations, and
falseif either the end of the file has been reached or if some other error occurred.
trueif 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.
truein 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.
trueif a file open for reading has reached the end.
falsein the same cases in which calling any of the previous functions would return
true. Note that
badare not exact opposites (
goodchecks more state flags at once).
clear()can be used to reset the state flags.
istream, keeps an internal get position with the location of the element to be read in the next input operation.
ostream, keeps an internal put position with the location where the next element has to be written.
fstream, keeps both, the get and the put position, like
streampos, which is a type representing the current get position (in the case of
tellg) or the put position (in the case of
seekg ( position );
seekp ( 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
seekg ( offset, direction );
seekp ( offset, direction );
offsetis of type
directionis 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:
|offset counted from the beginning of the stream|
|offset counted from the current position|
|offset counted from the end of the stream|
streamposis 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.
streamoff. These types are also defined as member types of the stream class:
|Defined as |
It can be converted to/from
|It is an alias of one of the fundamental integral types (such as |
>>) and functions like
getlineis not efficient, since we do not need to format any data and data is likely not formatted in lines.
read. The first one (
write) is a member function of
readis a member function of
ifstream). Objects of class
fstreamhave both. Their prototypes are:
memory_blockis of type
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
sizeparameter is an integer value that specifies the number of characters to be read or written from/to the memory block.
ios::ateflag, 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.
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.
sync()causes an immediate synchronization. This function returns an
intvalue 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