Get relative path from two absolute paths - c++

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.

Answer 2

As of version 1.60.0 boost.filesystem does support this. You're looking for the member function path lexically_relative(const path& p) const.
Original, pre-1.60.0 answer below.
Boost doesn't support this; it's an open issue — #1976 (Inverse function for complete) — that nevertheless doesn't seem to be getting much traction.
Here's a vaguely naive workaround that seems to do the trick (not sure whether it can be improved):
#include <boost/filesystem/path.hpp>
#include <boost/filesystem/operations.hpp>
#include <boost/filesystem/fstream.hpp>
#include <stdexcept>
/**
* https://svn.boost.org/trac/boost/ticket/1976#comment:2
*
* "The idea: uncomplete(/foo/new, /foo/bar) => ../new
* The use case for this is any time you get a full path (from an open dialog, perhaps)
* and want to store a relative path so that the group of files can be moved to a different
* directory without breaking the paths. An IDE would be a simple example, so that the
* project file could be safely checked out of subversion."
*
* ALGORITHM:
* iterate path and base
* compare all elements so far of path and base
* whilst they are the same, no write to output
* when they change, or one runs out:
* write to output, ../ times the number of remaining elements in base
* write to output, the remaining elements in path
*/
boost::filesystem::path
naive_uncomplete(boost::filesystem::path const p, boost::filesystem::path const base) {
using boost::filesystem::path;
using boost::filesystem::dot;
using boost::filesystem::slash;
if (p == base)
return "./";
/*!! this breaks stuff if path is a filename rather than a directory,
which it most likely is... but then base shouldn't be a filename so... */
boost::filesystem::path from_path, from_base, output;
boost::filesystem::path::iterator path_it = p.begin(), path_end = p.end();
boost::filesystem::path::iterator base_it = base.begin(), base_end = base.end();
// check for emptiness
if ((path_it == path_end) || (base_it == base_end))
throw std::runtime_error("path or base was empty; couldn't generate relative path");
#ifdef WIN32
// drive letters are different; don't generate a relative path
if (*path_it != *base_it)
return p;
// now advance past drive letters; relative paths should only go up
// to the root of the drive and not past it
++path_it, ++base_it;
#endif
// Cache system-dependent dot, double-dot and slash strings
const std::string _dot = std::string(1, dot<path>::value);
const std::string _dots = std::string(2, dot<path>::value);
const std::string _sep = std::string(1, slash<path>::value);
// iterate over path and base
while (true) {
// compare all elements so far of path and base to find greatest common root;
// when elements of path and base differ, or run out:
if ((path_it == path_end) || (base_it == base_end) || (*path_it != *base_it)) {
// write to output, ../ times the number of remaining elements in base;
// this is how far we've had to come down the tree from base to get to the common root
for (; base_it != base_end; ++base_it) {
if (*base_it == _dot)
continue;
else if (*base_it == _sep)
continue;
output /= "../";
}
// write to output, the remaining elements in path;
// this is the path relative from the common root
boost::filesystem::path::iterator path_it_start = path_it;
for (; path_it != path_end; ++path_it) {
if (path_it != path_it_start)
output /= "/";
if (*path_it == _dot)
continue;
if (*path_it == _sep)
continue;
output /= *path_it;
}
break;
}
// add directory level to both paths and continue iteration
from_path /= path(*path_it);
from_base /= path(*base_it);
++path_it, ++base_it;
}
return output;
}

Answer 3

I just wrote code that can translate an absolute path to a relative path. It works in all my use cases, but I can not guarantee it is flawless.
I have abreviated boost::filesystem to 'fs' for readability. In the function definition, you can use fs::path::current_path() as a default value for 'relative_to'.
fs::path relativePath( const fs::path &path, const fs::path &relative_to )
{
// create absolute paths
fs::path p = fs::absolute(path);
fs::path r = fs::absolute(relative_to);
// if root paths are different, return absolute path
if( p.root_path() != r.root_path() )
return p;
// initialize relative path
fs::path result;
// find out where the two paths diverge
fs::path::const_iterator itr_path = p.begin();
fs::path::const_iterator itr_relative_to = r.begin();
while( itr_path != p.end() && itr_relative_to != r.end() && *itr_path == *itr_relative_to ) {
++itr_path;
++itr_relative_to;
}
// add "../" for each remaining token in relative_to
if( itr_relative_to != r.end() ) {
++itr_relative_to;
while( itr_relative_to != r.end() ) {
result /= "..";
++itr_relative_to;
}
}
// add remaining path
while( itr_path != p.end() ) {
result /= *itr_path;
++itr_path;
}
return result;
}

Answer 4

I was just thinking about using boost::filesystem for the same task, but - since my application uses both Qt and Boost libraries, I decided to use Qt which does this task with one simple method QString QDir::relativeFilePath( const QString & fileName ):
QDir dir("/home/bob");
QString s;
s = dir.relativeFilePath("images/file.jpg"); // s is "images/file.jpg"
s = dir.relativeFilePath("/home/mary/file.txt"); // s is "../mary/file.txt"
It works like a charm and saved me a few hours of my life.

