C++ Ostream Buffers

ISO/IEC JTC1 SC22 WG21 N4187 - 2014-10-08

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

Introduction
Solution
Design
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.

N3978 proposed such an explicit buffering. This updated paper fixes some of the issues raised by SG1 and Daniel Krügler.

Solution

We propose a basic_ostream_buffer, that provides buffering output operations for a wrapped stream. The basic_ostream_buffer, will atomically transfer the contents of the internal stream buffer noteflush_stringbuf 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;
}
....

Design

We follow typical stream conventions of basic_ prefixes and typedefs.

The constructors for ostream_buffer take non-const references to either a basic_ostream or a basic_ostream_buffer. This non-const reference indicates that the destruction of the buffer may write to the stream associated with the argument.

The constructors for ostream_buffer from another ostream_buffer is not a copy constructor, but an attachment to the argument buffer. This approach enables nested passing an ostream_buffer to a function expecting an ostream, which then in turn uses an ostream_buffer on that ostream

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 output and pass the buffered content into an ostream on destruction or explicit request by calling clear_for_reuse() in one consecutive sequence.

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 = char_traits<charT>,
            class Allocator = 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 = char_traits<charT>,
            class Allocator = 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_ostream<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;

  basic_ostream_buffer(basic_ostream_buffer &osb);
  explicit basic_ostream_buffer(std::basic_ostream<charT,traits> &os);
  ~basic_ostream_buffer();
  void clear_for_reuse();
  noteflush_stringbuf *rdbuf() const;
private:
  std::basic_ostream<chart,traits>& out; // exposition only
};

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]

[Note: If the underlying stream can throw exceptions, use clear_for_reuse() to catch those, otherwise an exception of the underlying stream may terminate() your program. —end note]

27.8.8.1 Constructor [ostream.buffer.ctor]

Add a new section.

basic_ostream_buffer(basic_ostream_buffer &osb);

Effects: Stores a reference to the basic_ostream that is the base class of basic_ostream_buffer, and which will latter be the output stream. Constructs a noteflush_stringbuf. May construct a mutex.

[Note: This overload exists to allow wrapping a basic_ostream_buffer again with a basic_ostream_buffer. end note.]

basic_ostream_buffer(std::basic_ostream<charT,traits> &os);

Effects: Stores a reference to the basic_ostream, which will latter be the output stream. Constructs a noteflush_stringbuf. May construct a mutex.

27.8.8.2 Destructor [ostream.buffer.dtor]

Add a new section.

~basic_ostream_buffer() noexcept;

Effects: clear_for_reuse(). 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: 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.

Postcondition: !rdbuf()->flushed() && rdbuf()->str().empty().

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>
{
    bool needs_flush_=false; // exposition only
  public:
    using std::basic_stringbuf<charT, traits, Allocator>::basic_stringbuf;
  protected:
    bool flushed() const noexcept { return needs_flush_;} // exposition only
    virtual int sync();
};
The class template noteflush_stringbuf behaves as a basic_stringbuf except that flushes of the stream using it, which incur in calling the sync() member function are noted instead of a no-op.

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

Add a new section.

int sync() noexcept;

Effects: Notes the occurence of a flush within the object.

Postcondition: flushed().

[Note: The postcondition of sync() ensures that a subsequent call to clear_for_reuse() in the owning basic_ostream_buffer will flush the stream being buffered. -- end note]

Returns: 0.

Revisions

ISO/IEC JTC1 SC22 WG21 N4187 - 2014-10-08

This paper revises N4069 C++ Ostream Buffers

Added note to sync as suggested by BSI via email.

N4069 paper revised N3978 C++ Ostream Buffers

Added a Design section.
Clarify the reference capturing behavior of the ostream_buffer constructors.
Added noexcept and const as appropriate to members.
Added note on throwing wrapped streams.
Change the noteflush_stringbuf public member variable needsflush to a public member query function flushed.
Removed the public member function noteflush_stringbuf::clear.
Minor synopsis formatting changes.
Incorporated feedback from SG1 and Dietmar Kühl in specific in Rapperswil.

N3978 revised 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.

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