Filesystem Tutorial

| Filesystem Tutorial

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Introduction
Preliminaries
Reporting the size of a file - (tut1.cpp)
Using status queries to determine file existence and type - (tut2.cpp)
Directory iteration plus catching exceptions - (tut3.cpp)
Using path decomposition, plus sorting results - (tut4.cpp)
Class path: Constructors, including Unicode - (tut5.cpp)
Class path: Generic format vs. Native format
Class path: Iterators, observers, composition, decomposition, and query - (path_info.cpp)
Error reporting

## Introduction

This tutorial develops a little command line program to list information about files and directories - essentially a much simplified version of the POSIX `ls` or Windows `dir` commands. We'll start with the simplest possible version and progress to more complex functionality. Along the way we'll digress to cover topics you'll need to know about to understand Boost.Filesystem.

Source code for each of the tutorial programs is available, and you are encouraged to compile, test, and experiment with it. To conserve space, we won't always show boilerplate code here, but the provided source is complete and ready to build.

## Preliminaries

Install the Boost distribution if you haven't already done so. See the Boost Getting Started docs.

This tutorial assumes you are going to compile and test the examples using the provided scripts. That's highly recommended.

> If you are planning to compile and test the examples but not use the scripts, make sure your build setup knows where to locate or build the Boost library binaries.

Fire up your command line interpreter, and type the following commands:

Ubuntu Linux
    $ cd boost-root/libs/filesystem/example/test

    $ ./setup.sh
    Copying example programs...

    $ ./build.sh
    Compiling example programs...

    $ ./tut1
    Usage: tut1 path

Microsoft Windows
    >cd boost-root\libs\filesystem\example\test

    >setup
    Copying example programs...

    >build
    Compiling example programs...

    >tut1
    Usage: tut1 path

If the `tut1` command outputs "`Usage: tut1 path`", all is well. A set of tutorial example programs has been copied (by `setup`) to `boost-root``/libs/filesystem/example/test` and then built. You are encouraged to modify and experiment with them as the tutorial progresses. Just invoke the `build` script again to rebuild, or invoke `b2` directly.

If something didn't work right, here are some troubleshooting suggestions:

- If the `b2` program executable isn't being found, check your path environmental variable or see Boost Getting Started.
- Look at `b2.log` to try to spot an indication of the problem.

## Reporting the size of a file - (tut1.cpp)

Let's get started. Our first example program, tut1.cpp, reports the size of a file:

    #include <iostream>
    #include <boost/filesystem.hpp>
    using namespace boost::filesystem;

    int main(int argc, char* argv[])
    {
      if (argc < 2)
      {
        std::cout << "Usage: tut1 path\n";
        return 1;
      }
      std::cout << argv[1] << " " << file_size(argv[1]) << '\n';
      return 0;
    }

The Boost.Filesystem `file_size` function returns a `uintmax_t` containing the size of the file named by the argument. The declaration looks like this:

>     uintmax_t file_size(const path& p);

For now, all you need to know is that `class path` has constructors that take `const char *` and other string types. (If you can't wait to find out more, skip ahead to the class path section of the tutorial.)

Please take a minute to try out `tut1` on your system, using a file that is known to exist, such as `tut1.cpp`. Here is what the results look like on two different operating systems:

Ubuntu Linux
    $ ./tut1 tut1.cpp
    tut1.cpp 569
    $ ls -l tut1.cpp
    -rw-rw-r-- 1 beman beman 569 Jul 26 12:04 tut1.cpp

Microsoft Windows
    >tut1 tut1.cpp
    tut1.cpp 592

    >dir tut1.cpp
    ...
    07/26/2015 07:20 AM 592 tut1.cpp
    ...

So far, so good. The reported Linux and Windows sizes are different because the Linux tests used `"\n"` line endings, while the Windows tests used `"\r\n"` line endings. The sizes reported may differ from the above if changes have been made to `tut1.cpp`.

Now try again, but give a path that doesn't exist:

Ubuntu Linux
    $ ./tut1 foo
    terminate called after throwing an instance of 'boost::filesystem::filesystem_error'
    what(): boost::filesystem::file_size: No such file or directory: "foo"
    Aborted (core dumped)

Microsoft Windows
    >tut1 foo
An exception is thrown; the exact form of the response depends on Windows system options.

