Small C++ library for crossplatform Unicode path management
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README.md

PATHIE.

This is the Pathie project. It aims to provide a C++ library that covers all needs of pathname manipulation and filename fiddling, without having to worry about the underlying platform. That is, it is a glue library that allows you to create platform-independent filename handling code with special regard to Unicode path names.

Supported systems

Currently supported platforms are Linux and Windows, the latter via MSYS2 GCC. Any other compiler or system might or might not work. Mac OS should work as well, but I cannot test this due to lack of a Mac. I gladly accept contributions for any system or compiler.

Pathie's source code itself is written conforming to C++98. On UNIX systems, it assumes the system supports POSIX.1-2001. On Windows systems, the minimum supported Windows version is Windows Vista.

Installation

See INSTALL.md.

The library

The entire world is using UTF-8 as the primary Unicode encoding. The entire world? No, a little company from Redmond resists the temptation and instead uses UTF-16LE, causing cross-platform handling of Unicode pathes to be a nightmare.

One of the main problems the author ran into was compiler-dependant code that was not marked as such. Many sites on the Internet claim Unicode path handling on Windows is easy, but in fact, it only is if you define “development for Windows” as “development with MSVC”, Microsoft’s proprietary C/C++ compiler, which provides nonstandard interfaces to allow for handling UTF-16LE filenames. The Pathie library has been developed with a focus on MinGW and crosscompilation from Linux to Windows and thus does not suffer from this problem.

The Pathie library has been developed to release the programmer from the burden of handling the different encodings in use for filenames, and does so by focusing its API on UTF-8 regardless of the platform in use. Thus, if you use UTF-8 as your preferred encoding inside your program (take a look at the UTF8 Everywhere website for reasons why you should do that), Pathie will be of the most use for you, since it transparently converts whatever filesystem encoding is encountered to UTF-8 in its public interface. Likewise, any pathname you pass to the library is assumed to be UTF-8 and is transcoded transparently to the filesystem encoding before invoking the respective OS' filesystem access methods. Of course, explicit conversion functions are also provided, in case you do need a string in the native encoding or need to construct a path from a string in the native encoding.

General Usage

First thing is to include the main header:

#include <pathie/path.hpp>

Now consider the simple task to get all children of a directory, which have Unicode filenames. Doing that manually will result in you having to convert between UTF-8 and UTF-16 all the time. With pathie, you can just do this:

std::vector<Pathie::Path> children = your_path.children();

Done. Retrieving the parent directory of your directory is pretty easy:

Pathie::Path yourpath("foo/bar/baz");
Pathie::Path parent = yourpath.parent();

But Pathie is much more than just an abstraction of different filepath encodings. It is a utility library for pathname manipulation, i.e. it allows you to do things like finding the parent directory, expanding relative to absolute pathes, decomposing a filename into basename, dirname, and extension, and so on. See the documentation of the central Pathie::Path class on what you can do.

// Assume current directory is /tmp
Pathie::Path p("foo/bar/../baz");
p.expand(); // => /tmp/foo/baz

Or my personal favourite:

Pathie::Path p1("/tmp/foo/bar");
Pathie::Path p2("/tmp/bar/foo");
Pathie::Path p3 = p1.relative(p2); // => ../../foo/bar

It also provides you with commonly used pathes like the user’s configuration directory or the path to the running executable.

Pathie::Path configdir  = Pathie::Path::config_dir();
Pathie::Path exepath    = Pathie::Path::exe();

Pathie assumes that all string arguments passed are in UTF-8 and transparently converts to the native filesystem encoding internally.

Still, if you interface directly with the Windows API or other external libraries, you might want to retrieve the native representation from a Path or construct a Path from the native representation. Pathie doesn’t want to be in your way then. The following example constructs from and converts to the native representation on Windows, which is UTF-16LE:

// Contruct from native
wchar_t* utf16 = Win32ApiCall();
Path mypath = Path::from_native(utf16); // also accepts std::wstring

// Retrieve native (Note C++’ish std::wstring rather than
// raw wchar_t* on Windows)
std::wstring native_utf16 = mypath.native();

On UNIX, these methods work with normal strings (std::string instead of std::wstring) in the underlying filesystem encoding. In most cases, that will be UTF-8, but some legacy systems may still use something like ISO-8859-1 in which case that will differ.

