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<title>Boost Filesystem Library Design</title>
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<h1>
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<img border="0" src="../../../boost.png" align="center" width="277" height="86">Filesystem
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Library Design</h1>
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<p><a href="#Introduction">Introduction</a><br>
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<a href="#Requirements">Requirements</a><br>
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<a href="#Realities">Realities</a><br>
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<a href="#Rationale">Rationale</a><br>
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<a href="#Abandoned_Designs">Abandoned_Designs</a><br>
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<a href="#References">References</a></p>
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<h2><a name="Introduction">Introduction</a></h2>
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<p>The primary motivation for beginning work on the Filesystem Library was
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frustration with Boost administrative tools. Scripts were written in
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Python, Perl, Bash, and Windows command languages. There was no single
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scripting language familiar and acceptable to all Boost administrators. Yet they
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were all skilled C++ programmers - why couldn't C++ be used as the scripting
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language?</p>
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<p>The key feature C++ lacked for script-like applications was the ability to
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perform portable filesystem operations on directories and their contents. The
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Filesystem Library was developed to fill that void.</p>
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<p>The intent is not to compete with traditional scripting languages, but to
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provide a solution for situations where C++ is already the language
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of choice..</p>
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<h2><a name="Requirements">Requirements</a></h2>
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<ul>
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<li>Be able to write portable script-style filesystem operations in modern
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C++.<br>
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<br>
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Rationale: This is a common programming need. It is both an
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embarrassment and a hardship that this is not possible with either the current
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C++ or Boost libraries. The need is particularly acute
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when C++ is the only toolset allowed in the tool chain. File system
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operations are provided by many languages used on multiple platforms,
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such as Perl and Python, as well as by many platform specific scripting
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languages. All operating systems provide some form of API for filesystem
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operations, and the POSIX bindings are increasingly available even on
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operating systems not normally associated with POSIX, such as the Mac, z/OS,
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or OS/390.<br>
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</li>
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<li>Work within the <a href="#Realities">realities</a> described below.<br>
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<br>
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Rationale: This isn't a research project. The need is for something that works on
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today's platforms, including some of the embedded operating systems
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with limited file systems. Because of the emphasis on portability, such a
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library would be much more useful if standardized. That means being able to
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work with a much wider range of platforms that just Unix or Windows and their
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clones.<br>
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</li>
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<li>Avoid dangerous programming practices. Particularly, all-too-easy-to-ignore error notifications
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and use of global variables. If a dangerous feature is provided, identify it as such.<br>
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<br>
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Rationale: Normally this would be covered by "the usual Boost requirements...",
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but it is mentioned explicitly because the equivalent native platform and
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scripting language interfaces often depend on all-too-easy-to-ignore error
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notifications and global variables like "current
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working directory".<br>
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</li>
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<li>Structure the library so that it is still useful even if some functionality
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does not map well onto a given platform or directory tree. Particularly, much
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useful functionality should be portable even to flat
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(non-hierarchical) filesystems.<br>
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<br>
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Rationale: Much functionality which does not
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require a hierarchical directory structure is still useful on flat-structure
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filesystems. There are many systems, particularly embedded systems,
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where even very limited functionality is still useful.</li>
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</ul>
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<ul>
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<li>Interface smoothly with current C++ Standard Library input/output
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facilities. For example, paths should be
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easy to use in std::basic_fstream constructors.<br>
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<br>
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Rationale: One of the most common uses of file system functionality is to
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manipulate paths for eventual use in input/output operations.
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Thus the need to interface smoothly with standard library I/O.<br>
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</li>
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<li>Suitable for eventual standardization. The implication of this requirement
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is that the interface be close to minimal, and that great care be take
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regarding portability.<br>
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<br>
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Rationale: The lack of file system operations is a serious hole
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in the current standard, with no other known candidates to fill that hole.