What happens? There's no file named `foo` in the current directory, so by default an exception is thrown. See Error reporting to learn about error reporting via error codes rather than exceptions.

Try this:

Ubuntu Linux
    $ ./tut1 .
    terminate called after throwing an instance of 'boost::filesystem::filesystem_error'
    what(): boost::filesystem::file_size: Operation not permitted: "."
    Aborted (core dumped)

Microsoft Windows
    >tut1 .
An exception is thrown; the exact form of the response depends on Windows system options.

The current directory exists, but `file_size()` works on regular files, not directories, so again an exception is thrown.

We'll deal with those situations in `tut2.cpp`.

## Using status queries to determine file existence and type - (tut2.cpp)

Boost.Filesystem includes status query functions such as `exists`, `is_directory`, and `is_regular_file`. These return `bool`'s, and will return `true` if the condition described by their name is met. Otherwise they return `false`, including when any element of the path argument can't be found.

tut2.cpp uses several of the status query functions to cope with non-existent files and with different kinds of files:

    #include <iostream>
    #include <boost/filesystem.hpp>
    using namespace std;
    using namespace boost::filesystem;

    int main(int argc, char* argv[])
    {
      if (argc < 2)
      {
        cout << "Usage: tut2 path\n";
        return 1;
      }

      path p(argv[1]);  // avoid repeated path construction below

      if (exists(p))    // does path p actually exist?
      {
        if (is_regular_file(p))        // is path p a regular file?
          cout << p << " size is " << file_size(p) << '\n';

        else if (is_directory(p))      // is path p a directory?
          cout << p << " is a directory\n";

        else
          cout << p << " exists, but is not a regular file or directory\n";
      }
      else
        cout << p << " does not exist\n";

      return 0;
    }

Give it a try:

Ubuntu Linux
    $ ./tut2 tut2.cpp
    "tut2.cpp" size is 997

    $ ./tut2 foo
    "foo" does not exist

    $ ./tut2 .
    "." is a directory

Microsoft Windows
    >tut2 tut2.cpp
    tut2.cpp size is 1039

    >tut2 foo
    "foo" does not exist

    >tut2 .
    "." is a directory

Although tut2 works OK in these tests, the output is less than satisfactory for a directory. We'd typically like to see a list of the directory's contents. In `tut3.cpp` we will see how to iterate over directories.

But first, let's try one more test:

Ubuntu Linux
    $ ls /home/jane/foo
    ls: cannot access /home/jane/foo: No such file or directory

    $ ./tut2 /home/jane/foo
    terminate called after throwing an instance of 'boost::
    filesystem::filesystem_error>'
       what(): boost::filesystem::status: Permission denied:
         "/home/jane/foo"
    Aborted

Microsoft Windows
    >dir e:\
    The device is not ready.

    >tut2 e:\
An exception is thrown; the exact form of the response depends on Windows system options.

On the Linux system, the test was being run from an account that did not have permission to access `/home/jane/foo`. On the Windows system, `e:` was a Compact Disc reader/writer that was not ready. End users shouldn't have to interpret cryptic exceptions reports, so as we move on to `tut3.cpp` we will increase the robustness of the code, too.

## Directory iteration plus catching exceptions - (tut3.cpp)

Boost.Filesystem's `directory_iterator` class is just what we need here. It follows the general pattern of the standard library's `istream_iterator`. Constructed from a path, it iterates over the contents of the directory. A default constructed `directory_iterator` acts as the end iterator.

The value type of `directory_iterator` is `directory_entry`. A `directory_entry` object contains `path` and `file_status` information. A `directory_entry` object can be used directly, but can also be passed to `path` arguments in function calls.

The other need is increased robustness in the face of the many kinds of errors that can affect file system operations. We could do that at the level of each call to a Boost.Filesystem function (see Error reporting), but for simplicity tut3.cpp uses an overall try/catch block.