Temporary files and directories

There are two classes Pathie::Tempdir and Pathie::Tempfile that you can use if you need to work with temporary files or directories, respectively. Constructing instances of these classes creates a temporary entry, which is removed (recursively in case of directories) when the instance is destroyed again. Use TempEntry::path() to get access to the Path instance pointing to the created entry.

#include <pathie/tempdir.hpp>

//...

{
  srand(time(NULL)); // Needs random number generator
  Pathie::Tempdir tmpdir("foo"); // Pass a fragment to use as part of filename
  std::cout << "Temporary dir is: " << tmpdir.path() << std::endl;
}
// When `tmpdir' is destroyed, the destructor recursively
// deletes the directory that was created.

Opening a file with a Unicode path name

On Windows with GCC, it is not possible to open a file with Unicode pathname via C++'s usual std::ifstream and std::ofstream mechanism. There's a nonstandard extension provided by Microsoft's proprietary compiler that does this, but GCC does not have this extension. Consequently, code that is intended to compile on GCC (like Pathie) has to avoid it.

There is however a function in the Win32API that allows to open a file with a Unicode pathname and that returns a standard C FILE* handle, _wfopen(). The method Path::fopen() uses this function on Windows and a regular C fopen() on all other platforms, thus allowing you to just deal with your Unicode filename via the regular C I/O interface. If you urgently need C++ I/O streams, read on.

Stream replacements

Pathie mainly provides you with the means to handle pathes, compose, and decompose them. There is an experimental feature however that provides replacements for C++ file streams that work with instances of Pathie::Path instead of strings for opening a file. These replacements are neither elegant nor portable, because they don't nicely honour the template concept the STL is based on by directly subclassing the standard streams in the matter needed most frequently and additionally relying on vendor-specific details. For GCC, an internal (but at least documented) interface is used to exchange the file descriptor inside a stream, and for MSVC, a nonstandard (but documented) constructor is used. Other compilers are not supported by this feature (which most notably affects clang, where I have no idea on the interfaces I need to use for such a trick).

In one word, these replacements are hacky and I consider them experimental. If that does not strike you as problematic, you can enable this feature by passing -DPATHIE_BUILD_STREAM_REPLACEMENTS=ON when invoking cmake during the build process.

In order to use the replacements, include the respective header (either pathie_ifstream or pathie_ofstream) and use the Pathie::ifstream and Pathie::ofstream classes just like you would use std::ifstream and std::ofstream, with the only difference being that you construct them from a Pathie::Path instance instead of a string. See the documentation of Pathie::ofstream for more information.

#include <pathie/pathie_ofstream>

// ...

Pathie::Path p("Bärenstark.txt");
Pathie::ofstream file(p);
file << "Some content" << std::endl;
file.close()

There's also the inofficial boost::nowide, which is similar to this feature and maybe more reliable. It has recently been accepted into boost.

Dependencies and linking

Pathie is standalone, that is, it requires no other libraries except for those provided by your operating system. Note that there’s a caveat with this on Windows, which does provide the Shlwapi library by default, but MinGW's GCC does not automatically link it in. Be sure to link to this library explicitely when compiling for MinGW Windows by appending -lShlwapi to the end of your linking command line.

It is recommended to link in pathie as a dynamic library, because there are some problems with it when linked statically on certain operating systems (see Caveats below). If you are sure you aren’t affected by those problems, it is possible to link in pathie statically.

Caveats

This library assumes that under all UNIX systems out there (I also consider Mac OSX to be a UNIX system) the file system root always is / and the directory separator also always is /. This structure is mandatory as per POSIX -- in POSIX.1-2008, it’s specified in section 10.1. Systems which do neither follow POSIX directory structure, nor are Windows, are unsupported.

