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- <font size="7">Filesystem Version 3<br>
- Design</font></td>
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- <i><b>Contents</b></i></td>
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- <a href="#Introduction">Introduction</a><br>
- <a href="#Problem">Problem</a><br>
- <a href="#Solution">Solution</a><br>
- <a href="#Details">Details</a><br>
- <a href="#Other-changes">Other changes</a><br>
- <a href="#Acknowledgements">Acknowledgements</a></td>
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- <p><b>Caution:</b> This page documents thinking early in the V3 development
- process, and is intended to serve historical purposes. It is not updated to
- reflect the current state of the library.</p>
- <h2><a name="Introduction">Introduction</a></h2>
- <p>During the review of Boost.Filesystem.V2 (Internationalization), Peter Dimov
- suggested that the<code> basic_path</code> class template was unwieldy, and that a single
- path type that accommodated multiple character types and encodings would be more
- flexible. Although I wasn't willing to stop development at that time to
- explore how this idea might be implemented, or to break from the pattern for
- Internationalization used the C++ standard library, I've often thought about
- Peter's suggestion. With the advent of C++0x <code>char16_t</code> and <code>char32_t</code>
- character
- types, the <code>basic_path</code> class template approach becomes even more unwieldy, so it
- is time to revisit the problem in light of Peter's suggestion.</p>
- <h2><b><a name="Problem">Problem</a></b></h2>
- <p>With Filesystem.V2, a path argument to a user defined function that is to
- accommodate multiple character types and encodings must be written as a
- template. Do-the-right-thing overloads or template metaprogramming must be
- employed to allow arguments to be written as string literals. Here's what it
- looks like:</p>
- <blockquote>
- <pre>template<class Path>
- void foo( const Path & p );</pre>
- <pre>inline void foo( const path & p )
- {
- return foo<path>( p );
- }
- inline void foo( const wpath & p )
- {
- return foo<wpath>( p );
- }</pre>
- </blockquote>
- <p>That's really ugly for such a simple need, and there would be a combinatorial
- explosion if the function took multiple Path arguments and each could be either
- narrow or wide. It gets even worse if the C++0x <code>char16_t</code> and <code>
- char32_t</code> types are to be supported.</p>
- <h2><a name="Solution">Solution</a></h2>
- <p>Overview:</p>
- <ul>
- <li>A single, non-template, <code>class path</code>.</li>
- <li>Each member function is a template accommodating the various
- applicable character types, including user-defined character types.</li>
- <li>Hold the path internally in a string of the type used by the operating
- system API; <code>std::string</code> for POSIX, <code>std::wstring</code> for Windows.</li>
- </ul>
- <p>The signatures presented in <a href="#Problem">Problem</a> collapse to
- simply:</p>
- <blockquote>
- <pre>void foo( const path & p );</pre>
- </blockquote>
- <p>That's a signification reduction in code complexity. Specification becomes
- simpler, too. I believe it will be far easier to teach, and result in much more
- flexible user code.</p>
- <p>Other benefits:</p>
- <ul>
- <li>All the polymorphism still occurs at compile time.</li>
- <li>Efficiency is increased, in that conversions of the encoding, if required,
- only occur once at the time of creation, not each time the path is used.</li>
- <li>The size of the implementation code drops approximately in half and
- becomes much more readable.</li>
- </ul>
- <p>Possible problems:</p>
- <ul>
- <li>The combination of member function templates and implicit constructors can
- result in unclear error messages when the user makes simple commonplace coding
- errors. This should be much less of a problem with C++ concepts, but in the
- meantime work continues to restrict over aggressive templates via enable_if/disable_if.</li>
- </ul>
- <h2><a name="Details">Details</a></h2>
- <table border="1" cellpadding="4" cellspacing="0" style="border-collapse: collapse"
- bordercolor="#111111" width="100%">
- <tr>
- <td width="33%" colspan="3">
- <p align="center"><b><i>Encoding </i></b><i><b>Conversions</b></i></td>
- </tr>
- <tr>
- <td width="33%">
- <p align="center"><i><b>Host system</b></i></td>
- <td width="33%">
- <p align="center"><i><b>char string path arguments</b></i></td>
- <td width="34%">
- <p align="center"><i><b>wide string path arguments</b></i></td>
- </tr>
- <tr>
- <td width="33%">Systems with <code>char</code> as the native API path character type (i.e.
- POSIX-like systems)</td>
- <td width="33%">No conversion.</td>
- <td width="34%">Conversion occurs, performed by the current path locale's
- <code>codecvt</code> facet.</td>
- </tr>
- <tr>
- <td width="33%">Systems with <code>wchar_t</code> as the native API path character type
- (i.e. Windows-like systems).</td>
- <td width="33%">Conversion occurs, performed by the current path locale's
- <code>codecvt</code> facet.</td>
- <td width="34%">No conversion.</td>
- </tr>
- </table>
- <p>When a class path function argument type matches the operating system's
- API argument type for paths, no conversion is performed rather than conversion
- to a specified encoding such as one of the Unicode encodings. This avoids
- unintended consequences, etc.</p>
- <h2><a name="Other-changes">Other changes</a></h2>
- <p><b>Uniform hybrid error handling: </b>The hybrid error handling idiom has
- been consistently applied to all applicable functions.</p>
- <h2><a name="Acknowledgements">Acknowledgements</a></h2>
- <p>Peter Dimov suggested the idea of a single path class that could cope with
- multiple character types and encodings. Walter Landry contributed both the design
- and implementation of the copy_any,
- copy_directory, copy_symlink, and read_symlink functions.</p>
- <hr>
- <p>Revised
- <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2014<!--webbot bot="Timestamp" endspan i-checksum="38652" --></p>
- <p>© Copyright Beman Dawes, 2008</p>
- <p> Use, modification, and distribution are subject to the Boost Software
- License, Version 1.0. See <a href="http://www.boost.org/LICENSE_1_0.txt">
- www.boost.org/LICENSE_1_0.txt</a></p>
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