mirror of
https://github.com/boostorg/filesystem.git
synced 2025-05-12 13:41:47 +00:00
184 lines
12 KiB
HTML
184 lines
12 KiB
HTML
<title>Boost Filesystem FAQ</title>
|
||
<body bgcolor="#FFFFFF">
|
||
|
||
<h1>
|
||
<img border="0" src="../../../c++boost.gif" align="center" width="277" height="86">Filesystem
|
||
FAQ</h1>
|
||
<p><b>Why base the generic-path string format on POSIX?</b></p>
|
||
<p><a href="design.htm#POSIX-01">POSIX</a> is the basis for the most familiar path-string formats, including the
|
||
URL portion of URI's and the native Windows format. It is ubiquitous and
|
||
familiar. On many systems, it is very easy to implement because it is
|
||
either the native operating system format (Unix and Windows) or via a
|
||
operating system supplied
|
||
POSIX library (z/OS, OS/390, and many more.)</p>
|
||
<p><b>Why not use a full URI (Universal Resource Identifier) based path?</b></p>
|
||
<p><a href="design.htm#URI">URI's</a> would promise more than the Filesystem Library can actually deliver,
|
||
since URI's extend far beyond what most operating systems consider a file or a
|
||
directory. Thus for the primary "portable script-style file system
|
||
operations" requirement of the Filesystem Library, full URI's appear to be over-specification.</p>
|
||
<p><b>Why isn't <i>path</i> a base class with derived <i>directory_path</i> and
|
||
<i>file_path</i> classes?</b></p>
|
||
<p>Why bother? The behavior of all three classes is essentially identical.
|
||
Several early versions did require users to identify each path as a file or
|
||
directory path, and this seemed to increase errors and decrease code
|
||
readability. There was no apparent upside benefit.</p>
|
||
<p><b>Why are fully specified paths called <i>complete</i> rather than <i>
|
||
<a name="absolute">absolute</a></i>?</b></p>
|
||
<p>To avoid long-held assumptions (what do you mean, <i>"/foo"</i> isn't
|
||
absolute on some systems?) by programmers used to single-rooted filesystems.
|
||
Using an unfamiliar name for the concept and related functions causes
|
||
programmers to read the specs rather than just assuming the meaning is known.</p>
|
||
<p><b>Why do some function names have a "native_" prefix?</b></p>
|
||
<p>To alert users that the results are inherently non-portable. The names are
|
||
deliberately ugly to discourage use except where really necessary.</p>
|
||
<p><b>Why doesn't <i>path</i> supply operator== and other comparison operators?</b></p>
|
||
<p>There is no way to know if two <i>path</i> objects actually represent the
|
||
same path. For example, <i>path("/foo")</i> and <i>path("../foo")</i> may
|
||
represent the same path, even though they are textually different. If exact
|
||
textual comparison is desired, <i>
|
||
path::string()'s</i> can be used.</p>
|
||
<p><b>Why not support a concept of specific kinds of file systems, such as posix_file_system or windows_file_system.</b></p>
|
||
<p>Portability is one of the one or two most important requirements for the
|
||
library. Gaining some advantage by using features specific to particular
|
||
operating systems is not a requirement. There doesn't appear to be much need for
|
||
the ability to manipulate, say, a classic Mac OS path while running on an
|
||
OpenVMS machine.</p>
|
||
<p>Furthermore, concepts like "posix_file_system"
|
||
are very slippery. What happens when a NTFS or ISO 9660 file system is mounted
|
||
in directory on a machine running a POSIX-like operating system, for example?</p>
|
||
<p><b>Why not supply a 'handle' type, and let the file and directory operations
|
||
traffic in it?</b></p>
|
||
<p>It isn't clear there is any feasible way to meet the "portable script-style
|
||
file system operations" requirement with such a system. File systems exist where operations are usually performed on
|
||
some non-string handle type. The classic Mac OS has been mentioned explicitly as a case where
|
||
trafficking in paths isn't always natural. </p>
|
||
<p>The case for the "handle" (opaque data type to identify a file)
|
||
style may be strongest for directory iterator value type. (See Jesse Jones' Jan 28,
|
||
2002, Boost postings). However, as class path has evolved, it seems sufficient
|
||
even as the directory iterator value type.</p>
|
||
<p><b>Why aren't directories considered to be files?</b></p>
|
||
<p>Because
|
||
directories cannot portably and usefully be opened as files using the C++ Standard Library stdio or fstream
