从两条绝对路径中获取相对路径

对于C++17及其从boost进化而来的std::filesystem::relative，这是一个很容易的问题：

#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.
}

输出(第二个参数为基础(

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

它使用std::filesystem::path::lexically_relative进行比较。与纯词法函数的区别在于，std::filesystem::relative解析符号链接，并使用std::filesystem::weakly_canonical(它是为relative引入的(。

1.60.0版本的boost.filesystem确实支持此功能。您正在查找成员函数path lexically_relative(const path& p) const。

原始，1.60.0之前的答案如下。

Boost不支持这一点；这是一个悬而未决的问题#1976(完全反函数(—然而，这似乎并没有得到太多的支持。

这里有一个看似天真的变通方法，似乎可以奏效(不确定是否可以改进(：

#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;
}

我刚刚编写了可以将绝对路径转换为相对路径的代码。它适用于我的所有用例，但我不能保证它是完美无瑕的。

为了可读性，我将boost:：filesystem省略为"fs"。在函数定义中，可以使用fs:：path:：current_path((作为"relative_to"的默认值。

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;
}

我只是想将boost::filesystem用于同一任务，但由于我的应用程序同时使用Qt和Boost库，我决定使用Qt，它通过一个简单的方法QString QDir:：relativeFilePath(const QString&fileName(:来完成此任务

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"

它就像一个符咒，救了我几个小时的命。

以下是我在boost文件系统之上构建的库中的操作方法：

步骤1：确定"最深的共同根"。基本上，它就像是两条路径的最大公约数。例如，如果有两个路径是"C:\a\b\C\d"answers"C:\a\\b\C\l.txt"，那么它们共享的公共根是"C:\a\b\C\"。

要做到这一点，请将这两个路径转换为绝对形式，而不是规范形式(您希望能够为推测路径和符号链接执行此操作(。

步骤2：要从A转到B，您需要在A后面加上足够多的"../"副本，以将目录树向上移动到公共根，然后添加字符串，使B沿着目录树向下移动到它。在windows上，您可以有两个没有公共根的路径，因此从任何A到任何B都不可能。

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;

}

这将做你想做的事情-你从A向上，直到你找到它和B都是其后代的公共文件夹，然后向下到B。你可能不需要我的"commonDirsInOrder"返回，但"routeFrom1To2"返回是你想要的。

如果您计划实际将工作目录更改为"B"，则可以直接使用"routeFrom1To2"。请注意，尽管有".."部分，该函数仍将生成一个绝对路径，但这应该不是问题。

我已经为这个技巧写下了一个简单的解决方案。boost库上没有使用，只有STL的std::string、std::vector。

Win32平台已经过测试。

只是打电话：

strAlgExeFile = helper.GetRelativePath(PathA, PathB);

并且，它将返回从PathA到PathB的相对路径。

示例：

strAlgExeFile = helper.GetRelativePath((helper.GetCurrentDir()).c_str(), strAlgExeFile.c_str());
#ifdef _WIN32                                                                              
    #define STR_TOKEN "\"                                                                 
    #define LAST_FOLDER "..\"                                                             
    #define FOLDER_SEP "\"                                                                
    #define LINE_BREAK "rn"                                                              
#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;                                                              
}

我需要在没有Boost的情况下完成这项工作，而另一个基于std的解决方案不适合我，所以我重新实现了它。当我在做这项工作时，我意识到我以前也做过。。。

不管怎样，它不像其他一些那样完整，但可能对人们有用。它是特定于Windows的；使其成为POSIX的更改涉及字符串比较中的目录分隔符和区分大小写。

在我实现并工作后不久，我不得不将周围的功能转移到Python，所以所有这些都归结为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;
}