As has been discussed in many other threads here, 'cin ' doesn't remove the end-of-line from the stream after you read in the zero or one. So I'm thinking the getline at line 21 gets the now-empty line and writes it to the file, rather than waiting for a new line of input. Ifstream inFile; // input file stream object ofstream outFile; // output file stream object The types ifstream and ofstream are C stream classes designed to be connected to input or output files. File stream objects have all the member functions and manipulators possessed by the standard streams, cin and cout. Streams for File I/O. Feb 06, 2012 What application should be used to create an input or output file and how should it be saved for use in Xcode. I am using C located in the Command Line Utility. Basically, I need to know just about everything related to creating and using an input or output file with C in Xcode. Each stream is associated with an external physical device (file, standard input stream, printer, serial port, etc). I/O streams can be used for both unformatted and formatted input and output. They are locale-sensitive and may perform wide/multibyte conversions as necessary. Manipulators are helper functions that make it possible to control input/output streams using operator. (since C20) The manipulators that are invoked with arguments (e.g. Controls whether leading whitespace is skipped on input (function).
- File Input Output C++
- C++ File Input Output Example
- C++ Input File Data
- C++ Output To File
- C++ Input File Example
C++ Standard Library |
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Containers |
C standard library |
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In the C++programming language, input/output library refers to a family of class templates and supporting functions in the C++ Standard Library that implement stream-based input/output capabilities.[1][2] It is an object-oriented alternative to C's FILE-based streams from the C standard library.[3][4]
History[edit]
Bjarne Stroustrup, the creator of C++, wrote the first version of the stream I/O library in 1984, as a type-safe and extensible alternative to C's I/O library.[5] The library has undergone a number of enhancements since this early version, including the introduction of manipulators to control formatting, and templatization to allow its use with character types other than
char
.Standardization in 1998 saw the library moved into the
std
namespace, and the main header changed from <iostream.h>
to <iostream>
. It is this standardized version that is covered in the rest of the article.Overview[edit]
Most of the classes in the library are actually very generalized class templates. Each template can operate on various character types, and even the operations themselves, such as how two characters are compared for equality, can be customized. However, the majority of code needs to do input and output operations using only one or two character types, thus most of the time the functionality is accessed through several typedefs, which specify names for commonly used combinations of template and character type.
For example,
basic_fstream<CharT,Traits>
refers to the generic class template that implements input/output operations on file streams. It is usually used as fstream
which is an alias for basic_fstream<char,char_traits<char>>
, or, in other words, basic_fstream
working on characters of type char
with the default character operation set.The classes in the library could be divided into roughly two categories: abstractions and implementations. Classes, that fall into abstractions category, provide an interface which is sufficient for working with any type of a stream. The code using such classes doesn't depend on the exact location the data is read from or is written to. For example, such code could write data to a file, a memory buffer or a web socket without a recompilation. The implementation classes inherit the abstraction classes and provide an implementation for concrete type of data source or sink. The library provides implementations only for file-based streams and memory buffer-based streams.
The classes in the library could also be divided into two groups by whether it implements low-level or high-level operations. The classes that deal with low-level stuff are called stream buffers. They operate on characters without providing any formatting functionality. These classes are very rarely used directly. The high-level classes are called streams and provide various formatting capabilities. They are built on top of stream buffers.
The following table lists and categorizes all classes provided by the input-output library.
Class | Explanation | Typedefs |
---|---|---|
Stream buffers (low level functionality) | ||
basic_streambuf | provides abstract low level input/output interface, that can be implemented for concrete data sources or sinks. Rarely used directly. |
|
basic_filebuf | implements low level input/output interface for file-based streams. Rarely used directly. |
|
basic_stringbuf | implements low level input/output interface for string-based streams. Rarely used directly. |
|
Support classes | ||
ios_base | manages formatting information and exception state | N/A |
basic_ios | manages a stream buffer |
|
Input streams buffers (high level functionality) | ||
basic_istream | wraps an abstract stream buffer and provides high level input interface, such as formatting capabilities. |
|
basic_ifstream | an input stream that wraps a file stream buffer. Provides functions to open or close a file in addition to those of generic input stream |
|
basic_istringstream | an input stream that wraps a string stream buffer. Provides functions to access the underlying string in addition to those of generic input stream |
|
Output streams buffers (high level functionality) | ||
basic_ostream | wraps an abstract stream buffer and provides high level output interface, such as formatting capabilities. |
|
basic_ofstream | an output stream that wraps a file stream buffer. Provides functions to open or close a file in addition to those of generic output stream |
|
basic_ostringstream | an output stream that wraps a string stream buffer. Provides functions to access the underlying string in addition to those of generic output stream |
|
Input/output streams buffers (high level functionality) | ||
basic_iostream | wraps an abstract stream buffer and provides high level input/output interface, such as formatting capabilities. |
|
basic_fstream | an input/output stream that wraps a file stream buffer. Provides functions to open or close a file in addition to those of generic input/output stream |
|
basic_stringstream | an input/output stream that wraps a string stream buffer. Provides functions to access the underlying string in addition to those of generic input/output stream |
|
Header files[edit]
The classes of the input/output library reside in several headers.