|
||
I/O facilities. An important additional rationale is that separating the concept
|
||
of directories and files makes exposition and specification clearer. A
|
||
particular problem is the naming and description of function arguments.</p>
|
||
<div align="center">
|
||
<center>
|
||
<table border="1" cellpadding="5" cellspacing="0">
|
||
<tr>
|
||
<td colspan="3">
|
||
<p align="center"><b>Meaningful Names for Arguments</b></td>
|
||
</tr>
|
||
<tr>
|
||
<td><b>Argument Intent</b></td>
|
||
<td><b>Meaningful name if<br>
|
||
directories are files</b></td>
|
||
<td><b>Meaningful name if<br>
|
||
directories aren't files</b></td>
|
||
</tr>
|
||
<tr>
|
||
<td>A path to either a directory or a non-directory</td>
|
||
<td align="center"><i>path</i></td>
|
||
<td align="center"><i>path</i></td>
|
||
</tr>
|
||
<tr>
|
||
<td>A path to a directory, but not to a non-directory</td>
|
||
<td align="center"><i>directory_path</i></td>
|
||
<td align="center"><i>directory_path</i></td>
|
||
</tr>
|
||
<tr>
|
||
<td>A path to a non-directory, but not a directory</td>
|
||
<td align="center"><i>non_directory_path</i></td>
|
||
<td align="center"><i>file_path</i></td>
|
||
</tr>
|
||
</table>
|
||
</center>
|
||
</div>
|
||
<p>The problem is that when directories are considered files, <i>
|
||
non_directory_path</i> as an argument name, and the corresponding "non-directory
|
||
path" in documentation, is ugly and lengthy, and so is shortened to just <i>path</i>,
|
||
causing the code and documentation to be confusing if not downright wrong. The
|
||
names which result from the "directories aren't files" approach are more
|
||
acceptable and less likely to be used incorrectly. </p>
|
||
<p><b>Why are the operations.hpp non-member functions so low-level?</b></p>
|
||
<p>To provide a toolkit from which higher-level functionality can be created.</p>
|
||
<p>An
|
||
extended attempt to add convenience functions on top of, or as a replacement
|
||
for, the low-level functionality failed because there is no widely acceptable
|
||
set of simple semantics for most convenience functions considered.
|
||
Attempts to provide alternate semantics, via either run-time options or
|
||
compile-time polices, became overly complicated in relation to the value
|
||
delivered, or became contentious. OTOH, the specific functionality needed for several trial
|
||
applications was very easy for the user to construct from the lower-level
|
||
toolkit functions. See <a href="#Failed Attempts">Failed Attempts</a>.</p>
|
||
<p><b>Isn't it inconsistent then to provide a few convenience functions?</b></p>
|
||
<p>Yes, but experience with both this library, POSIX, and Windows indicates
|
||
the utility of certain convenience functions, and that it is possible to provide
|
||
simple, yet widely acceptable, semantics for them. For example, remove_all.</p>
|
||
<p><b>Why are library functions so picky about errors?</b></p>
|
||
<p>Safety. The default is to be safe rather than sorry. This is particularly
|
||
important given the reality that on many computer systems files and directories
|
||
are <a href="#global">globally shared</a> resources, and thus subject to
|
||
unexpected errors.</p>
|
||
<p><b>Why are errors reported by exception rather than return code or error
|
||
notification variable?</b></p>
|
||
<p>Safety. Return codes or error notification variables are often ignored
|
||
by programmers. Exceptions are much harder to ignore, provided desired
|
||
default behavior (program termination) if not caught, yet allow error recovery
|
||
if desired.</p>
|
||
<p><b>Why are attributes accessed via named functions rather than property maps?</b></p>
|
||
<p>For a few commonly used attributes (existence, directory or file, emptiness),
|
||
simple syntax and guaranteed presence outweigh other considerations. Because
|
||
access to virtually all other attributes is inherently system dependent,
|
||
property maps are viewed as the best hope for access and modification, but it is
|
||
better design to provide such functionality in a separate library. (Historical
|
||
note: even the apparently simple attribute "read-only" turned out to be so
|
||
system depend as to be disqualified as a "guaranteed presence" operation.)</p>
|
||
<p><b>Why isn't there a set_current_directory function?</b></p>
|
||
<p>Global variables are considered harmful [<a href="design.htm#Wulf-Shaw-73">wulf-shaw-73</a>].