    #include <iostream>
    #include <boost/filesystem.hpp>
    using std::cout;
    using namespace boost::filesystem;

    int main(int argc, char* argv[])
    {
      if (argc < 2)
      {
        cout << "Usage: tut3 path\n";
        return 1;
      }

      path p (argv[1]);

      try
      {
        if (exists(p))
        {
          if (is_regular_file(p))
            cout << p << " size is " << file_size(p) << '\n';

          else if (is_directory(p))
          {
            cout << p << " is a directory containing:\n";

            for (directory_entry& x : directory_iterator(p))
              cout << "    " << x.path() << '\n';
          }
          else
            cout << p << " exists, but is not a regular file or directory\n";
        }
        else
          cout << p << " does not exist\n";
      }

      catch (const filesystem_error& ex)
      {
        cout << ex.what() << '\n';
      }

      return 0;
    }

Give `tut3` a try, passing it a path to a directory as a command line argument. Here is a run on a checkout of the Boost Git develop branch, followed by a repeat of the test cases that caused exceptions on Linux and Windows:

Ubuntu Linux
    $ ./tut3 ~/boost/develop
        "/home/beman/boost/develop" is a directory containing:
        "/home/beman/boost/develop/rst.css"
        "/home/beman/boost/develop/boost"
        "/home/beman/boost/develop/boost.png"
        "/home/beman/boost/develop/libs"
        "/home/beman/boost/develop/doc"
        "/home/beman/boost/develop/project-config.jam.2"
        "/home/beman/boost/develop/.gitmodules"
        "/home/beman/boost/develop/boostcpp.py"
        "/home/beman/boost/develop/.travis.yml"
        "/home/beman/boost/develop/.gitattributes"
        "/home/beman/boost/develop/index.htm"
        "/home/beman/boost/develop/index.html"
        "/home/beman/boost/develop/bjam"
        "/home/beman/boost/develop/project-config.jam.1"
        "/home/beman/boost/develop/LICENSE_1_0.txt"
        "/home/beman/boost/develop/.git"
        "/home/beman/boost/develop/tools"
        "/home/beman/boost/develop/stage"
        "/home/beman/boost/develop/boostcpp.jam"
        "/home/beman/boost/develop/Jamroot"
        "/home/beman/boost/develop/.gitignore"
        "/home/beman/boost/develop/INSTALL"
        "/home/beman/boost/develop/more"
        "/home/beman/boost/develop/bin.v2"
        "/home/beman/boost/develop/project-config.jam"
        "/home/beman/boost/develop/boost-build.jam"
        "/home/beman/boost/develop/bootstrap.bat"
        "/home/beman/boost/develop/bootstrap.sh"
        "/home/beman/boost/develop/status"
        "/home/beman/boost/develop/boost.css"

Microsoft Windows
    >tut3 \boost\develop
    "\boost\develop" is a directory containing:
        "\boost\develop\.git"
        "\boost\develop\.gitattributes"
        "\boost\develop\.gitignore"
        "\boost\develop\.gitmodules"
        "\boost\develop\.travis.yml"
        "\boost\develop\bin.v2"
        "\boost\develop\boost"
        "\boost\develop\boost-build.jam"
        "\boost\develop\boost.css"
        "\boost\develop\boost.png"
        "\boost\develop\boostcpp.jam"
        "\boost\develop\boostcpp.py"
        "\boost\develop\bootstrap.bat"
        "\boost\develop\bootstrap.sh"
        "\boost\develop\doc"
        "\boost\develop\index.htm"
        "\boost\develop\index.html"
        "\boost\develop\INSTALL"
        "\boost\develop\Jamroot"
        "\boost\develop\libs"
        "\boost\develop\LICENSE_1_0.txt"
        "\boost\develop\more"
        "\boost\develop\project-config.jam"
        "\boost\develop\rst.css"
        "\boost\develop\stage"
        "\boost\develop\status"
        "\boost\develop\tools"
    >tut3 e:\
    boost::filesystem::status: The device is not ready: "e:\"

Not bad, but we can make further improvements:

- The listing would be much easier to read if only the filename was displayed, rather than the full path.
- The Linux listing isn't sorted. That's because the ordering of directory iteration is unspecified. Ordering depends on the underlying operating system API and file system specifics. So we need to sort the results ourselves.