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Libraries with elaborate interfaces and difficult to port specifications are much less likely to be accepted for
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standardization.<br>
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</li>
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<li>The usual Boost <a href="http://www.boost.org/more/lib_guide.htm">requirements and
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guidelines</a> apply.<br>
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</li>
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<li>Encourage, but do not require, portability in path names.<br>
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<br>
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Rationale: For paths which originate from user input it is unreasonable to
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require portable path syntax.<br>
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</li>
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<li>Avoid giving the illusion of portability where portability in fact does not
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exist.<br>
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<br>
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Rationale: Leaving important behavior unspecified or "implementation defined" does a
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great disservice to programmers using a library because it makes it appear
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that code relying on the behavior is portable, when in fact there is nothing
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portable about it. The only case where such under-specification is acceptable is when both users and implementors know from
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other sources exactly what behavior is required, yet for some reason it isn't
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possible to specify it exactly.</li>
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</ul>
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<h2><a name="Realities">Realities</a></h2>
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<ul>
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<li>Some operating systems have a single directory tree root, others have
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multiple roots.<br>
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</li>
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<li>Some file systems provide both a long and short form of filenames.<br>
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</li>
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<li>Some file systems have different syntax for file paths and directory
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paths.<br>
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</li>
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<li>Some file systems have different rules for valid file names and valid
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directory names.<br>
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</li>
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<li>Some file systems (ISO-9660, level 1, for example) use very restricted
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(so-called 8.3) file names.<br>
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</li>
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<li>Some operating systems allow file systems with different
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characteristics to be "mounted" within a directory tree. Thus an
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ISO-9660 or Windows
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file system may end up as a sub-tree of a POSIX directory tree.<br>
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</li>
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<li>Wide-character versions of directory and file operations are available on some operating
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systems, and not available on others.<br>
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</li>
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<li>There is no law that says directory hierarchies have to be specified in
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terms of left-to-right decent from the root.<br>
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</li>
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<li>Some file systems have a concept of file "version number" or "generation
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number". Some don't.<br>
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</li>
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<li>Not all operating systems use single character separators in path names. Some use
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paired notations. A typical fully-specified OpenVMS filename
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might look something like this:<br>
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<br>
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<code> DISK$SCRATCH:[GEORGE.PROJECT1.DAT]BIG_DATA_FILE.NTP;5<br>
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</code><br>
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The general OpenVMS format is:<br>
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<br>
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<i>Device:[directories.dot.separated]filename.extension;version_number</i><br>
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</li>
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<li>For common file systems, determining if two descriptors are for same
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entity is extremely difficult or impossible. For example, the concept of
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equality can be different for each portion of a path - some portions may be
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case or locale sensitive, others not. Case sensitivity is a property of the
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pathname itself, and not the platform. Determining collating sequence is even
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worse.<br>
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</li>
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<li>Race-conditions may occur. Directory trees, directories, files, and file attributes are in effect shared between all threads, processes, and computers which have access to the
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filesystem. That may well include computers on the other side of the
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world or in orbit around the world. This implies that file system operations
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may fail in unexpected ways. For example:<br>
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<br>
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<code> assert( exists("foo") == exists("foo") );
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// may fail!<br>
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assert( is_directory("foo") == is_directory("foo");
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// may fail!<br>
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</code><br>
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In the first example, the file may have been deleted between calls to
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exists(). In the second example, the file may have been deleted and then
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replaced by a directory of the same name between the calls to is_directory().<br>
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</li>
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<li>Even though an application may be portable, it still will have to traffic
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in system specific paths occasionally; user provided input is a common
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example.<br>
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</li>
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<li><a name="symbolic-link-use-case">Symbolic</a> links cause canonical and
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normal form of some paths to represent different files or directories. For
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example, given the directory hierarchy <code>/a/b/c</code>, with a symbolic
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link in <code>/a</code> named <code>x</code> pointing to <code>b/c</code>,
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then under POSIX Pathname Resolution rules a path of <code>"/a/x/.."</code>
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should resolve to <code>"/a/b"</code>. If <code>"/a/x/.."</code> were first
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normalized to <code>"/a"</code>, it would resolve incorrectly. (Case supplied
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by Walter Landry.)</li>
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</ul>
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<h2><a name="Rationale">Rationale</a></h2>
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<p>The <a href="#Requirements">Requirements</a> and <a href="#Realities">
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Realities</a> above drove much of the C++ interface design. In particular,
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the desire to make script-like code straightforward caused a great deal of
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effort to go into ensuring that apparently simple expressions like <i>exists( "foo"
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)</i> work as expected.</p>
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<p>See the <a href="faq.htm">FAQ</a> for the rationale behind many detailed
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design decisions.</p>
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<p>Several key insights went into the <i>path</i> class design:</p>
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<ul>
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<li>Decoupling of the input formats, internal conceptual (<i>vector<string></i>
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or other sequence)
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model, and output formats.</li>
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<li>Providing two input formats (generic and O/S specific) broke a major
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design deadlock.</li>
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<li>Providing several output formats solved another set of previously
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intractable problems.</li>
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<li>Several non-obvious functions (particularly decomposition and composition)
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are required to support portable code. (Peter Dimov, Thomas Witt, Glen
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Knowles, others.)</li>
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</ul>
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<p>Error checking was a particularly difficult area. One key insight was that
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with file and directory names, portability isn't a universal truth.
