ISO/IEC JTC1 SC22 WG21 N3978 - 2014-05-26
Lawrence Crowl, [email protected]
Peter Sommerlad, [email protected]
Introduction
Solution
Wording
27.8.1 Overview [string.stream.overview]
27.8.8 Class template basic_ostream_buffer [ostream.buffer]
27.8.8.1 Constructor [ostream.buffer.ctor]
27.8.8.2 Destructor [ostream.buffer.dtor]
27.8.8.2 Member Functions [ostream.buffer.mfun]
27.8.9 Class template noteflush_stringbuf [ostream.noteflush.stringbuf]
27.8.9.1 Member Functions [ostream.noteflush.stringbuf.mfun]
Revisions
References
At present, stream output operations guarantee that they will not produce race conditions, but do not guarantee that the effect will be sensible. Some form of external synchronization is required. Unfortunately, without a standard mechanism for synchronizing, independently developed software will be unable to synchronize.
N3535 C++ Stream Mutexes proposed a standard mechanism for finding and sharing a mutex on streams. At the Spring 2013 standards meeting, the Concurrency Study Group requested a change away from a full mutex definition to a definition that also enabled buffering.
N3678 C++ Stream Guards proposed a standard mechanism for batching operations on a stream. That batching may be implemented as mutexees, as buffering, or some combination of both. It was the response to the Concurrency Study Group. A draft of that paper was reviewed in the Library Working Group, who found too many open issues on what was reasonably exposed to the 'buffering' part.
N3665 Uninterleaved Sring Output Streaming
proposed making streaming of strings of length less than BUFSIZ
appear uninterleaved in the output.
It was a "minimal" functionality change to the existing standard
to address the problem.
The full Committee chose not to adopt that minimal solution.
The general consensus in the July 2013 meeting of the Concurrency Study Group was that buffering should be explicit. This paper proposes such an explicit buffering.
We propose a basic_ostream_buffer,
that provides access to a matching basic_ostringstream
for buffering output operations.
The basic_ostream_buffer,
will atomically transfer the contents
of the basic_ostringstream to an ostream
on destruction of the basic_ostream_buffer.
The transfer on destruction simplifies the code and ensures at least some output in the presence of an exception.
The intent is that the basic_ostream_buffer
is an automatic-duration variable
with a relatively small scope
which constructs the text to appear uninterleaved.
For example,
....
{
std::ostream_buffer bout(std::cout);
bout << "Hello, " << "World!" << std::endl;
}
....
The wording below
permits implementation of basic_ostream_buffer
with either a stream_mutex
from
N3535
or with implementations suitable for
N3665,
e.g. with Posix file locks
[PSL]
This wording is relative to N3691.
Edit within paragraph 1 as follows.
The header
<sstream>defines four class templates and eight types that associate stream buffers with objects of classbasic_string, as described in 21.3. It also defines a class templatebasic_ostream_bufferto buffer abasic_ostringstreaminto anostreamand the corresponding string buffer.
Edit within the synopsis as follows.
template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_stringstream; typedef basic_stringstream<char> stringstream; typedef basic_stringstream<wchar_t> wstringstream; template <class charT, class traits = std::char_traits<charT>, class Allocator = std::allocator<charT> > class basic_ostream_buffer typedef basic_ostream_buffer<char> ostream_buffer; typedef basic_ostream_buffer<wchar_t> wostream_buffer; template <class charT, class traits = std::char_traits<charT>, class Allocator = std::allocator<charT> > class noteflush_stringbuf typedef basic_ostream_buffer<char> ostream_buffer; typedef basic_ostream_buffer<wchar_t> wostream_buffer;
basic_ostream_buffer [ostream.buffer]Add a new section.
template <class charT, class traits = std::char_traits<charT>, class Allocator = std::allocator<charT> > class basic_ostream_buffer : public std::basic_ostringstream<charT,traits,Allocator> { public: typedef charT char_type; typedef traits traits_type; typedef typename traits_type::int_type int_type; typedef typename traits_type::pos_type pos_type; typedef typename traits_type::off_type off_type; typedef Allocator allocator_type; typedef std::basic_string<char_type, traits_type, allocator_type> string_type; basic_ostream_buffer(basic_ostream_buffer &osb); explicit basic_ostream_buffer(std::basic_ostream<charT,traits> &os); ~basic_ostream_buffer(); };The class template
basic_ostream_buffersupports buffering into anoteflush_stringbufand then indivisibly transfering the contents of thenoteflush_stringbufto abasic_ostream.[Example:
....{ std::ostream_buffer bout(std::cout); bout << "Hello, " << "World!" << std::endl; }....—end example]
Add a new section.
basic_ostream_buffer(basic_ostream_buffer &osb);Effects: Constructs a
noteflush_stringbuf. May construct a mutex.basic_ostream_buffer(std::basic_ostream<charT,traits> &os);Effects: Constructs a
noteflush_stringbuf. May construct a mutex.
Add a new section.
~basic_ostream_buffer();Effects: Transfers the contents of the
noteflush_stringbufto the stream specified in the constructor as an indivisible uninterleaved sequence of characters, with respect to all other uses ofbasic_ostream_bufferon that stream. If and only if a flush was requested on thenoteflush_stringbuf, the stream will be flushed. May destroy a lock.Synchronization: May or may not acquire a mutex while transfering characters.
Add a new section.
void clear_for_reuse();Effects: As if the
ostream_bufferwere destroyed and recreated.Synchronization: May or may not acquire a mutex while transfering characters.
noteflush_stringbuf [ostream.noteflush.stringbuf]Add a new section.
template <class charT, class traits, class Allocator> class noteflush_stringbuf : public std::basic_stringbuf<charT,traits,Allocator> { public: using std::basic_stringbuf<charT, traits, Allocator>::basic_stringbuf; bool needsflush = false; void clear(); protected: virtual int sync(); };The class template
noteflush_stringbufbehaves as abasic_stringbufexcept that flushes are only noted instead of effective.
Add a new section.
void clear();Effects: Empties the contents.
int sync();Effects: Notes the occurence of a flush in
needsflush.Returns: 0.
This paper revises N3892 C++ Ostream Buffers
Flush the ostream if and only if the ostream_buffer was flushed.
Add the clear_for_reuse function.
Change the design from inheriting from basic_ostream
to using a noteflush_stringbuf,
which is a slightly modified basic_stringbuf.
The modification is to note the flush rather than act upon it.
N3892 revised N3750 C++ Ostream Buffers
Change name to basic_ostream_buffer
and add the usual typedefs.
Change interface to inherit from basic_ostringstream
rather than provide access to a member of that type.
Add a Revisions section.