Input/output library

C++ includes the following input/output libraries: an OOP-style stream-based I/O library, print-based family of functions (since C++23), and the standard set of C-style I/O functions.

Stream-based I/O

The stream-based input/output library is organized around abstract input/output devices. These abstract devices allow the same code to handle input/output to files, memory streams, or custom adaptor devices that perform arbitrary operations (e.g. compression) on the fly.

Most of the classes are templated, so they can be adapted to any basic character type. Separate typedefs are provided for the most common basic character types (char and wchar_t). The classes are organized into the following hierarchy:

Abstraction
Defined in header <ios>
ios_base	manages formatting flags and input/output exceptions (class)
basic_ios	manages an arbitrary stream buffer (class template)
Defined in header <streambuf>
basic_streambuf	abstracts a raw device (class template)
Defined in header <ostream>
basic_ostream	wraps a given abstract device (std::basic_streambuf) and provides high-level output interface (class template)
Defined in header <istream>
basic_istream	wraps a given abstract device (std::basic_streambuf) and provides high-level input interface (class template)
basic_iostream	wraps a given abstract device (std::basic_streambuf) and provides high-level input/output interface (class template)
File I/O implementation
Defined in header <fstream>
basic_filebuf	implements raw file device (class template)
basic_ifstream	implements high-level file stream input operations (class template)
basic_ofstream	implements high-level file stream output operations (class template)
basic_fstream	implements high-level file stream input/output operations (class template)
String I/O implementation
Defined in header <sstream>
basic_stringbuf	implements raw string device (class template)
basic_istringstream	implements high-level string stream input operations (class template)
basic_ostringstream	implements high-level string stream output operations (class template)
basic_stringstream	implements high-level string stream input/output operations (class template)
Array I/O implementations
Defined in header <spanstream>
basic_spanbuf (C++23)	implements raw fixed character buffer device (class template)
basic_ispanstream (C++23)	implements fixed character buffer input operations (class template)
basic_ospanstream (C++23)	implements fixed character buffer output operations (class template)
basic_spanstream (C++23)	implements fixed character buffer input/output operations (class template)
Defined in header <strstream>
strstreambuf (deprecated in C++98)	implements raw character array device (class)
istrstream (deprecated in C++98)	implements character array input operations (class)
ostrstream (deprecated in C++98)	implements character array output operations (class)
strstream (deprecated in C++98)	implements character array input/output operations (class)
Synchronized output
Defined in header <syncstream>
basic_syncbuf (C++20)	synchronized output device wrapper (class template)
basic_osyncstream (C++20)	synchronized output stream wrapper (class template)

Typedefs

The following typedefs for common character types are provided in namespace std:

Type	Definition
Defined in header <ios>
std::ios	std::basic_ios<char>
std::wios	std::basic_ios<wchar_t>
Defined in header <streambuf>
std::streambuf	std::basic_streambuf<char>
std::wstreambuf	std::basic_streambuf<wchar_t>
Defined in header <istream>
std::istream	std::basic_istream<char>
std::wistream	std::basic_istream<wchar_t>
std::iostream	std::basic_iostream<char>
std::wiostream	std::basic_iostream<wchar_t>
Defined in header <ostream>
std::ostream	std::basic_ostream<char>
std::wostream	std::basic_ostream<wchar_t>
Defined in header <fstream>
std::filebuf	std::basic_filebuf<char>
std::wfilebuf	std::basic_filebuf<wchar_t>
std::ifstream	std::basic_ifstream<char>
std::wifstream	std::basic_ifstream<wchar_t>
std::ofstream	std::basic_ofstream<char>
std::wofstream	std::basic_ofstream<wchar_t>
std::fstream	std::basic_fstream<char>
std::wfstream	std::basic_fstream<wchar_t>
Defined in header <sstream>
std::stringbuf	std::basic_stringbuf<char>
std::wstringbuf	std::basic_stringbuf<wchar_t>
std::istringstream	std::basic_istringstream<char>
std::wistringstream	std::basic_istringstream<wchar_t>
std::ostringstream	std::basic_ostringstream<char>
std::wostringstream	std::basic_ostringstream<wchar_t>
std::stringstream	std::basic_stringstream<char>
std::wstringstream	std::basic_stringstream<wchar_t>
Defined in header <spanstream>
std::spanbuf (C++23)	std::basic_spanbuf<char>
std::wspanbuf (C++23)	std::basic_spanbuf<wchar_t>
std::ispanstream (C++23)	std::basic_ispanstream<char>
std::wispanstream (C++23)	std::basic_ispanstream<wchar_t>
std::ospanstream (C++23)	std::basic_ospanstream<char>
std::wospanstream (C++23)	std::basic_ospanstream<wchar_t>
std::spanstream (C++23)	std::basic_spanstream<char>
std::wspanstream (C++23)	std::basic_spanstream<wchar_t>
Defined in header <syncstream>
std::syncbuf (C++20)	std::basic_syncbuf<char>
std::wsyncbuf (C++20)	std::basic_syncbuf<wchar_t>
std::osyncstream (C++20)	std::basic_osyncstream<char>
std::wosyncstream (C++20)	std::basic_osyncstream<wchar_t>

Predefined standard stream objects

Defined in header <iostream>
cinwcin	reads from the standard C input stream stdin (global object)
coutwcout	writes to the standard C output stream stdout (global object)
cerrwcerr	writes to the standard C error stream stderr, unbuffered (global object)
clogwclog	writes to the standard C error stream stderr (global object)

I/O Manipulators

The stream-based I/O library uses I/O manipulators (e.g. std::boolalpha, std::hex, etc.) to control how streams behave.

Types

The following auxiliary types are defined:

Defined in header <ios>
streamoff	represents relative file/stream position (offset from fpos), sufficient to represent any file size (typedef)
streamsize	represents the number of characters transferred in an I/O operation or the size of an I/O buffer (typedef)
fpos	represents absolute position in a stream or a file (class template)

The following typedef names for std::fpos<std::mbstate_t> are provided:

Defined in header <iosfwd>
Type	Definition
std::streampos	std::fpos<std::char_traits<char>::state_type>
std::wstreampos	std::fpos<std::char_traits<wchar_t>::state_type>
std::u8streampos (C++20)	std::fpos<std::char_traits<char8_t>::state_type>
std::u16streampos (C++11)	std::fpos<std::char_traits<char16_t>::state_type>
std::u32streampos (C++11)	std::fpos<std::char_traits<char32_t>::state_type>

Error category interface

Defined in header <ios>
io_errc (C++11)	the IO stream error codes (enum)
iostream_category (C++11)	identifies the iostream error category (function)

Print functions (since C++23)

The Unicode-aware print-family functions that perform formatted I/O on text that is already formatted. They bring all the performance benefits of std::format, are locale-independent by default, reduce global state, avoid allocating a temporary std::string object and calling operator<<, and in general make formatting more efficient compared to iostreams and stdio.

The following print-like functions are provided:

Defined in header <print>
print (C++23)	prints to stdout or a file stream using formatted representation of the arguments (function template)
println (C++23)	same as std::print except that each print is terminated by additional new line (function template)
vprint_unicode (C++23)	prints to Unicode capable stdout or a file stream using type-erased argument representation (function)
vprint_nonunicode (C++23)	prints to stdout or a file stream using type-erased argument representation (function)

C-style I/O

C++ also includes the input/output functions defined by C, such as std::fopen, std::getc, etc.