HOWTO: Directory recursion in Boost (and other tips)

Boost’s Filesystem library is an incredible resource: it abstracts paths, directories, and stat results. It simplifies coding shell problems in C++, it’s portable, and is maintained by a large community of contributors. The one downside of Boost is that some of its newer libraries are poorly documented.¹ Until I have time to get involved in the Boost project, I’m going to post examples here.

Traversing a directory tree

This is the coolest feature I’ve found in boost::filesystem so far. It treats directory elements like iterators, and has a convenience iterator that flattens the problem of iterating through a directory tree recursively. The only examples I found for it were in their extensive test sources, (See convenience tests and operations tests for more examples.) which are a bit light on comments.

#include "boost/filesystem.hpp"
#include <iostream>;

for ( boost::filesystem::recursive_directory_iterator end, dir("./");
       dir != end; ++dir ) {
       cout &lt;&lt; *dir &lt;&lt; std::endl;
}

The example starts in the current working directory, and prints all of the file names (and directories) in inode order.²

Notes:

• I don’t alias the namespace here (but I recommend doing so in production code, see below)
• Creating an instance of recursive_directory_iterator sets it to the .end() element by default.
• The path type supports all standard string paths, including relative paths.

Aliasing Boost namespaces

Here’s how Boost’s own source recommends aliasing their namespaces:

namespace fs = boost::filesystem;
namespace sys = boost::system;

Other cool bits

I’ll write more about these later, but for now here are a few things you’ll find in the library:

• fs::exists( boost_root / "libs" ), a static function to check if a file exists (-e)
• fs::current_path() that returns the application’s cwd
• fs::create_directories( "xx/yy/zz" ) is equivalent to mkdir -p xx/yy/zz
• fs::is_directory( "xx" ) is the same as Perl/*sh’s -d
• fs::change_extension("a.txt", ".tex") does the obvious
• fs::extension("a/b.txt") == "txt" is used to check file extensions
• fs::remove_all( "x/" ); deletes everything in "x/"
• ifstream file2( arg_path / "foo" / "bar" ); shows the overloaded / operator!

Things that are great about boost::filesystem’s approach:

• Static functions are used for ‘helper’ stuff. Take note C++ devs: this is a great balance for types, it keeps the types noise-free while still providing a great deal of utility.
• It mirrors Perl/*sh functionality, something most developers should know well.
• It throws errors (filesystem_error), allowing for some really clean, transactional code.
• And, it abstracts paths that work on Win32, OSX, Unix, and Linux variants.