The next sections show how how those changes play out, so read on!

## Using path decomposition, plus sorting results - (tut4.cpp)

For directories, tut4.cpp builds a `std::vector` of all the entries and then sorts it before writing to `cout`.

    #include <iostream>
    #include <vector>
    #include <algorithm>
    #include <boost/filesystem.hpp>
    using std::cout;
    using namespace boost::filesystem;

    int main(int argc, char* argv[])
    {
      if (argc < 2)
      {
        cout << "Usage: tut4 path\n";
        return 1;
      }

      path p (argv[1]);

      try
      {
        if (exists(p))
        {
          if (is_regular_file(p))
            cout << p << " size is " << file_size(p) << '\n';

          else if (is_directory(p))
          {
            cout << p << " is a directory containing:\n";

            std::vector<path> v;

            for (auto&& x : directory_iterator(p))
              v.push_back(x.path());

            std::sort(v.begin(), v.end());

            for (auto&& x : v)
              cout << "    " << x.filename() << '\n';
          }
          else
            cout << p << " exists, but is not a regular file or directory\n";
        }
        else
          cout << p << " does not exist\n";
      }

      catch (const filesystem_error& ex)
      {
        cout << ex.what() << '\n';
      }

      return 0;
    }

The only difference between `tut3.cpp` and `tut4.cpp` is what happens for directories. We changed:

>     for (const directory_entry& x : directory_iterator(p))
>       cout << " " << x.path() << '\n';

to:

>     std::vector<path> v;

>     for (auto&& x : directory_iterator(p))
>       v.push_back(x.path());

>     std::sort(v.begin(), v.end());

>     for (auto&& x : v)
>       cout << " " << x.filename() << '\n';

`filename()` is one of several class `path` decomposition functions. It extracts the filename portion from a path (i.e. `"index.html"` from `"/home/beman/boost/trunk/index.html"`). These decomposition functions are more fully explored in the Path iterators, observers, composition, decomposition and query portion of this tutorial.

The above was written as two lines of code for clarity. It could have been written more concisely as:

>     v.push_back(it->path().filename()); // we only care about the filename

Here is the output from a test of `tut4.cpp`:

Ubuntu Linux
    $ ./tut4  v

Microsoft Windows
    $ ./tut4 ~/boost/develop
    "/home/beman/boost/develop" is a directory containing:
        .git
        .gitattributes
        .gitignore
        .gitmodules
        .travis.yml
        INSTALL
        Jamroot
        LICENSE_1_0.txt
        bin.v2
        boost
        boost-build.jam
        boost.css
        boost.png
        boostcpp.jam
        boostcpp.py
        bootstrap.bat
        bootstrap.sh
        doc
        index.htm
        index.html
        libs
        more
        project-config.jam
        project-config.jam.1
        project-config.jam.2
        rst.css
        stage
        status
        tools

That completes the main portion of this tutorial. If you haven't already worked through the Class path sections of this tutorial, dig into them now. The Error reporting section may also be of interest, although it can be skipped unless you are deeply concerned about error handling issues.

## Class path: Constructors, including Unicode - (tut5.cpp)

Traditional C interfaces pass paths as `const char*` arguments. C++ interfaces may add `const std::string&` overloads, but adding overloads becomes untenable if wide characters, containers, and iterator ranges need to be supported.

Passing paths as `const path&` arguments is far simpler, yet far more flexible because class `path` itself is far more flexible:

1. Class `path` supports multiple character types and encodings, including Unicode, to ease internationalization.
2. Class `path` supports multiple source types, such as iterators for null terminated sequences, iterator ranges, containers (including `std::basic_string`), and `directory_entry`'s, so functions taking paths don't need to provide several overloads.
3. Class `path` supports both native and generic pathname formats, so programs can be portable between operating systems yet use native formats where desirable.
4. Class `path` supplies a full set of iterators, observers, composition, decomposition, and query functions, making pathname manipulations easy, convenient, reliable, and portable.

