ofstream
: Stream class to write on filesifstream
: Stream class to read from filesfstream
: Stream class to both read and write from/to files.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:
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[file example.txt] Writing this to a file. |
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.myfile
) and any input or output operation performed on this stream object will be applied to the physical file associated to it.open
:
open (filename, mode);
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. |
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). 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
:
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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 |
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).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.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:
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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:
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close
. This member function takes flushes the associated buffers and closes the file:
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close
.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.cout
:
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[file example.txt] This is a line. This is another line. |
cin
:
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This is a line. This is another line. |
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.bool
value): bad()
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()
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()
true
if a file open for reading has reached the end.good()
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).clear()
can be used to reset the state flags.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.fstream
, keeps both, the get and the put position, like iostream
.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 ( 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 tellg
and tellp
.
seekg ( offset, direction );
seekp ( offset, direction );
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 |
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size is: 40 bytes. |
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.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 ). |
<<
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.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: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.
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the entire file content is in memory |
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.
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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.flush
and endl
.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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