Question

I have written a very simple file managing database that basicly looks like this:

class FileDB
{
public:
    FileDB(std::string dir) : rootDir(dir) { }

    void loadFile(std::string filename, File &file) const;
    void saveFile(std::string filename, const File &file) const;

private:
    std::string rootDir;
}

Now I would like to iterate through all files contained in the database like using a std::iterator:

void iterateFiles()
{
    FileDB filedb("C:\\MyFiles");

    for (FileDB::iterator file_it = filedb.begin(); file_it != filedb.end(); ++file_it)
    {
        File f = *file_it;
        // do something with file
    }
}

I’ve read answers to similar questions, some suggesting to derive std::iterator, some to use std::iterator_traits, but I don’t really understand how to do that. What can possibly go wrong when trying to implement a custom iterator? And what’s a simple yet elegant way to do it?

EDIT:
Please don’t consider using boost, my question is of more conceptual nature.

EDIT 2:

The FileDB works like this:

• rootDir

  • foo1
    • bar1
      • foo1bar1_1.txt
      • foo1bar1_2.txt
    • bar2
      • foo1bar2_1.txt
      • foo1bar2_2.txt

  • foo2

  • fooN

    • barM

      • fooNBarM_x.txt

So basicly, I can find a file by its name.

As my container is not in memory, I don’t have pointers to its data. So my idea was to store the file’s path in the iterator. This way, I can implement operator== with a string comparison, as the paths should be unique. The iterator returned from fileDB.end() would be an empty string and operator* would call fileDB::loadFile() with its filepath.

My biggest concern is about operator++. Having the filename, I can find out the containing directory and search for the next file, but this is really ineffective. Any ideas on how to do that? Or am I completely wrong with my whole concept?

Answer 1

class FileDB
{
class iterator;

public:
    FileDB(std::string dir) : m_rootDir(dir) { m_files = getFileTreeList(); }

    void loadFile(std::string filename, File &file) const;
    void saveFile(std::string filename, const File &file) const;


    iterator begin()
    {
      return iterator(m_files.begin(), *this);
    }
    iterator end()
    {
      return iterator(m_files.end(), *this);
    }

private:
    std::list<std::string> getFileTreeList() const;

private:    
    std::string m_rootDir;
    std::list<std::string> m_files;
}



class FileDB::iterator
{
public:
  iterator(std::list<std::string>::iterator pos, FileDB& owner) : m_pos(pos), m_owner(owner){}

  bool operator==(const iterator& rhs) {return m_pos == rhs.m_pos;}
  bool operator!=(const iterator& rhs) {return m_pos != rhs.m_pos;}
  //etc

  void operator++() {++m_pos;}

  File operator*()
  {
    return openFile(*m_pos); // for example open some file descriptor
  }

  ~iterator()
  {
    closeFile(*m_pos); // clean up!
  }

private:
  std::list<std::string>::iterator& m_pos;
  FileDB& m_owner;
};