Here is how (1) and (2) work. Class path constructors, assignments, and appends have member templates for sources. For example, here are the constructors that take sources:

>     template <class Source>
>       path(Source const& source);

>     template <class InputIterator>
>       path(InputIterator begin, InputIterator end);

Let's look at a little program that shows how comfortable class `path` is with both narrow and wide characters in C-style strings, C++ strings, and via C++ iterators:

    #include <boost/filesystem/fstream.hpp>
    #include <string>
    #include <list>
    namespace fs = boost::filesystem;

    int main()
    {
      // \u263A is "Unicode WHITE SMILING FACE = have a nice day!"
      std::string narrow_string ("smile2");
      std::wstring wide_string (L"smile2\u263A");
      std::list<char> narrow_list;
      narrow_list.push_back('s');
      narrow_list.push_back('m');
      narrow_list.push_back('i');
      narrow_list.push_back('l');
      narrow_list.push_back('e');
      narrow_list.push_back('3');
      std::list<wchar_t> wide_list;
      wide_list.push_back(L's');
      wide_list.push_back(L'm');
      wide_list.push_back(L'i');
      wide_list.push_back(L'l');
      wide_list.push_back(L'e');
      wide_list.push_back(L'3');
      wide_list.push_back(L'\u263A');

      { fs::ofstream f("smile"); }
      { fs::ofstream f(L"smile\u263A"); }
      { fs::ofstream f(narrow_string); }
      { fs::ofstream f(wide_string); }
      { fs::ofstream f(narrow_list); }
      { fs::ofstream f(wide_list); }
      narrow_list.pop_back();
      narrow_list.push_back('4');
      wide_list.pop_back();
      wide_list.pop_back();
      wide_list.push_back(L'4');
      wide_list.push_back(L'\u263A');
      { fs::ofstream f(fs::path(narrow_list.begin(), narrow_list.end())); }
      { fs::ofstream f(fs::path(wide_list.begin(), wide_list.end())); }

      return 0;
    }

Testing `tut5`:

Ubuntu Linux
    $ ./tut5

    $ ls smile*
    smile smile☺ smile2 smile2☺ smile3 smile3☺ smile4 smile4☺

Microsoft Windows
    >tut5

    >dir /b smile*
    smile
    smile2
    smile2☺
    smile3
    smile3☺
    smile4
    smile4☺
    smile☺

The exact appearance of the smiling face will depend on the font, font size, and other settings for your command line window. The above tests were run with out-of-the-box Ubuntu 14.04 and Windows 7, US Edition. If you don't get the above results, take a look at the `boost-root/libs/filesystem/example/test` directory with your system's GUI file browser, such as Linux Nautilus, Mac OS X Finder, or Windows Explorer. These tend to be more comfortable with international character sets than command line interpreters.

Class `path` takes care of whatever character type or encoding conversions are required by the particular operating system. Thus as `tut5` demonstrates, it's no problem to pass a wide character string to a Boost.Filesystem operational function even if the underlying operating system uses narrow characters, and visa versa. And the same applies to user supplied functions that take `const path&` arguments.

Class `path` also provides path syntax that is portable across operating systems, element iterators, and observer, composition, decomposition, and query functions to manipulate the elements of a path. The next section of this tutorial deals with path syntax.

## Class path: Generic format vs. Native format

Class `path` deals with two different pathname formats - generic format and native format. For POSIX-like file systems, these formats are the same. But for users of Windows and other non-POSIX file systems, the distinction is important. Even programmers writing for POSIX-like systems need to understand the distinction if they want their code to be portable to non-POSIX systems.

The generic format is the familiar `/my_directory/my_file.txt` format used by POSIX-like operating systems such as the Unix variants, Linux, and Mac OS X. Windows also recognizes the generic format, and it is the basis for the familiar Internet URL format. The directory separator character is always one or more slash characters.

The native format is the format as defined by the particular operating system. For Windows, either the slash or the backslash can be used as the directory separator character, so `/my_directory\my_file.txt` would work fine. Of course, if you write that in a C++ string literal, it becomes `"/my_directory\\my_file.txt"`.

