C++ Ostream Buffers

ISO/IEC JTC1 SC22 WG21 N3978 - 2014-05-26

Lawrence Crowl, Lawrence@Crowl.org
Peter Sommerlad, Peter.Sommerlad@hsr.ch

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

Introduction

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.

Solution

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]

Wording

This wording is relative to N3691.

27.8.1 Overview [string.stream.overview]

Edit within paragraph 1 as follows.

The header <sstream> defines four class templates and eight types that associate stream buffers with objects of class basic_string, as described in 21.3. It also defines a class template basic_ostream_buffer to buffer a basic_ostringstream into an ostream and 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;

27.8.8 Class template 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_buffer supports buffering into a noteflush_stringbuf and then indivisibly transfering the contents of the noteflush_stringbuf to a basic_ostream.

[Example:

....
{
  std::ostream_buffer bout(std::cout);
  bout << "Hello, " << "World!" << std::endl;
}
....

end example]

27.8.8.1 Constructor [ostream.buffer.ctor]

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.

27.8.8.2 Destructor [ostream.buffer.dtor]

Add a new section.

~basic_ostream_buffer();

Effects: Transfers the contents of the noteflush_stringbuf to the stream specified in the constructor as an indivisible uninterleaved sequence of characters, with respect to all other uses of basic_ostream_buffer on that stream. If and only if a flush was requested on the noteflush_stringbuf, the stream will be flushed. May destroy a lock.

Synchronization: May or may not acquire a mutex while transfering characters.

27.8.8.3 Member Functions [ostream.buffer.mfun]

Add a new section.

void clear_for_reuse();

Effects: As if the ostream_buffer were destroyed and recreated.

Synchronization: May or may not acquire a mutex while transfering characters.

27.8.9 Class template 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_stringbuf behaves as a basic_stringbuf except that flushes are only noted instead of effective.

27.8.9.1 Member Functions [ostream.noteflush.stringbuf.mfun]

Add a new section.

void clear();

Effects: Empties the contents.

int sync();

Effects: Notes the occurence of a flush in needsflush.

Returns: 0.

Revisions

This paper revises N3892 C++ Ostream Buffers

N3892 revised N3750 C++ Ostream Buffers

References

[PSL]
The Open Group Base Specifications Issue 6, IEEE Std 1003.1, 2004 Edition, functions, flockfile, http://pubs.opengroup.org/onlinepubs/009695399/functions/flockfile.html