The iostream library is an object-oriented library that provides input and output functionality using streams.
A stream is an abstraction that represents a device on which input and ouput operations are performed. A stream can basically be represented as a source or destination of characters of indefinite length.
Streams are generally associated to a physical source or destination of characters, like a disk file, the keyboard, or the console, so the characters gotten or written to/from our abstraction called stream are physically input/output to the physical device. For example, file streams are C++ objects to manipulate and interact with files; Once a file stream is used to open a file, any input or output operation performed on that stream is physically reflected in the file.
To operate with streams, C++ provides the standard iostream library, which contains the following elements:
Basic class templates
The base of the iostream library is the hierarchy of class templates. The class templates provide most of the functionality of the library in a type-independent fashion.
This is a set of class templates, each one having two template parameters: the char type (charT) parameter, that determines the type of elements that are going to be manipulated and the traits parameter, that provides additional characteristics specific for a particular type of elements.
The class templates in this class hierarchy have the same name as their char-type instantiations but with the prefix basic_. For example, the class template which istream is instantiated from is called basic_istream, the one from which fstream is is called basic_fstream, and so on... The only exception is ios_base, which is by itself type-independent, and therefore is not based on a template, but is a regular class.
Class template instantiations
The library incorporates two standard sets of instantiations of the entire iostream class template hierarchy: one is narrow-oriented, to manipulate elements of type char and another one, wide-oriented, to manipulate elements of type wchar_t.
The narrow-oriented (char type) instantiation is probably the better known part of the iostream library. Classes like ios, istream and ofstream are narrow-oriented. The diagram on top of this page shows the names and relationships of narrow-oriented classes.
The classes of the wide-oriented (wchar_t) instatiation follow the same naming conventions as the narrow-oriented instantiation but with the name of each class and object prefixed with a w character, forming wios, wistream and wofstream, as an example.
Standard objects
As part of the iostream library, the header file
They are divided in two sets: narrow-oriented objects, which are the popular cin, cout, cerr and clog and their wide-oriented counterparts, declared as wcin, wcout, wcerr and wclog.
Types
The iostream classes barely use fundamental types on their member's prototypes. They generally use defined types that depend on the traits used in their instantiation. For the default char and wchar_t instantiations, types streampos, streamoff and streamsize are used to represent positions, offsets and sizes, respectively.
Manipulators
Manipulators are global functions designed to be used together with insertion (<<) and extraction (>>) operators performed on iostream stream objects. They generally modify properties and formatting settings of the streams. endl, hex and scientific are some examples of manipulators.
Source : http://www.cplusplus.com/reference/iostream/
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