std::filesystem::path in a Linux/POSIX environment: root_drive vs root_directory vs root_path

Given an instance of std::filesytem::path in a Linux environment, what exactly do the root_name, root_directory, and root_path methods return?

I understand that in the Linux/POSIX environment, the UNC notation is not used. Therefore, is it true that in the Linux/POSIX environment, root_name has no function or meaning?

While cppreference does take a bit of reading to find out what the string would actually be, you can always do a bit of testing yourself:

#include <filesystem>
#include <iostream>

int main()
{
    std::filesystem::path path="/tmp/foo/bar";
    std::cout << "full: "  << path << '\n';
    std::cout << "root name: "  << path.root_name() << '\n';
    std::cout << "root path: "  << path.root_path() << '\n';
    std::cout << "root directory: "  << path.root_directory() << '\n';
}

stieber@gatekeeper:~ $ g++ Test.cpp && ./a.out
full: "/tmp/foo/bar"
root name: ""
root path: "/"
root directory: "/"