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Rather, the programmer must think out the question "What operating systems do I
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want this path to be portable to?" By providing support for several
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answers to that question, the Filesystem Library alerts programmers of the need
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to ask it in the first place.</p>
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<h2><a name="Abandoned_Designs">Abandoned Designs</a></h2>
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<h3>operations.hpp</h3>
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<p>Dietmar Kühl's original <code>dir_it</code> design and implementation supported
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wide-character file and directory names. It was abandoned after extensive
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discussions among Library Working Group members failed to identify portable
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semantics for wide-character names on systems not providing native support. See
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<a href="faq.htm#wide-character_names">FAQ</a>.</p>
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<p>Previous iterations of the interface design used explicitly named functions providing a
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large number of convenience operations, with no compile-time or run-time
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options. There were so many function names that they were very confusing to use,
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and the interface was much larger. Any benefits seemed theoretical rather than
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real. </p>
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<p>Designs based on compile time (rather than runtime) flag and option selection
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(via policy, enum, or int template parameters) became so complicated that they
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were abandoned, often after investing quite a bit of time and effort. The need
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to qualify attribute or option names with namespaces, even aliases, made use in
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template parameters ugly; that wasn't fully appreciated until actually writing
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real code.</p>
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<p>Yet another set of convenience functions ( for example, <i>remove</i> with
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permissive, prune, recurse, and other options, plus predicate, and possibly
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other, filtering features) were abandoned because the details became both
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complex and contentious.</p>
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<p>What is left is a toolkit of low-level operations from which the user can
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create more complex convenience operations, plus a very small number of
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convenience functions which were found to be useful enough to justify inclusion.</p>
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<h3>path.hpp</h3>
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<p>There were so many abandoned path designs, I've lost track. Policy-based
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class templates in several flavors, constructor supplied runtime policies,
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operation specific runtime policies, they were all considered, often
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implemented, and ultimately abandoned as far too complicated for any small
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benefits observed.</p>
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<p>Additional design considerations apply to <a href="v3_design.html">Internationalization</a>. </p>
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<h3>error checking</h3>
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<p>A number of designs for the error checking machinery were abandoned, some
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after experiments with implementations. Totally automatic error checking was
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attempted in particular. But automatic error checking tended to make the overall
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library design much more complicated.</p>
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<p>Some designs associated error checking mechanisms with paths. Some with
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operations functions. A policy-based error checking template design was
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partially implemented, then abandoned as too complicated for everyday
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script-like programs.</p>
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<p>The final design, which depends partially on explicit error checking function
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calls, is much simpler and straightforward, although it does depend to
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some extent on programmer discipline. But it should allow programmers who
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are concerned about portability to be reasonably sure that their programs will
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work correctly on their choice of target systems.</p>
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<h2><a name="References">References</a></h2>
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<table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="100%">
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<tr>
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<td width="13%" valign="top">[<a name="IBM-01">IBM-01</a>]</td>
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<td width="87%">IBM Corporation, <i>z/OS V1R3.0 C/C++ Run-Time
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Library Reference</i>, SA22-7821-02, 2001,
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<a href="http://www-1.ibm.com/servers/eserver/zseries/zos/bkserv/">
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www-1.ibm.com/servers/eserver/zseries/zos/bkserv/</a></td>
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</tr>
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<tr>
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<td width="13%" valign="top">[<a name="ISO-9660">ISO-9660</a>]</td>
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<td width="87%">International Standards Organization, 1988</td>
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</tr>
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<tr>
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<td width="13%" valign="top">[<a name="Kuhn">Kuhn</a>]</td>
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<td width="87%">UTF-8 and Unicode FAQ for Unix/Linux,
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<a href="http://www.cl.cam.ac.uk/~mgk25/unicode.html">
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www.cl.cam.ac.uk/~mgk25/unicode.html</a></td>
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</tr>
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<tr>
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<td width="13%" valign="top">[<a name="MSDN">MSDN</a>] </td>
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<td width="87%">Microsoft Platform SDK for Windows, Storage Start
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Page,
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<a href="http://msdn.microsoft.com/library/en-us/fileio/base/storage_start_page.asp">
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msdn.microsoft.com/library/en-us/fileio/base/storage_start_page.asp</a></td>
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</tr>
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<tr>
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<td width="13%" valign="top">[<a name="POSIX-01">POSIX-01</a>]</td>
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<td width="87%">IEEE Std 1003.1-2001, ISO/IEC 9945:2002, and The Open Group Base Specifications, Issue 6. Also known as The
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Single UNIX® Specification, Version 3.
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Available from each of the organizations involved in its creation. For
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example, read online or download from
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<a href="http://www.unix.org/single_unix_specification/">
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www.unix.org/single_unix_specification/</a>.</font> The ISO JTC1/SC22/WG15 - POSIX
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homepage is <a href="http://www.open-std.org/jtc1/sc22/WG15/">
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www.open-std.org/jtc1/sc22/WG15/</a></td>
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</tr>
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<tr>
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<td width="13%" valign="top">[<a name="URI">URI</a>]</td>
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<td width="87%">RFC-2396, Uniform Resource Identifiers (URI): Generic
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Syntax, <a href="http://www.ietf.org/rfc/rfc2396.txt">
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www.ietf.org/rfc/rfc2396.txt</a></td>
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</tr>
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<tr>
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<td width="13%" valign="top">[<a name="UTF-16">UTF-16</a>]</td>
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<td width="87%">Wikipedia, UTF-16,
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<a href="http://en.wikipedia.org/wiki/UTF-16">
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en.wikipedia.org/wiki/UTF-16</a></td>
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</tr>
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<tr>
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<td width="13%" valign="top">[<a name="Wulf-Shaw-73">Wulf-Shaw-73</a>]</td>
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<td width="87%">William Wulf, Mary Shaw, <i>Global
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Variable Considered Harmful</i>, ACM SIGPLAN Notices, 8, 2, 1973, pp. 23-34</td>
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</tr>
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</table>
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<hr>
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<p>© Copyright Beman Dawes, 2002</p>
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<p> Use, modification, and distribution are subject to the Boost Software
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License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt">
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LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
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www.boost.org/LICENSE_1_0.txt</a>)</p>
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