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Input/output (C++)

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C++ Standard Library
Containers
C standard library

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 @@

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. 

   @@History @@

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.[6] 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.

Stream buffers

There are twelve stream buffer classes defined in the C++ language as the table.

Support classes

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.[7][8]

Typedefs

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

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

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 objects cin, 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.[9]

The cout object is of type ostream, which overloads the left bit-shift operator 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.

Output formatting

Methods

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

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

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,[10] 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,[11] 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>.[12]

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.[13]

Naming conventions

Main article: Standard streams

Examples

The canonical "Hello, World!" program can be expressed as follows: 

#include <iostream>

int main()
{
    std::cout << "Hello, world!" << std::endl;
}

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. 

#include <fstream>
int main()
{
    std::ofstream file("file.txt");
    file << "Hello, world!" << std::endl;
}

References

  1. ^ ISO/IEC 14882:2003 Programming Languages — C++. [lib.string.streams]/1
  2. ^ Stanley B. Lippman, Josee Lajoie (1999). C++ Primer (third ed.). Massachusetts: Addison-Wesley. pp. 1109–1112. ISBN 0-201-82470-1.
  3. ^ Bjarne Stroustrup (1997). The C++ programming language (third ed.). Addison-Wesley. pp. 637–640. ISBN 0-201-88954-4.
  4. ^ Stanley B. Lippman, Josee Lajoie (1999). C++ Primer (third ed.). Massachusetts: Addison-Wesley. pp. 1063–1067. ISBN 0-201-82470-1.
  5. ^ Bjarne Stroustrup. "A History of C++: 1979–1991" (PDF).
  6. ^ Bjarne Stroustrup. "A History of C++: 1979–1991" (PDF).
  7. ^ Stanley B. Lippman, Josee Lajoie (1999). C++ Primer (third ed.). Massachusetts: Addison-Wesley. pp. 1112–1120. ISBN 0-201-82470-1.
  8. ^ "<ios> Visual Studio 2010". Microsoft MSDN: Visual Studio 2010. Retrieved 28 September 2011.
  9. ^ Holzner, Steven (2001). C++ : Black Book. Scottsdale, Ariz.: Coriolis Group. p. 584. ISBN 1-57610-777-9. ...endl, which flushes the output buffer and sends a newline to the standard output stream.
  10. ^ "MinGW.org: Large executables". Retrieved 22 April 2009.
  11. ^ GNU libstdc++ source code, bits/ios_base.h
  12. ^ C++ vs. C - Pin Eight
  13. ^ "uClibc++ C++ library". Retrieved 6 January 2012.
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