NAME¶

std::basic_syncbuf - std::basic_syncbuf

Synopsis¶

Defined in header <syncstream>
template<

class CharT,
class Traits = std::char_traits<CharT>, (since C++20)
class Allocator = std::allocator<CharT>

> class basic_syncbuf : public std::basic_streambuf<CharT, Traits>

std::basic_syncbuf is a wrapper for a std::basic_streambuf (provided at construction
time as a pointer). It accumulates output in its own internal buffer, and atomically
transmits its entire contents to the wrapped buffer on destruction and when
explicitly requested, so that they appear as a contiguous sequence of characters. It
guarantees that there are no data races and no interleaving of characters sent to
the wrapped buffer as long as all other outputs made to the same buffer are made
through, possibly different, instances of std::basic_syncbuf.

Typical implementation of std::basic_syncbuf holds a pointer to the wrapped
std::basic_streambuf, a boolean flag indicating whether the buffer will transmit its
contents to the wrapped buffer on sync (flush), a boolean flag indicating a pending
flush when the policy is to not emit on sync, an internal buffer that uses Allocator
(such as std::string), and a pointer to a mutex used to synchronize emit between
multiple threads accessing the same wrapped stream buffer (these mutexes may be in a
hash map with pointers to std::basic_streambuf objects used as keys).

Like other streambuf classes, std::basic_syncbuf is normally only accessed through
the corresponding stream, std::osyncstream, not directly.

Several typedefs for common character types are provided:

Defined in header <syncstream>
Type Definition
std::syncbuf std::basic_syncbuf<char>
std::wsyncbuf std::basic_syncbuf<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
allocator_type Allocator
streambuf_type std::basic_streambuf<CharT, Traits>

Member functions¶

Public member functions¶

constructor constructs a basic_syncbuf object
(public member function)
operator= assigns a basic_syncbuf object
(public member function)
swap swaps two basic_syncbuf objects
(public member function)
destructor destroys the basic_syncbuf and emits its internal buffer
(public member function)
atomically transmits the entire internal buffer to the wrapped
emit streambuf
(public member function)
get_wrapped retrieves the wrapped streambuf pointer
(public member function)
get_allocator retrieves the allocator used by this basic_syncbuf
(public member function)
set_emit_on_sync changes the current emit-on-sync policy
(public member function)

Protected member functions¶

either emits, or records a pending flush, depending on the current
sync emit-on-sync policy
(public member function)

Non-member functions¶

std::swap(std::basic_syncbuf) specializes the std::swap algorithm
(C++20) (function template)

Inherited from std::basic_streambuf

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 of
std::basic_streambuf<CharT,Traits>)

Locales¶

invokes imbue()
pubimbue (public member function of std::basic_streambuf<CharT,Traits>)

obtains a copy of the associated locale
getloc (public member function of std::basic_streambuf<CharT,Traits>)

Positioning¶

invokes setbuf()
pubsetbuf (public member function of std::basic_streambuf<CharT,Traits>)

invokes seekoff()
pubseekoff (public member function of std::basic_streambuf<CharT,Traits>)

invokes seekpos()
pubseekpos (public member function of std::basic_streambuf<CharT,Traits>)

invokes sync()
pubsync (public member function of std::basic_streambuf<CharT,Traits>)

Get area¶

obtains the number of characters immediately available in the get
in_avail area
(public member function of std::basic_streambuf<CharT,Traits>)

advances the input sequence, then reads one character without
snextc advancing again
(public member function of std::basic_streambuf<CharT,Traits>)

sbumpc reads one character from the input sequence and advances the
stossc sequence
(removed in C++17) (public member function of std::basic_streambuf<CharT,Traits>)

reads one character from the input sequence without advancing the
sgetc sequence
(public member function of std::basic_streambuf<CharT,Traits>)

invokes xsgetn()
sgetn (public member function of std::basic_streambuf<CharT,Traits>)

Put area¶

writes one character to the put area and advances the next
sputc pointer
(public member function of std::basic_streambuf<CharT,Traits>)

invokes xsputn()
sputn (public member function of std::basic_streambuf<CharT,Traits>)

Putback¶

puts one character back in the input sequence
sputbackc (public member function of std::basic_streambuf<CharT,Traits>)

moves the next pointer in the input sequence back by one
sungetc (public member function of std::basic_streambuf<CharT,Traits>)

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 of
std::basic_streambuf<CharT,Traits>)

Positioning¶

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

Get area¶

obtains the number of characters available for input in the associated
showmanyc input sequence, if known
[virtual] (virtual protected member function of
std::basic_streambuf<CharT,Traits>)
underflow reads characters from the associated input sequence to the get area
[virtual] (virtual protected member function of
std::basic_streambuf<CharT,Traits>)
reads characters from the associated input sequence to the get area
uflow and advances the next pointer
[virtual] (virtual protected member function of
std::basic_streambuf<CharT,Traits>)
xsgetn reads multiple characters from the input sequence
[virtual] (virtual protected member function of
std::basic_streambuf<CharT,Traits>)
eback returns a pointer to the beginning, current character and the end of
gptr 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 of
std::basic_streambuf<CharT,Traits>)
overflow writes characters to the associated output sequence from the put area
[virtual] (virtual protected member function of
std::basic_streambuf<CharT,Traits>)
pbase returns a pointer to the beginning, current character and the end of
pptr 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¶

puts a character back into the input sequence, possibly modifying the
pbackfail input sequence
[virtual] (virtual protected member function of
std::basic_streambuf<CharT,Traits>)

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_syncbuf 201803L (C++20) Synchronized buffered ostream (std::syncbuf,
std::osyncstream) and manipulators