There have been requests for a Filesystem library relative function for at least ten years.
The requested functionality seems simple - given two paths with a common prefix, return the non-common suffix portion of one of the paths such that it is relative to the other path.
In terms of the Filesystem library,
path rel = relative(p, base); // the requested function
cout << rel << endl; // outputs "c"
assert(absolute(rel, base) == p);
If that was all there was to it, the Filesystem library would have had a relative function years ago.
Critical issues: Clashing requirements, symlinks, directory placeholders (dot, dot-dot), user-expectations, corner cases.
A paper by Jamie Allsop, Additions to Filesystem supporting Relative Paths, is what broke my mental logjam. Much of [my proposal] is based directly on Jamie's analysis and proposal. The weakly_canonical function and aspects of the semantic specifications are my contributions. Mistakes, of course, are mine.
The full proposal is at http://boostorg.github.io/filesystem/relative_proposal.html
A preliminary implementation is available in the feature/relative2 branch of the Boost Filesystem Git repository. See github.com/boostorg/filesystem/tree/feature/relative2
The overall plan is to get feedback from boost developers, then merge to develop for boost 1.60, then propose to the C++ committee for addition to the Filesystem TS.