iostream

C++ Standard Library
fstream iomanip ios iostream sstream string
Standard Template Library
algorithm bitset deque functional iterator list map set stack queue vector
C++11
array forward_list unordered_map unordered_set
C Standard Library
Data types Character classification Strings Mathematics File input/output Date/time Localization Memory allocation Program control Miscellaneous headers: <assert.h> <errno.h> <iso646.h> <setjmp.h> <signal.h> <stdarg.h> <wchar.h>
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iostream is a header file which is used for input/output in the C++ programming language. It 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 error (buffered) respectively. As part of the C++ standard library, these objects are a part of the std namespace.

Example usage

The canonical Hello world program can be expressed as follows:

#include <iostream>
 
int main()
{
    std::cout << "Hello,world!\n";
    return 0;
}

This program would output "Hello, world!" followed by a newline and standard output stream buffer flush.

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. 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.

An alternative to the newline character \n is endl, which is used as follows:

#include <iostream>
 
int main()
{
    std::cout << "Hello, world!" << std::endl;
    return 0;
}

endl is an output manipulator that writes a newline and flushes the buffer, ensuring that the data is output immediately. Several other manipulators are listed below.

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

Example:

using namespace std;
 
cout.width(10);
cout << "ten" << "four" << "four";

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
dec	changes the output format of number to be in decimal format
oct	changes the output format of number to be in octal format
hex	changes the output format of number to be in hexadecimal format
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,^[1] 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,^[2] 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>.^[3] 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.^[4]

Naming conventions

Please refer to Standard streams.

References

1. ^ MinGW.org: Large executables. Accessed April 22, 2009.
2. ^ GNU libstdc++ source code, bits/ios_base.h
3. ^ Pin Eight: RAnT (Rants, Articles, and Treatises)
4. ^ uClibc++ C++ library

External links

• C++ reference for input/output library
• Apache C++ Standard Library Iostreams
• Comprehensive tutorial on formatting output in C++.