Answer 5

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.

Answer 6

I have write down one simple solution for this trick.
There's no usage on boost libraries, only STL's std::string, std::vector.
The Win32 platform has been tested.
Just calling:
strAlgExeFile = helper.GetRelativePath(PathA, PathB);
And, it would return relative path from PathA to PathB.
Example:
strAlgExeFile = helper.GetRelativePath((helper.GetCurrentDir()).c_str(), strAlgExeFile.c_str());
#ifdef _WIN32
#define STR_TOKEN "\\"
#define LAST_FOLDER "..\\"
#define FOLDER_SEP "\\"
#define LINE_BREAK "\r\n"
#else
#define STR_TOKEN "/"
#define LAST_FOLDER "../"
#define FOLDER_SEP "/"
#define LINE_BREAK "\n"
#endif // _WIN32
void CHelper::SplitStr2Vec(const char* pszPath, vector<string>& vecString)
{
char * pch;
pch = strtok (const_cast < char*> (pszPath), STR_TOKEN );
while (pch != NULL)
{
vecString.push_back( pch );
pch = strtok (NULL, STR_TOKEN );
}
}
string& CHelper::GetRelativePath(const char* pszPath1,const char* pszPath2)
{
vector<string> vecPath1, vecPath2;
vecPath1.clear();
vecPath2.clear();
SplitStr2Vec(pszPath1, vecPath1);
SplitStr2Vec(pszPath2, vecPath2);
size_t iSize = ( vecPath1.size() < vecPath2.size() )? vecPath1.size(): vecPath2.size();
unsigned int iSameSize(0);
for (unsigned int i=0; i<iSize; ++i)
{
if ( vecPath1[i] != vecPath2[i])
{
iSameSize = i;
break;
}
}
m_strRelativePath = "";
for (unsigned int i=0 ; i< (vecPath1.size()-iSameSize) ; ++i)
m_strRelativePath += const_cast<char *> (LAST_FOLDER);
for (unsigned int i=iSameSize ; i<vecPath2.size() ; ++i)
{
m_strRelativePath += vecPath2[i];
if( i < (vecPath2.size()-1) )
m_strRelativePath += const_cast<char *> (FOLDER_SEP);
}
return m_strRelativePath;
}

Answer 7

I needed to do this without Boost and the other std based solution didn't do it for me so I reimplemented it. As I was working on this I realized that I'd done it before too...
Anyway, it's not as complete as some of the others but might be useful to people. It's Windows-specific; changes to make it POSIX involve directory separator and case sensitivity in the string compare.
Shortly after I got this implemented and working I had to transfer the surrounding functionality to Python so all of this just boiled down to os.path.relpath(to, from).
static inline bool StringsEqual_i(const std::string& lhs, const std::string& rhs)
{
return _stricmp(lhs.c_str(), rhs.c_str()) == 0;
}
static void SplitPath(const std::string& in_path, std::vector<std::string>& split_path)
{
size_t start = 0;
size_t dirsep;
do
{
dirsep = in_path.find_first_of("\\/", start);
if (dirsep == std::string::npos)
split_path.push_back(std::string(&in_path[start]));
else
split_path.push_back(std::string(&in_path[start], &in_path[dirsep]));
start = dirsep + 1;
} while (dirsep != std::string::npos);
}
/**
* Get the relative path from a base location to a target location.
*
* \param to The target location.
* \param from The base location. Must be a directory.
* \returns The resulting relative path.
*/
static std::string GetRelativePath(const std::string& to, const std::string& from)
{
std::vector<std::string> to_dirs;
std::vector<std::string> from_dirs;
SplitPath(to, to_dirs);
SplitPath(from, from_dirs);
std::string output;
output.reserve(to.size());
std::vector<std::string>::const_iterator to_it = to_dirs.begin(),
to_end = to_dirs.end(),
from_it = from_dirs.begin(),
from_end = from_dirs.end();
while ((to_it != to_end) && (from_it != from_end) && StringsEqual_i(*to_it, *from_it))
{
++to_it;
++from_it;
}
while (from_it != from_end)
{
output += "..\\";
++from_it;
}
while (to_it != to_end)
{
output += *to_it;
++to_it;
if (to_it != to_end)
output += "\\";
}
return output;
}