If a drive specifier or a backslash appears in a pathname on a Windows system, it is always treated as the native format.

Class `path` has observer functions that allow you to obtain the string representation of a path object in either the native format or the generic format. See the next section for how that plays out.

The distinction between generic format and native format is important when communicating with native C-style API's and with users. Both tend to expect paths in the native format and may be confused by the generic format. The generic format is great, however, for writing portable programs that work regardless of operating system.

The next section covers class `path` observers, composition, decomposition, query, and iteration over the elements of a path.

## Class path: Iterators, observers, composition, decomposition, and query - (path_info.cpp)

The `path_info.cpp` program is handy for learning how class `path` iterators, observers, composition, decomposition, and query functions work on your system. It is one of the programs built by the `build.sh` and `build.bat` scripts:

    #include <iostream>
    #include <boost/filesystem.hpp>
    using namespace std;
    using namespace boost::filesystem;

    const char * say_what(bool b) { return b ? "true" : "false"; }

    int main(int argc, char* argv[])
    {
      if (argc < 2)
      {
        cout << "Usage: path_info path-element [path-element...]\n"
                "Composes a path via operator/= from one or more path-element arguments\n"
                "Example: path_info foo/bar baz\n"
    #            ifdef BOOST_POSIX_API
                "         would report info about the composed path foo/bar/baz\n";
    #            else  // BOOST_WINDOWS_API
                "         would report info about the composed path foo/bar\\baz\n";
    #            endif
        return 1;
      }

      path p;
      for (; argc > 1; --argc, ++argv)
        p /= argv[1];  // compose path p from the command line arguments

      cout  <<  "\ncomposed path:\n";
      cout  <<  "  operator<<()---------: " << p << "\n";
      cout  <<  "  make_preferred()-----: " << p.make_preferred() << "\n";

      cout << "\nelements:\n";
      for (auto element : p)
        cout << "  " << element << '\n';

      cout  <<  "\nobservers, native format:" << endl;
    # ifdef BOOST_POSIX_API
      cout  <<  "  native()-------------: " << p.native() << endl;
      cout  <<  "  c_str()--------------: " << p.c_str() << endl;
    # else  // BOOST_WINDOWS_API
      wcout << L"  native()-------------: " << p.native() << endl;
      wcout << L"  c_str()--------------: " << p.c_str() << endl;
    # endif
      cout  <<  "  string()-------------: " << p.string() << endl;
      wcout << L"  wstring()------------: " << p.wstring() << endl;

      cout  <<  "\nobservers, generic format:\n";
      cout  <<  "  generic_string()-----: " << p.generic_string() << endl;
      wcout << L"  generic_wstring()----: " << p.generic_wstring() << endl;

      cout  <<  "\ndecomposition:\n";
      cout  <<  "  root_name()----------: " << p.root_name() << '\n';
      cout  <<  "  root_directory()-----: " << p.root_directory() << '\n';
      cout  <<  "  root_path()----------: " << p.root_path() << '\n';
      cout  <<  "  relative_path()------: " << p.relative_path() << '\n';
      cout  <<  "  parent_path()--------: " << p.parent_path() << '\n';
      cout  <<  "  filename()-----------: " << p.filename() << '\n';
      cout  <<  "  stem()---------------: " << p.stem() << '\n';
      cout  <<  "  extension()----------: " << p.extension() << '\n';

      cout  <<  "\nquery:\n";
      cout  <<  "  empty()--------------: " << say_what(p.empty()) << '\n';
      cout  <<  "  is_absolute()--------: " << say_what(p.is_absolute()) << '\n';
      cout  <<  "  has_root_name()------: " << say_what(p.has_root_name()) << '\n';
      cout  <<  "  has_root_directory()-: " << say_what(p.has_root_directory()) << '\n';
      cout  <<  "  has_root_path()------: " << say_what(p.has_root_path()) << '\n';
      cout  <<  "  has_relative_path()--: " << say_what(p.has_relative_path()) << '\n';
      cout  <<  "  has_parent_path()----: " << say_what(p.has_parent_path()) << '\n';
      cout  <<  "  has_filename()-------: " << say_what(p.has_filename()) << '\n';
      cout  <<  "  has_stem()-----------: " << say_what(p.has_stem()) << '\n';
      cout  <<  "  has_extension()------: " << say_what(p.has_extension()) << '\n';

      return 0;
    }

Run the examples below on your system, and try some different path arguments as we go along. Here is the invocation we will talk about in detail:

