Filesystem FAQ

Why base the generic-path string format on POSIX?

[POSIX-01] is an ISO Standard. It 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.)

Why not use a full URI (Universal Resource Identifier) based path?

URI's 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.

Why isn't path a base class with derived directory_path and file_path classes?

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.

Why are fully specified paths called complete rather than absolute?

To avoid long-held assumptions (what do you mean, "/foo" 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.

Why not support a concept of specific kinds of file systems, such as posix_file_system or windows_file_system.

Portability is one of the 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.

Furthermore, concepts like "file system" are very slippery. What happens when a NTFS or FAT file system is mounted in directory on a machine running a POSIX-like operating system, for example? Some of the POSIX API's may return very un-POSIX like results.

Why not supply a 'handle' type, and let the file and directory operations traffic in it?

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.

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.

Why are the operations.hpp non-member functions so low-level?

To provide a toolkit from which higher-level functionality can be created.

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 Failed Attempts.

Isn't it inconsistent then to provide a few convenience functions?

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.

Why are there basic_directory_iterator<> overloads for operations.hpp predicate functions? Isn't two ways to do the same thing poor design?

Yes, two ways to do the same thing is often a poor design practice. But the iterator versions are often much more efficient. Calling status() during iteration over a directory containing 15,000 files took 6 seconds for the path overload, and 1 second for the iterator overload, for tests on a freshly booted machine. Times were .90 seconds and .30 seconds, for tests after prior use of the directory. This performance gain is large enough to justify deviating from preferred design practices. Neither overload alone meets all needs.

Why are library functions so picky about errors?

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 globally shared resources, and thus subject to unexpected errors.

Why are errors reported by exception rather than return code or error notification variable?

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. Non-throwing versions of functions are provided where experience indicates the need.

Why are attributes accessed via named functions rather than property maps?

For commonly used attributes (existence, directory or file, emptiness), simple syntax and guaranteed presence outweigh other considerations. Because access to many 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.)

Why isn't there a set_current_directory function?

Global variables are considered harmful [wulf-shaw-73]. 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.

Why aren't wide-character names supported? Why not std::wstring or even a templated type?

They are supported, starting with version 1.33. See Internationalization.

Why isn't automatic name portability error detection provided?

A number (at least six) of designs for name validity error detection were evaluated, including at least four complete implementations. While the details for rejection differed, all of the more powerful name validity checking designs distorted other otherwise simple aspects of the library. Even the simple name checking provided in prior library versions was a constant source of user complaints. While name checking can be helpful, it isn't important enough to justify added a lot of additional complexity.

Why are paths sometimes manipulated by member functions and sometimes by non-member functions?

The design rule is that purely lexical operations are supplied as class basic_path member functions, while operations performed by the operating system are provided as free functions.

Why is path normalized form different from canonical form?

On operating systems such as POSIX which allow symbolic links to directories, the normalized form of a path can represent a different location than the canonical form. See use case from Walter Landry.

Revised 06 February, 2006

Use, modification, and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at www.boost.org/LICENSE_1_0.txt)