Here's how I do it in the library I build on top of boost filesystem:
Step 1: Determine "deepest common root". Basically, its like the greatest common denominator for 2 paths. For example, if you're 2 paths are "C:\a\b\c\d" and "C:\a\b\c\l.txt" then the common root they both share is "C:\a\b\c\".
To get this, convert both paths into absolute- NOT canonical- form (you'll want to be able to do this for speculative paths & symlinks).
Step 2: To go from A to B, you suffix A with enough copies of "../" to shift up the directory tree to the common root, then add the string for B to travel down the tree to it. On windows you can have 2 paths with no common root, so going from any A to any B is not always possible.
namespace fs = boost::filesystem;
bool GetCommonRoot(const fs::path& path1,
const fs::path& path2,
fs::path& routeFrom1To2,
std::vector<fs::path>& commonDirsInOrder)
{
fs::path pathA( fs::absolute( path1));
fs::path pathB( fs::absolute( path2));
// Parse both paths into vectors of tokens. I call them "dir" because they'll
// be the common directories unless both paths are the exact same file.
// I also Remove the "." and ".." paths as part of the loops
fs::path::iterator iter;
std::vector<fs::path> dirsA;
std::vector<fs::path> dirsB;
for(iter = pathA.begin(); iter != pathA.end(); ++iter) {
std::string token = (*iter).string();
if(token.compare("..") == 0) { // Go up 1 level => Pop vector
dirsA.pop_back();
}
else if(token.compare(".") != 0) { // "." means "this dir" => ignore it
dirsA.push_back( *iter);
}
}
for(iter = pathB.begin(); iter != pathB.end(); ++iter) {
std::string token = (*iter).string();
if(token.compare("..") == 0) { // Go up 1 level => Pop vector
dirsB.pop_back();
}
else if(token.compare(".") != 0) { // "." means "this dir" => ignore it
dirsB.push_back( *iter);
}
}
// Determine how far to check in each directory set
size_t commonDepth = std::min<int>( dirsA.size(), dirsB.size());
if(!commonDepth) {
// They don't even share a common root- no way from A to B
return false;
}
// Match entries in the 2 vectors until we see a divergence
commonDirsInOrder.clear();
for(size_t i=0; i<commonDepth; ++i) {
if(dirsA[i].string().compare( dirsB[i].string()) != 0) { // Diverged
break;
}
commonDirsInOrder.push_back( dirsA[i]); // I could use dirsB too.
}
// Now determine route: start with A
routeFrom1To2.clear();
for(size_t i=0; i<commonDepth; ++i) {
routeFrom1To2 /= dirsA[i];
}
size_t backupSteps = dirsA.size() - commonDepth; // # of "up dir" moves we need
for(size_t i=0; i<backupSteps; ++i) {
routeFrom1To2 /= "../";
}
// Append B's path to go down to it from the common root
for(size_t i=commonDepth; i<dirsB.size(); ++i) {
routeFrom1To2 /= dirsB[i]; // ensures absolutely correct subdirs
}
return true;
}
This will do what you want- you go up from A until you hit the common folder it and B are both descendants of, then go down to B. You probably don't need the "commonDirsInOrder" return that I have, but the "routeFrom1To2" return IS the one you're asking for.
If you plan to actually change the working directory to "B" you can use "routeFrom1To2" directly. Be aware that this function will produce an absolute path despite all the ".." parts, but that shouldn't be a problem.