|
||
While we can't prevent people from shooting themselves in the foot, we aren't
|
||
about to hand them a loaded gun pointed right at their big toe.</p>
|
||
<p><b>Why aren't there query functions for compound conditions like existing_directory?</b></p>
|
||
<p>After several attempts, named queries for multi-attribute proved a
|
||
slippery-slope; where do you stop?</p>
|
||
<p><b>Why aren't <a name="wide-character_names">wide-character names</a> supported? Why not std::wstring or even
|
||
a templated type?</b></p>
|
||
<p>Wide-character names would provide an illusion of portability where
|
||
portability does not in fact exist. Behavior would be completely different on
|
||
operating systems (Windows, for example) that support wide-character names, than
|
||
on systems which don't (POSIX). Providing functionality that appears to provide
|
||
portability but in fact delivers only implementation-defined behavior is highly
|
||
undesirable. Programs would not even be portable between library implementations
|
||
on the same operating system, let alone portable to different operating systems.</p>
|
||
<p>The C++ standards committee Library Working Group discussed this in some
|
||
detail both on the committee's library reflector and at the Spring, 2002, <font face="Times New Roman"> meeting</font>, and feels that (1) names based on types other than char are
|
||
extremely non-portable, (2) there are no agreed upon semantics for conversion
|
||
between wide-character and narrow-character names for file systems which do not support
|
||
wide-character name, and
|
||
(3) even the committee members most interested in wide-character names are
|
||
unsure that they are a good idea in the context of a portable library.</p>
|
||
<p>[October, 2002 - PJ Plauger has suggested a locale based conversion
|
||
scheme. Others have indicated support for such an experiment.]</p>
|
||
<p><b>Why aren't file and directory name portability errors detected automatically,
|
||
rather than by separate function calls?</b></p>
|
||
<p>Applications mix use of portable and non-portable names, and the situations
|
||
where non-portable names constitute errors vary widely. For example, a
|
||
non-portable name found by a <i>directory_iterator</i> may or may not constitute
|
||
an application error. In another example, for an application copying selected
|
||
native directories and files for later use restricted to ISO-6990 filesystem,
|
||
conditions for error are very different between the source and the target.</p>
|
||
<p>A number (at least six) of designs for automatic name validity error
|
||
detection were evaluated, including at least four complete implementations.
|
||
While the details for rejection differed, all automatic name validity checking
|
||
designs tended to distort the
|
||
otherwise simple design of the rest of the library.</p>
|
||
<p><b>Why are paths sometimes manipulated by member functions and sometimes by
|
||
non-member functions?</b></p>
|
||
<p>The design rule is that purely lexical operations are supplied as <i>class
|
||
path</i> member
|
||
functions, while operations performed by the operating system are provided as
|
||
free functions.</p>
|
||
<hr>
|
||
<p><EFBFBD> Copyright Beman Dawes, 2002</p>
|
||
<p>Revised
|
||
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->11 August, 2003<!--webbot bot="Timestamp" endspan i-checksum="34471" --></p> |