<ios>
contains the definitions ofios_base
andbasic_ios
classes, that manage formatting information and the associated stream-buffer.<istream>
contains the definition ofbasic_istream
class template, which implements formatted input.<ostream>
contains the definition ofbasic_ostream
class template, which implements formatted output.<iostream>
contains the definition ofbasic_iostream
class template, which implements formatted input and output, and includes<ios>
,<istream>
and<ostream>
.<fstream>
contains the definitions ofbasic_ifstream
,basic_ofstream
andbasic_fstream
class templates which implement formatted input, output and input/output on file streams.<sstream>
contains the definitions ofbasic_istringstream
,basic_ostringstream
andbasic_stringstream
class templates which implement formatted input, output and input/output on string-based streams.<iomanip>
contains formatting manipulators.<iosfwd>
contains forward declarations of all classes in the input/output library.
Stream buffers[edit]
There are twelve stream buffer classes defined in the C++ language as the table.
Support classes[edit]
ios_base
and basic_ios
are two classes that manage the lower-level bits of a stream. ios_base
stores formatting information and the state of the stream. basic_ios
manages the associated stream-buffer. basic_ios
is commonly known as simply ios
or wios
, which are two typedefs for basic_ios
with a specific character type. basic_ios
and ios_base
are very rarely used directly by programmers. Usually, their functionality is accessed through other classes such as iostream
which inherit them.[6][7]Typedefs[edit]
Name | description |
---|---|
ios | convenience typedef for a basic_ios working with characters of type char |
wios | convenience typedef for a basic_ios working with characters of type wchar_t |
streamoff | supports internal operations. |
streampos | holds the current position of the buffer pointer or file pointer. |
wstreampos | holds the current position of the buffer pointer or file pointer. |
streamsize | specifies the size of the stream. |
Formatting manipulators[edit]
Name | Description |
---|---|
boolalpha / noboolalpha | specifies whether variables of type bool appear as true and false or as 0 and 1 in the stream. |
skipws / noskipws | specifies whether the white space is skipped in input operations |
showbase / noshowbase | specifies whether the notational base of the number is displayed |
showpoint / noshowpoint | specifies whether to display the fractional part of a floating point number, when the fractional part is zero |
showpos / noshowpos | specifies whether to display + for positive numbers |
unitbuf / nounitbuf | specifies whether the output should be buffered |
uppercase / nouppercase | specifies whether uppercase characters should be used in hexadecimal integer and floating-point output |
left / right / internal | specifies how a number should be justified |
dec / oct / hex | specifies the notation an integer number should be displayed in |
fixed / scientific /hexfloat (C++11) / defaultfloat (C++11) | specifies the notation a floating-point number should be displayed in |
Input/output streams[edit]
C++input/output streams are primarily defined by
iostream
, a header file that is part of the C++ standard library (the name stands for Input/Output Stream). In C++ and its predecessor, the C programming language, there is no special syntax for streaming data input or output. Instead, these are combined as a library of functions. Like the cstdio
header inherited from C's stdio.h, iostream
provides basic input and output services for C++ programs. iostream uses the objectscin
, cout
, cerr
, and clog
for sending data to and from the standard streams input, output, error (unbuffered), and log (buffered) respectively. As part of the C++ standard library, these objects are a part of the std
namespace.[8]The
cout
object is of type ostream
, which overloads the left bit-shiftoperator to make it perform an operation completely unrelated to bitwise operations, and notably evaluate to the value of the left argument, allowing multiple operations on the same ostream object, essentially as a different syntax for method cascading, exposing a fluent interface. The cerr
and clog
objects are also of type ostream
, so they overload that operator as well. The cin
object is of type istream
, which overloads the right bit-shift operator. The directions of the bit-shift operators make it seem as though data is flowing towards the output stream or flowing away from the input stream. Download food games.Output formatting[edit]
Methods[edit]
width(int x) | minimum number of characters for next output |
fill(char x) | character used to fill with in the case that the width needs to be elongated to fill the minimum. |
precision(int x) | sets the number of significant digits for floating-point numbers |
