Apr 15, 2020 I want to ask the user for the file name to open. I have tried to use the basic open file code from cplusplus.com and it won't work unless I specify the file name in the code. If I have them enter a file name, I then need to add 's to it? Either way, I get errors if I use char or string for the input file name. Can anyone show me how to do this?
In C++, you open a file, you must first obtain a stream. There are the following three types of streams:
To create an input stream, you must declare the stream to be of class ifstream. Here is the syntax:
To create an output stream, you must declare it as class ofstream. Here is an example:
Streams that will be performing both input and output operations must be declared as class fstream. Here is an example:
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 :
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.
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.
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
Here is an example,
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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.
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 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.
Following table lists the filemodes available in C++ with their meaning :
|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.
|ios :: ate||This seeks to end-of-file upon opening of the file.|
I/O operations can still occur anywhere within the file.
|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.
|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.
|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.
|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.
|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.
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.,
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 :
Here is an example given, for the complete understanding on:
Let's look at this program.
Here is the sample run of the above C++ program:
Here are some example programs of C++ listed, that you can go for. These programs uses file:
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