# 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

- 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