Manipulators[edit]
Manipulators are objects that can modify a stream using the
<<
or >>
operators.endl | 'end line': inserts a newline into the stream and calls flush. |
ends | 'end string': inserts a null character into the stream and calls flush. |
flush | forces an output stream to write any buffered characters |
ws | causes an inputstream to 'eat' whitespace |
showpoint | tells the stream to show the decimal point and some zeros with whole numbers |
Other manipulators can be found using the header
iomanip
.Criticism[edit]
Some environments do not provide a shared implementation of the C++ library. These include embedded systems and Windows systems running programs built with MinGW. Under these systems, the C++ standard library must be statically linked to a program, which increases the size of the program,[9] or distributed as a shared library alongside the program.Some implementations of the C++ standard library have significant amounts of dead code. For example, GNU libstdc++ automatically constructs a locale when building an
ostream
even if a program never uses any types (date, time or money) that a locale affects,[10]and a statically linked 'Hello, World!' program that uses <iostream>
of GNU libstdc++ produces an executable an order of magnitude larger than an equivalent program that uses <cstdio>
.[11]There exist partial implementations of the C++ standard library designed for space-constrained environments; their
<iostream>
may leave out features that programs in such environments may not need, such as locale support.[12]Naming conventions[edit]
Examples[edit]
The canonical 'Hello, World!' program can be expressed as follows:
This program would output 'Hello, world!' followed by a newline and standard output stream buffer flush.
The following example creates a file called 'file.txt' and puts the text 'Hello, world!' followed by a newline into it. Boot camp download for mac.
References[edit]
- ^ISO/IEC 14882:2003 Programming Languages — C++. [lib.string.streams]/1
- ^Stanley B. Lippman, Josee Lajoie (1999). C++ Primer (third ed.). Massachusetts: Addison-Wesley. pp. 1109–1112. ISBN0-201-82470-1.
- ^Bjarne Stroustrup (1997). The C++ programming language (third ed.). Addison-Wesley. pp. 637–640. ISBN0-201-88954-4.
- ^Stanley B. Lippman, Josee Lajoie (1999). C++ Primer (third ed.). Massachusetts: Addison-Wesley. pp. 1063–1067. ISBN0-201-82470-1.
- ^Bjarne Stroustrup. 'A History of C++: 1979–1991'(PDF).
- ^Stanley B. Lippman, Josee Lajoie (1999). C++ Primer (third ed.). Massachusetts: Addison-Wesley. pp. 1112–1120. ISBN0-201-82470-1.
- ^'<ios> Visual Studio 2010'. Microsoft MSDN: Visual Studio 2010. Retrieved 28 September 2011.
- ^Holzner, Steven (2001). C++ : Black Book. Scottsdale, Ariz.: Coriolis Group. p. 584. ISBN1-57610-777-9.
..endl, which flushes the output buffer and sends a newline to the standard output stream.
- ^'MinGW.org: Large executables'. Retrieved 22 April 2009.
- ^GNU libstdc++ source code,
bits/ios_base.h
- ^C++ vs. C - Pin Eight
- ^'uClibc++ C++ library'. Retrieved 6 January 2012.
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Input/output_(C%2B%2B)&oldid=949056496'
C++ provides the following classes to perform output and input of characters to/from files: ofstream
: Stream class to write on filesifstream
: Stream class to read from filesfstream
: 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.
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 wasmyfile
) 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 functionclose
. 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
.Text files
Text file streams are those where theios::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 abool
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 asbad()
, 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 returntrue
. Note thatgood
andbad
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 typestreampos
, 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 File Input Output C++
end
: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 ). |
C++ File Input Output Example
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 typestreambuf
. This buffer object may represent a memory block that acts as an intermediary between the stream and the physical file. For example, with an C++ Input File Data
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.
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
C++ Output To File
and takes place under any of the following circumstances:C++ Input File Example
- 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
andendl
. - Explicitly, with member function sync(): Calling the stream's member function
sync()
causes an immediate synchronization. This function returns anint
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 returns0
.
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