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CS253 Filesystem

Major filesystem classes

#include <filesystem> introduces a lot of new symbols, quarantined to their own namespace std::filesystem to avoid conflicts, so put using namespace std::filesystem; in your code.

<filesystem> defines several classes, including:

• path: a filesystem path (plain file, directory, symlink, etc.)
• filesystem_error: an exception thrown on file system errors
• directory_entry: one entry in a directory
• directory_iterator: loop over the contents of the directory
• recursive_directory_iterator: like directory_iterator, but deals with subdirectories also
• file_status: file type & permissions

path

• The path class represents a single path in a filesystem, whether relative (foo, ./bar) or absolute (/etc/group).
• It has several methods to extract parts of the path. All return a path, except .native(), which returns a string (or a wstring on Windows?).
• A path displays with "quotes", to protect funny characters.

path p("/usr/bin/cat");
cout << "path:        " << p               << '\n'
     << "native:      " << p.native()      << '\n'
     << "parent_path: " << p.parent_path() << '\n'
     << "filename:    " << p.filename()    << '\n'
     << "stem:        " << p.stem()        << '\n'
     << "extension:   " << p.extension()   << '\n';

path:        "/usr/bin/cat"
native:      /usr/bin/cat
parent_path: "/usr/bin"
filename:    "cat"
stem:        "cat"
extension:   ""

filesystem_error

Many filesystem functions take an optional error_code argument. If that argument isn’t given, then the function throws a filesystem_error object if things go bad, which has a nice error message.

try {
    cout << "/etc/group: " << file_size("/etc/group") << endl;
    cout << "doh!: " << file_size("doh!") << endl;
}
catch (const filesystem_error &oops) {
    cerr << oops.what() << '\n';
}

/etc/group: 1535
doh!: filesystem error: cannot get file size: No such file or directory [doh!]

error_code

If the optional error_code argument is given, nothing is thrown:

for (auto fn : {"/bin/cc", "/bin/cat"}) {
    error_code ec;
    string s = read_symlink(fn, ec);
    if (ec)
        cerr << fn << ": " << ec.message() << '\n';
    else
        cout << fn << " -> " << s << '\n';
}

/bin/cc -> gcc
/bin/cat: Invalid argument

An error_code in a boolean context is true iff the last operation was bad.

Iteration

path home(getenv("HOME"));
path pub(home / "pub");
for (directory_entry d : directory_iterator(pub))
    if (d.path().extension() == ".txt")
        cout << d.path().filename() << '\n';

"abcdefg.txt"
"hamlet.txt"
"common-words.txt"

• class directory_iterator yields one directory_entry object for each item in the directory (no recursion).
• Alas, <filesystem> has no way to get your home directory, so use getenv() instead.
• Note the use of the / operator, which is overloaded for path objects.

Recursive Iteration

path home(getenv("HOME"));
path pub(home / "pub");
for (auto &d : recursive_directory_iterator(pub))
    if (d.path().stem() == "README")
        cout << d << '\n';

"/s/bach/a/class/cs253/pub/Example/README.txt"

A path is printed with "quotes" so we can read & write paths containing whitespace.

file_status

$ ls -lhog /etc/group
-rw-r--r-- 1 1.5K Nov 30  2022 /etc/group

status() returns a file_status object:

file_status s = status("/etc/group");
file_type t = s.type();
perms p = s.permissions();
if ((p & perms::owner_read) != perms::none)
    cout << "readable by owner\n";
if (t == file_type::regular)
    cout << "a regular file\n";

readable by owner
a regular file

path and fstream

const path home(getenv("HOME"));
auto fname(home / "pub/dwarfs");
if (ifstream in(fname); in.good())
    for (string s; getline(in, s); )
        cout << s << '\n';
else
    cerr << "can’t open " << fname << " for reading.\n";

Bashful
Doc
Dopey
Grumpy
Happy
Sleepy
Sneezy

• The fstream ctor takes a const char *, a string, or now, a path.
• Note the unnecessary if (declaration; condition).
• Purists would insist on home / "pub" / "dwarfs".
  • True, but both \ and / work on Windows.
  • The HOME environment variable already broke this on Windows, which would like USERPROFILE or HOMEDRIVE+HOMEPATH, I think.

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