Ubuntu Linux
    $ ./path_info /foo bar baa.txt

    composed path:
      operator<<()---------: "/foo/bar/baa.txt"
      make_preferred()-----: "/foo/bar/baa.txt"

    elements:
      "/"
      "foo"
      "bar"
      "baa.txt"

    observers, native format:
      native()-------------: /foo/bar/baa.txt
      c_str()--------------: /foo/bar/baa.txt
      string()-------------: /foo/bar/baa.txt
      wstring()------------: /foo/bar/baa.txt

    observers, generic format:
      generic_string()-----: /foo/bar/baa.txt
      generic_wstring()----: /foo/bar/baa.txt

    decomposition:
      root_name()----------: ""
      root_directory()-----: "/"
      root_path()----------: "/"
      relative_path()------: "foo/bar/baa.txt"
      parent_path()--------: "/foo/bar"
      filename()-----------: "baa.txt"
      stem()---------------: "baa"
      extension()----------: ".txt"

    query:
      empty()--------------: false
      is_absolute()--------: true
      has_root_name()------: false
      has_root_directory()-: true
      has_root_path()------: true
      has_relative_path()--: true
      has_parent_path()----: true
      has_filename()-------: true
      has_stem()-----------: true
      has_extension()------: true

Microsoft Windows
    >path_info \foo bar baa.txt

    composed path:
    operator<<()---------: "\foo\bar\baa.txt"
    make_preferred()-----: "\foo\bar\baa.txt"

    elements:
      "/"
      "foo"
      "bar"
      "baa.txt"

    observers, native format:
    native()-------------: \foo\bar\baa.txt
    c_str()--------------: \foo\bar\baa.txt
    string()-------------: \foo\bar\baa.txt
    wstring()------------: \foo\bar\baa.txt

    observers, generic format:
    generic_string()-----: /foo/bar/baa.txt
    generic_wstring()----: /foo/bar/baa.txt

    decomposition:
    root_name()----------: ""
    root_directory()-----: "\"
    root_path()----------: "\"
    relative_path()------: "foo\bar\baa.txt"
    parent_path()--------: "\foo\bar"
    filename()-----------: "baa.txt"
    stem()---------------: "baa"
    extension()----------: ".txt"

    query:
    empty()--------------: false
    is_absolute()--------: false
    has_root_name()------: false
    has_root_directory()-: true
    has_root_path()------: true
    has_relative_path()--: true
    has_parent_path()----: true
    has_filename()-------: true
    has_stem()-----------: true
    has_extension()------: true

We will go through the above code in detail to gain a better understanding of what is going on.

A common need is to compose a path from its constituent directories. Class `path` uses `/` and `/=` operators to append elements. That's a reminder that these operations append the operating system's preferred directory separator if needed. The preferred directory separator is a slash on POSIX-like systems, and a backslash on Windows-like systems.

That's what this code does before displaying the resulting `path p` using the `class path` stream inserter:

      path p;
      for (; argc > 1; --argc, ++argv)
        p /= argv[1];  // compose path p from the command line arguments

      cout  <<  "\ncomposed path:\n";
      cout  <<  "  operator<<()---------: " << p << "\n";
      cout  <<  "  make_preferred()-----: " << p.make_preferred() << "\n";

One abstraction for thinking about a path is as a sequence of elements, where the elements are directory and file names. To support this abstraction, class `path` provides STL-like iterators and also `begin()` and `end()` functions.

