Python-like abspath for c++

Tue, 06 May 2025 09:51:17 -0000
pdo, devel, eng, c++

Python's os.path.abspath or Path.absolute are great: you give them a path, which might not exist, and you get a path you can use regardless of the current directory. os.path.abspath will also normalize it, while Path will not by default because with Paths a normal form is less needed.

This is great to normalize input, regardless of if it's an existing file you're needing to open, or a new file you're needing to create.

In C++17, there is a filesystem library with methods with enticingly similar names, but which are almost, but not quite, totally unlike Python's abspath.

Because in my C++ code I need to normalize input, regardless of if it's an existing file I'm needing to open or a new file I'm needing to create, here's an apparently working Python-like abspath for C++ implemented on top of the std::filesystem library:

std::filesystem::path abspath(const std::filesystem::path& path)
{
    // weakly_canonical is defined as "the result of calling canonical() with a
    // path argument composed of the leading elements of p that exist (as
    // determined by status(p) or status(p, ec)), if any, followed by the
    // elements of p that do not exist."
    //
    // This means that if no lead components of the path exist then the
    // resulting path is not made absolute, and we need to work around that.
    if (!path.is_absolute())
        return abspath(std::filesystem::current_path() / path);

    // This is further and needlessly complicated because we need to work
    // around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118733
    unsigned retry = 0;
    while (true)
    {
        std::error_code code;
        auto result = std::filesystem::weakly_canonical(path, code);
        if (!code)
        {
            // fprintf(stderr, "%s: ok in %u tries\n", path.c_str(), retry+1);
            return result;
        }

        if (code == std::errc::no_such_file_or_directory)
        {
            ++retry;
            if (retry > 50)
                throw std::system_error(code);
        }
        else
            throw std::system_error(code);
    }

    // Alternative implementation that however may not work on all platforms
    // since, formally, "[std::filesystem::absolute] Implementations are
    // encouraged to not consider p not existing to be an error", but they do
    // not mandate it, and if they did, they might still be affected by the
    // undefined behaviour outlined in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118733
    //
    // return std::filesystem::absolute(path).lexically_normal();
}

I added it to my wobble code repository, which is the thin repository of components I use to ease my C++ systems programming.