My late discovery of std::filesystem - Part I - Buy Cookies Online: Your Source for News

I know that this is not a new topic at all. But this blog in its roots is to document what I learn, and I haven’t used the filesystem library up until a few weeks ago. After the initial encounter, I deliberately spent a bit more time exploring it and I want to share what I found.

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I don’t want to go over the C++ Reference documentation and I also don’t want to simply repeat what Bartek already shared here.

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I rarely use C++ to manipulate the filesystem. That usually comes up with Python. So I decided to go through my Python use cases and see how that would in C++ with the std::filesystem library. Which was introduced in C++17, but its roots are back in the boost::filesystem library.

Get the current filename

By current file, I don’t mean the executable file, but the source code file. This is not something that we are going to achieve with std::filesystem. You either need to use the standard __FILE__ macro, or you need to use std::source_location::current()::file_name. Let’s see a little example:

Even though we don’t do this with std::filesystem, I wanted to include it, because requiring the current filename is often needed for the following use cases.

Get the directory of a file

Now that we have the absolute path of the current file, let’s see how to get the directory of it.

Nothing is simpler than that!

Get the absolute path

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First of all, if we need the current directory, we can use std::filesystem::current_path(). But if that’s not the case and we already have an absolute path of a file anywhere even just as a string, we can use remove_filename().

Get the relative path

You might say that you are not interested in the absolute path, but rather in the relative. Let’s assume that we have a file called /Users/sandor/personal/dev/dojos/Racing-Car-Katas/Cpp/TextConverter/tests/HtmlTextConverter_Test.cpp. Your first idea might be to try std::filesystem::path::relative_path() but then you realize that it doesn’t take any parameters! What does relative mean then? Well, it only removes the prefix that signals the root, e.g. / or C:…

If you want to get the relative path compared to another path, then instead of the above member function we have to use a free function called std::filesystem::relative.

As we can see, the only question is which path goes first. The correct way to read this is “give me the relative path of the left-hand path compared to the right-hand path”.

In my opinion, a nice API could have used a member function, such as std::filesystem::path::relative_to(otherPath). But there must have been other considerations that I’m not aware of.

Step up a directory

If you have a path at your hands and you want the path of the parent directory, you don’t have a difficult task, just use the parent_path() member function.

Check if a file is a file or directory

Let’s check now if a file or directory exists. To do so, we leave behind the RacingKar katas and we are going to use the standard library to create some files and directories. With std::filesystem::create_directory, it’s easy to create a new folder. With std::ofstream we can create a file(stream) and with std::ofstream::put we can add a character to it.

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As you can see, by calling std::filesystem::exists(path), it’s easy to check whether a file or directory exists and there are different additional query functions available to check if a file object is a directory, a block, character or regular file. These file types are defined in the POSIX standard.

Copy or rename a file

In the next example, after creating some files and directories, we are going to first copy a directory, then some files and then we will rename a file. To facilitate our example, we’ll also use std::filesystem::remove to remove directories and files.

In the “copy a file to…?” part, we can observe that when we want to copy a file, the destination is not the destination directory, but the destination path. If you don’t pay attention, you might copy a file to a place which was meant to be a directory.

But it can only happen if you haven’t created the destination directory yet. It’s important to note, that you cannot use std::filesystem::copy to copy to a non-existing directory. You have to make sure that it exists. Even though as a third, optional parameter, it takes std::filesystem::copy_options, seemingly there is no option to create automatically the needed directory.

While std::filesystem::copy can be used to copy both files and directories, there is also std::filesystem::copy_file which can only copy a single file. Its name is more expressive, but that’s not the only difference. While copy is a void function, copy_file returns a boolean to show if a copy was successful (true) or not (false).

Conclusion

Today, I shared with you a part of what I learned about the std::filesystem library. I must tell you that I found it pretty usable, despite the fact that sometimes I could have imagined a more intuitive API. We had a look into how to navigate up on a path, how to remove a filename from it and also we also checked how to copy, rename or delete files.

Next week, we are going to discuss how to iterate over a directory structure, a quite common operation when you have to apply some changes to a whole repository. Stay tuned!