Here is the code that produced the list of elements in the above output listing:

    cout << "\nelements:\n";
    for (auto element : p)
      cout << " " << element << '\n';

Let's look at class path observer functions:

      cout  <<  "\nobservers, native format:" << endl;
    # ifdef BOOST_POSIX_API
      cout  <<  "  native()-------------: " << p.native() << endl;
      cout  <<  "  c_str()--------------: " << p.c_str() << endl;
    # else  // BOOST_WINDOWS_API
      wcout << L"  native()-------------: " << p.native() << endl;
      wcout << L"  c_str()--------------: " << p.c_str() << endl;
    # endif
      cout  <<  "  string()-------------: " << p.string() << endl;
      wcout << L"  wstring()------------: " << p.wstring() << endl;

      cout  <<  "\nobservers, generic format:\n";
      cout  <<  "  generic_string()-----: " << p.generic_string() << endl;
      wcout << L"  generic_wstring()----: " << p.generic_wstring() << endl;

Native format observers should be used when interacting with the operating system or with users; that's what they expect.

Generic format observers should be used when the results need to be portable and uniform regardless of the operating system.

`path` objects always hold pathnames in the native format, but otherwise leave them unchanged from their source. The preferred() function will convert to the preferred form, if the native format has several forms. Thus on Windows, it will convert slashes to backslashes.

Moving on to decomposition:

      cout  <<  "\ndecomposition:\n";
      cout  <<  "  root_name()----------: " << p.root_name() << '\n';
      cout  <<  "  root_directory()-----: " << p.root_directory() << '\n';
      cout  <<  "  root_path()----------: " << p.root_path() << '\n';
      cout  <<  "  relative_path()------: " << p.relative_path() << '\n';
      cout  <<  "  parent_path()--------: " << p.parent_path() << '\n';
      cout  <<  "  filename()-----------: " << p.filename() << '\n';
      cout  <<  "  stem()---------------: " << p.stem() << '\n';
      cout  <<  "  extension()----------: " << p.extension() << '\n';

And, finally, query functions:

      cout  <<  "\nquery:\n";
      cout  <<  "  empty()--------------: " << say_what(p.empty()) << '\n';
      cout  <<  "  is_absolute()--------: " << say_what(p.is_absolute()) << '\n';
      cout  <<  "  has_root_name()------: " << say_what(p.has_root_name()) << '\n';
      cout  <<  "  has_root_directory()-: " << say_what(p.has_root_directory()) << '\n';
      cout  <<  "  has_root_path()------: " << say_what(p.has_root_path()) << '\n';
      cout  <<  "  has_relative_path()--: " << say_what(p.has_relative_path()) << '\n';
      cout  <<  "  has_parent_path()----: " << say_what(p.has_parent_path()) << '\n';
      cout  <<  "  has_filename()-------: " << say_what(p.has_filename()) << '\n';
      cout  <<  "  has_stem()-----------: " << say_what(p.has_stem()) << '\n';
      cout  <<  "  has_extension()------: " << say_what(p.has_extension()) << '\n';

These are pretty self-evident, but do note the difference in the result of `is_absolute()` between Linux and Windows. Because there is no root name (i.e. drive specifier or network name), a lone slash (or backslash) is a relative path on Windows but an absolute path on POSIX-like operating systems.

## Error reporting

The Boost.Filesystem `file_size` function, like many of the operational functions, has two overloads:

>     uintmax_t file_size(const path& p);
>     uintmax_t file_size(const path& p, system::error_code& ec);

The only significant difference between the two is how they report errors.

The first signature will throw exceptions to report errors. A `filesystem_error` exception will be thrown on an operational error. `filesystem_error` is derived from `std::runtime_error`. It has a member function to obtain the `error_code` reported by the source of the error. It also has member functions to obtain the path or paths that caused the error.

> Motivation for the second signature: Throwing exceptions on errors was the entire error reporting story for the earliest versions of Boost.Filesystem, and indeed throwing exceptions on errors works very well for many applications. But user reports trickled in that some code became so littered with try and catch blocks as to be unreadable and unmaintainable. In some applications I/O errors aren't exceptional, and that's the use case for the second signature.

Functions with a `system::error_code&` argument set that argument to report operational error status, and so do not throw exceptions when I/O related errors occur. For a full explanation, see Error reporting in the reference documentation.

© Copyright Beman Dawes 2010, 2015

Distributed under the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt

Revised 07 August 2017