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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 directly-included method for streaming data input or output. Instead, these are combined as a library. iostream handles basic input and output for C++, whereas stdio.h is used in C. 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.

The canonical Hello world program can be as follows:

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

This program would output "Hello, world!" followed by a newline.

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.

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. A commonly-used alternative to the newline character \n is endl, which is used as follows:

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

Example:

cout.width(10);
cout << "ten" << "four" << "four";

Manipulators are objects that can modify a stream using the << or >> operators.

Other manipulators can be found using the header iomanip.

Some C++ compiler toolchains are incapable of removing unreachable code when producing an executable that includes through static linking. For instance, a hello world program with the GNU implementation of the C++ standard library produces an executable an order of magnitude larger than an equivalent program that uses ,^[1] due partly to a deficiency in GNU ld.^[2] There exist partial implementations of the C++ standard library designed for space-constrained environments; their may leave out features that programs in such environments may not need, such as locale support.^[3]

• Apache C++ Standard Library Iostreams
• Comprehensive tutorial on formatting output in C++.

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