On POSIX-compliant systems other than Mac OS X, the filesystem encoding generally is unspecified. Pathnames are merely byte blobs which do not contain NUL bytes, and components are separated by /. It’s up to the applications, including utilities like a shell or the ls(1) program, to make something of those byte streams. Therefore, it is perfectly possible that on one system, user A uses ISO-8859-1 filenames and user B uses UTF-8 filenames. Even the same user could use differently encoded filenames. Programs that have to interpret the byte blobs in pathnames on these systems look at the locale environment variables, namely LANG and LC_ALL, see section 7 of POSIX.1-2008. As a consequence, it may happen you want to create filenames with characters not supported in the user’s pathname encoding. For example, if you want to create a file with a hebrew filename and the user’s pathname encoding is ISO-8859-1, there’s a problem, because ISO-8859-1 has no hebrew characters in it, but in UTF-8, which is the encoding you are advised to use and which is what Pathie’s API expects from you, they are available. There is no sensible solution to this problem that the Pathie library could dictate; the iconv() function used by pathie just replaces characters that are unavailable in the target encoding with a system-defined default (probably “?”). Note that on systems which have a Unicode pathname encoding, especially modern Linuxes with UTF-8, such a situation can’t ever arise, because the Unicode encodings (UTF-*) cover all characters you can ever use.

At least on FreeBSD, calling the POSIX iconv() function fails with the cryptic error message “Service unavailable” if a program is linked statically. I’ve reported a bug on this. This means that you currently can’t link in pathie statically on FreeBSD and systems which don’t allow statically linked executables to call iconv().

On Linux systems, it is recommended to set your program’s locale to the environment’s locale before you call any functions the Pathie library provides, because this will allow Pathie to use the correct encoding for filenames. This is relevant where the environment’s encoding is not UTF-8, e.g. with $LANG set to de_DE.ISO-8859-1. You can do this as follows (the "" locale always refers to the locale of the environment):

#include <locale>
std::locale::global(std::locale(""));

This is not required on Windows nor on Mac OS X, because these operating systems always use UTF-16LE (Windows) or UTF-8 (Mac OS X) as the filesystem encoding, regardless of the user's locale. It however does not hurt to call this either, it simply makes no difference for Pathie on these systems. If you urgently need to avoid this call on Linux, you need to compile pathie with the special build option PATHIE_ASSUME_UTF8_ON_UNIX, which will force Pathie to assume that UTF-8 is used as the filesystem encoding under any UNIX-based system.

Links

Contributing

Feel free to submit any contributions you deem useful. Try to make separate branches for your new features, give a description on what you changed, etc.

Don’t you duplicate boost::filesystem?

Yes and no. boost::filesystem provides many methods pathie provides, but has a major problem with Unicode path handling if you are not willing to do the UTF-8/UTF-16 conversion manually. boost::filesystem always uses UTF-8 to store the pathes on UNIX, and, which is the problem, always uses UTF-16LE to store the pathes on a Windows system. There is no way to override this, although there is a hidden documentation page that claims to solve the problem. I have wasted a great amount of time to persuade boost::filesystem to automatically convert all std::string input it receives into UTF-16LE, but failed to succeed. Each time I wanted to create a file with a Unicode filename, the test failed on Windows by producing garbage filenames. Finally I found out that the neat trick shown in the documentation above indeed does work -- but only if you use the Microsoft Visual C++ compiler (MSVC) to compile your code. I don’t, I generally use g++ via the MinGW toolchain. boost::filesystem fails with g++ via MinGW with regard to Unicode filenames on Windows as of this writing (September 2014).

Apart from that, pathie provides some additional methods, especially with regard to finding out where the user’s pathes are. It is modelled after Ruby’s popular Pathname class, but it doesn’t entirely duplicate its interface (which wouldn’t be idiomatic C++).

Also, pathie is a small library. Adding it to your project shouldn’t hurt too much, while boost::filesystem is quite a large dependency.

License

Pathie is BSD-licensed; see the file “LICENSE” for the exact license conditions.