NAME¶

std::basic_streambuf - std::basic_streambuf

Synopsis¶

Defined in header <streambuf>
template<

class CharT,
class Traits = std::char_traits<CharT>

> class basic_streambuf;

The class basic_streambuf controls input and output to a character sequence. It
includes and provides access to

1. The controlled character sequence, also called the buffer, which may contain
input sequence (also called get area) for buffering the input operations and/or
output sequence (also called put area) for buffering the output operations.
2. The associated character sequence, also called source (for input) or sink (for
output). This may be an entity that is accessed through OS API (file, TCP
socket, serial port, other character device), or it may be an object
(std::vector, array, string literal), that can be interpreted as a character
source or sink.

The I/O stream objects std::basic_istream and std::basic_ostream, as well as all
objects derived from them (std::ofstream, std::stringstream, etc), are implemented
entirely in terms of std::basic_streambuf.

The controlled character sequence is an array of CharT which, at all times,
represents a subsequence, or a "window" into the associated character sequence. Its
state is described by three pointers:

1. The beginning pointer, always points at the lowest element of the buffer
2. The next pointer, points at the element that is the next candidate for reading
or writing
3. The end pointer, points one past the end of the buffer.

A basic_streambuf object may support input (in which case the buffer described by
the beginning, next, and end pointers is called get area), output (put area), or
input and output simultaneously. In latter case, six pointers are tracked, which may
all point to elements of the same character array or two individual arrays.

If the next pointer is less than the end pointer in the put area, a write position
is available. The next pointer can be dereferenced and assigned to.

If the next pointer is less than the end pointer in the get area, a read position is
available. The next pointer can be dereferenced and read from.

If the next pointer is greater than the beginning pointer in a get area, a putback
position is available, and the next pointer may be decremented, dereferenced, and
assigned to, in order to put a character back into the get area.

The character representation and encoding in the controlled sequence may be
different from the character representations in the associated sequence, in which
case a std::codecvt locale facet is typically used to perform the conversion. Common
examples are UTF-8 (or other multibyte) files accessed through std::wfstream
objects: the controlled sequence consists of wchar_t characters, but the associated
sequence consists of bytes.

Typical implementation of the std::basic_streambuf base class holds only the six
CharT* pointers and a copy of std::locale as data members. In addition,
implementations may keep cached copies of locale facets, which are invalidated
whenever imbue() is called. The concrete buffers such as std::basic_filebuf or
std::basic_stringbuf are derived from std::basic_streambuf.

std-streambuf.svg

Two convenience typedefs are provided by the standard library

Defined in header <streambuf>
Type Definition
streambuf basic_streambuf<char>
wstreambuf basic_streambuf<wchar_t>

Member types¶

Member type Definition
char_type CharT
traits_type Traits; the program is ill-formed if Traits::char_type is not CharT.
int_type Traits::int_type
pos_type Traits::pos_type
off_type Traits::off_type

Member functions¶

destructor destructs the basic_streambuf object
[virtual] (virtual public member function)

Locales¶

pubimbue invokes imbue()
(public member function)
getloc obtains a copy of the associated locale
(public member function)

Positioning¶

pubsetbuf invokes setbuf()
(public member function)
pubseekoff invokes seekoff()
(public member function)
pubseekpos invokes seekpos()
(public member function)
pubsync invokes sync()
(public member function)

Get area¶

obtains the number of characters immediately available in the get
in_avail area
(public member function)
advances the input sequence, then reads one character without
snextc advancing again
(public member function)
sbumpc reads one character from the input sequence and advances the
stossc sequence
(removed in C++17) (public member function)
reads one character from the input sequence without advancing the
sgetc sequence
(public member function)
sgetn invokes xsgetn()
(public member function)

Put area¶

writes one character to the put area and advances the next
sputc pointer
(public member function)
sputn invokes xsputn()
(public member function)

Putback¶

sputbackc puts one character back in the input sequence
(public member function)
sungetc moves the next pointer in the input sequence back by one
(public member function)

Protected member functions¶

constructor constructs a basic_streambuf object
(protected member function)
operator= replaces a basic_streambuf object
(C++11) (protected member function)
swap swaps two basic_streambuf objects
(C++11) (protected member function)

Locales¶

imbue changes the associated locale
[virtual] (virtual protected member function)

Positioning¶

setbuf replaces the buffer with user-defined array, if permitted
[virtual] (virtual protected member function)
seekoff repositions the next pointer in the input sequence, output
[virtual] sequence, or both, using relative addressing
(virtual protected member function)
seekpos repositions the next pointer in the input sequence, output
[virtual] sequence, or both using absolute addressing
(virtual protected member function)
sync synchronizes the buffers with the associated character sequence
[virtual] (virtual protected member function)

Get area¶

showmanyc obtains the number of characters available for input in the
[virtual] associated input sequence, if known
(virtual protected member function)
underflow reads characters from the associated input sequence to the get
[virtual] area
(virtual protected member function)
uflow reads characters from the associated input sequence to the get
[virtual] area and advances the next pointer
(virtual protected member function)
xsgetn reads multiple characters from the input sequence
[virtual] (virtual protected member function)
eback returns a pointer to the beginning, current character and the end
gptr of the get area
egptr (protected member function)
gbump advances the next pointer in the input sequence
(protected member function)
repositions the beginning, next, and end pointers of the input
setg sequence
(protected member function)

Put area¶

xsputn writes multiple characters to the output sequence
[virtual] (virtual protected member function)
overflow writes characters to the associated output sequence from the put
[virtual] area
(virtual protected member function)
pbase returns a pointer to the beginning, current character and the end
pptr of the put area
epptr (protected member function)
pbump advances the next pointer of the output sequence
(protected member function)
repositions the beginning, next, and end pointers of the output
setp sequence
(protected member function)

Putback¶

pbackfail puts a character back into the input sequence, possibly modifying
[virtual] the input sequence
(virtual protected member function)

See also¶

object type, capable of holding all information needed to control a C I/O
FILE stream
(typedef)