C++ Experimental filesystem has no "relative" function? - c++

Question

I'm stuck with GCC7.1 for which I have to use #include <experimental/filesystem> instead of #include <filesystem>.
So I have namespace fs = std::experimental::filesystem; and then in the code I use fs::relative(p, base) which gives me error: ‘relative’ is not a member of ‘fs’.
It was working with normal C++17 filesystem, but doesn't seem to be present in experimental::filesystem. How can I use relative function with experimental::filesystem?

Answer 1

In addition to https://stackoverflow.com/a/63900421/9323999, it would be better to call weakly_canonical() backport instead of absolute() to properly handle '.' and '..' in paths. And as the answer above "(no complete solution - some extra edge cases need to be handled)":
[[nodiscard]] inline path weakly_canonical(path p)
{
// 1. convert p to an absolute path
p = absolute(p);
// 2. declare special strings to handle
using path_sv = std::basic_string_view<path::value_type>;
static constexpr path_sv dot{"."};
static constexpr path_sv dotdot{".."};
// 3. iterate `p` to handle its parts
path ret;
for (const auto &part : p)
if (part == dot)
continue; // skip '.'
else if (part == dotdot)
ret = ret.parent_path(); // move to parent path on '..'
else
ret /= part; // add other entries to the result
return ret;
}

Answer 2

You have to use third-party implementation (for ex. boost) or provide your own, something like (no complete solution - some extra edge cases need to be handled):
path relative(path p, path base)
{
// 1. convert p and base to absolute paths
p = fs::absolute(p);
base = fs::absolute(base);
// 2. find first mismatch and shared root path
auto mismatched = std::mismatch(p.begin(), p.end(), base.begin(), base.end());
// 3. if no mismatch return "."
if (mismatched.first == p.end() && mismatched.second == base.end())
return ".";
auto it_p = mismatched.first;
auto it_base = mismatched.second;
path ret;
// 4. iterate abase to the shared root and append "../"
for (; it_base != base.end(); ++it_base) ret /= "..";
// 5. iterate from the shared root to the p and append its parts
for (; it_p != p.end(); ++it_p) ret /= *it_p;
return ret;
}