I/O Streams

Overview

The CRT provides  printf() variants to emit formatted output of data such as integers, boolean, strings etc. on different targets such as console, character buffers, files. 

Similarly, CRT also provides  scanf() variants to extract input from console, character buffers, files for data such as integers, boolean, strings etc. 

This is demonstrated in this example.

The standard library Input and Output Stream classes provides an alternative for console, disk and string IO  instead of CRT functions.

Details

IO Stream classes provides the same functionality in a structured and object oriented way.  It uses manipulators to achieve formatting. Overloaded operator >>() and operator <<() along with dedicated functions to do the Input and Output.  Additional functionalities  that is not possible using CRT  can also be achieved. Examples: custom formatting manipulators or I/O of any user defined or standard types. 

Just like CRT, IO Stream classes can be used for IO with  console, file and strings in memory.

This is demonstrated in this example 2.

A locale different from the current locale can be injected into IO stream classes to encode/decode independently while doing I/O.

Class Structure

The following depicts the class structure. Note that classes that start with the letter w are defined for whcar_t and other classes are defined for char. char16_t and char32_t are not supported.

Class Organization➹

The classes can be grouped into following categories - Primitive base, Intermediate base , disk based streams and String based streams.

Primitive Base classes

The following describes each of the bases classes in detail.

Input and Output Preparation

ios_base➹

ios_base class describes the storage and member functions common to both input and output streams that don't depend on the template parameters. 

basic_ios➹

basic_ios class describes the storage and member functions common to both input and output streams that don't depend on the template parameters. 

Serialization

These base classes support read / write stream operations.

basic_streambuf➹

basic_streambuf class describes the mechanism of of the low level layer of reading and writing streams to devices like console, file and string.

Intermediate Base classes

These define intermediate base classes.  It also includes console I/O stream objects.

Input and Output Preparation

basic_istream➹

basic_istream is the base class of all the input classes designed to receive inputs from various sources such as console, files and strings.

basic_ostream➹

basic_ostream is the base class of all the output classes designed to publish outputs to various targets such as console, files and strings.

basic_iostream➹

basic_iostream is the base class of all the classes that perform input and output operations on  file and string streams.

Predefined Console IO Streams

These  predefined console based streams are derived from istream and ostream.

The CRT provides three predefined file objects stdin mapped to console input; stdout and stderr mapped to console output.

Similarly,  IO Stream classes also defines 2 sets (char_t and wchar_t) of 8  predefined streams as below.

Name	class	CRT	Description
cin	istream	stdin	Buffered Console input
wcin	wistream	stdin	Buffered Console input
cout	ostream	stdout	Buffered Console output
wcout	wostream	stdout	Buffered Console output
cerr	ostream	stderr	Buffered Console output
wcerr	wostream	stderr	Buffered Console output
clog	ostream	stderr	Unbuffered Console output
wclog	wostream	stderr	Unbuffered Console output

IO Stream extraction and insertion operators and Manipulators➹

istreams defines extraction operators that are basically overloaded operator >>() functions to extract values from the inputs 

Similarly, ostreams defines insertion operators that are basically overloaded operator<<() functions to insert values into the output. 

Manipulators are helper functions that prepare the input/output streams before using operator << or operator >>.

Disk based Streams

These provides stream classes  that can be used Input only, Output only or Both with files.

Classes that does input only are derived from basic_istream. Similarly classes that does output only are derived from basic_ostream. Classes that do both input and output are derived from basic_iostream class.

Input and Output Preparation

basic_ifstream➹

basic_ifstream class is designed to receive inputs from sources such as file system.

basic_ofstream➹

basic_ofstream class is designed to send outputs to sources such as file system.

basic_fstream➹

basic_fstream class is designed to receive inputs from and send outputs to sources such as file system.

Serialization

basic_filebuf➹

basic_filebuf is a template based class used for read from or/and write to files in the file system. It's used by ifstream, ofstream and fstream classes to read from or/and write to files.

The example 8 demonstrates its usage.

String based streams

These provides stream classes  that can be used Input only, Output only or Both with string.

Classes that does input only are derived from basic_istream. Similarly classes that does output only are derived from basic_ostream. Classes that do both input and output are derived from basic_iostream class.

Input and Output Preparation

basic_istringstream➹

basic_istringstream class is designed to receive inputs from sources such as strings in the memory.

basic_ostringstream➹

basic_ostringstream class is designed to send outputs to sources such as strings in the memory.

basic_stringstream➹

basic_stringstream class is designed to receive inputs from and send outputs to sources such as strings in the memory.

Serialization

basic_stringbuf➹

The example 9 demonstrates its usage.

Stream iterators

istream_iterator and ostream_iterator can be used for typed input and  output. They are faster as they skip construction of sentry objects.

istream_iterator➹
istream iterators are input iterators that read successive elements from an input stream (such as cin).
The example 14 demonstrates its usage.

ostream_iterator➹

ostream iterators are output iterators that write successive elements from an output stream (such as cout). 

Streambuf iterators
streambuf defines input iterator and output iterator for untyped input and output.
istreambuf_iterator and ostreambuf_iterator can be used instead of raw i/o apis.

istreambuf_iterator➹
istreambuf iterators are input iterators that read successive characters from an input stream (such as cin).

ostreambuf_iterator➹
ostreambuf iterators are output iterators that write successive characters from an output stream (such as cout).
The example 15 demonstrates its usage.

Synchronizations and Redirection

tie()

An input stream and output stream can be tied such that output stream is always flushed before input stream is fetched for input. 

The example 10 demonstrates use of tie() operation as seen in its console output.

Here,  EOF is received when input is fetched before tie(). After tie() the output is flushed and same is fetched during input.

Redirection of stream

Two streams can also be made to reuse same streambuf by instantiating the second stream with the streambuf of the first.

The example 11 demonstrates reuse of same streambuf to print same number in hex and decimal as seen in its console output.

Streambuf reuse 

The contents of a predefined stream such as clog can be redirected to another stream such as a file.

This example 12 redirects output of clog to a file as seen in its console output.

Redirection using streambuf

Contents of  a stream can also be outputted to cout.

This  example 13 populating a istringstream with some text and later print it on console using its streambuf  as seen its console output.

Summary of Examples

Name	Category	Description	Github	WandBox
Example	CRT Demo	stdin,stdout	source  output	source + output
Example 2	IO Stream Demo	cin,cout	source  output	source + output
Example 3	Custom Stream IO operator	operator >>, operator <<	source  output	source + output
Example 4	Stream Manipulators	ostream	source  output	source + output
Example 5	Stream Manipulators	istream	source  output	source + output
Example 6	Stream Manipulators	custom beeper	source  output	source + output
Example 7	Stream State management and error handling	IO Flags	source  output	source + output
Example 8	Disk IO	ifstream, ofstream	source  output	source + output
Example 9	Memory IO	istringstream, ostringstream	source  output	source + output
Example 10	Stream Iterator	istream_iterator, ostream_iterator	source  output	source + output
Example 11	Streambuf Iterator	istreambuf_iterator, ostreambuf_iterator	source  output	source + output
Example 10	Stream Synchronizations  and Redirection	tie	source  output	source + output
Example 13	Stream Synchronizations  and Redirection	StreamBuf reuse	source  output	source + output
Example 14	Stream Synchronizations  and Redirection	redirection to clog	source  output	source + output
Example 15	Stream Synchronizations  and Redirection	redirection to cout	source  output	source + output