Get relative path from two absolute paths

Question

I have two absolute filesystem paths (A and B), and I want to generate a third filesystem path that represents "A relative from B".

Use case:

• Media player managing a playlist.
• User adds file to playlist.
• New file path added to playlist relative to playlist path.
• In the future, entire music directory (including playlist) moved elsewhere.
• All paths still valid because they are relative to the playlist.

boost::filesystem appears to have complete to resolve relative ~ relative => absolute, but nothing to do this in reverse (absolute ~ absolute => relative).

I want to do it with Boost paths.

Answer 1

With C++17 and its std::filesystem::relative, which evolved from boost, this is a no-brainer:

#include <filesystem> #include <iostream> namespace fs = std::filesystem; int main() {     const fs::path base("/is/the/speed/of/light/absolute");     const fs::path p("/is/the/speed/of/light/absolute/or/is/it/relative/to/the/observer");     const fs::path p2("/little/light/races/in/orbit/of/a/rogue/planet");     std::cout << "Base is base: " << fs::relative(p, base).generic_string() << '\n'               << "Base is deeper: " << fs::relative(base, p).generic_string() << '\n'               << "Base is orthogonal: " << fs::relative(p2, base).generic_string();     // Omitting exception handling/error code usage for simplicity. } 

Output (second parameter is base)

Base is base: or/is/it/relative/to/the/observer Base is deeper: ../../../../../../.. Base is orthogonal: ../../../../../../little/light/races/in/orbit/of/a/rogue/planet 

It uses std::filesystem::path::lexically_relative for comparison. The difference to the pure lexical function is, that std::filesystem::relative resolves symlinks and normalizes both paths using std::filesystem::weakly_canonical (which was introduced for relative) before comparison.